COMET ISON NEWS

It seems like a portion of Comet ISON may have survived, but it is difficult to say if it will amount to much. At the moment, it seems like ISON might fade. It was between magnitude 4 and 5 as of Friday, November 29th in the evening. However, it could continue to surprise us, but we will have to wait and see.

Early obituary?

The most epic thing about Comet ISON was that we were able to see the journey of an ancient comet that existed prior to the dawn of mankind. It met its destiny with the sun, and came out victoriously much to the surprise of everyone, but was severely damaged. The elemental contents of Comet ISON share the exact elements that compose almost every object in the Solar System, whether it’s the sun, the moon, the Earth or even yourself. Think about that. It’s very fascinating. Whatever is left of ISON will safely continue on its orbit and eventually go back home to the Oort cloud… if it does not disintegrate long before then.

Comet ISON has survived perihelion, but only a small part of it of it seems leftover. Several videos have been updated featuring imagery from the past few days and converted into videos. It seems like ISON might fade. It was between magnitude 4 and 5 as of Friday, November 29 in the evening. However, it could continue to surprise us, but we will have to wait and see.

Remarkable Days

Here is a list of all the information published within the last month, starting out with the date and a link for that information. Here is an archive for October 2013.
1 - Several observations of Comet ISON from various sources suggest it is now viewable in 10x50 binoculars, but faint
2 - Comet ISON update for November 2, 2013
3 - Partial solar eclipse shot in Queens, New York
5 - Comet ISON moves into the area of the constellation Virgo
5 - Comet ISON update for November 5, 2013
10 - Comet ISON now has two long tails
13 - Reports suggest ISON’s inner coma brightness has increased by one magnitude. Is ISON fragmenting or brightening up?
14 - Observation reports suggest that Comet ISON is now visible to the naked eye in good viewing conditions as a faint magnitude 6 object
16 - Interference from the moon’s light resumes.
18 - Comet ISON update for November 18
22 - Comet ISON update for November 22
28 - Comet ISON reaches perihelion. Will ISON survive perihelion? Will ISON disintegrate? Will ISON shine brighter than the planet Venus?

Resources

Universe Today
Space.com
Sky & Telescope
pleine-lune.org
Astronomy
ISONblog
Comet ISON Observing Campaign (CIOC)
Remanzacco Observatory in Italy
SpaceWeather.com
CloudyNights.com
Astroblog
Waiting for ISON
Comet ISON Observations by Bruce Gary
Komet ISON (German)
Kometen.info (German)

For Fun

Art Bell | Dark Matter
Science Fantastic with Michio Kaku (Podcast)
Explorations in Science with Michio Kaku (Podcast)
Coast to Coast AM

FAQ

How Big is Comet ISON?

“Images captured June 13 with Spitzer’s Infrared Array Camera indicate carbon dioxide is slowly and steadily “fizzing” away from the so-called “soda-pop comet,” along with dust, in a tail about 186,400 miles (300,000 kilometers) long.

“We estimate ISON is emitting about 2.2 million pounds (1 million kilograms) of what is most likely carbon dioxide gas and about 120 million pounds (54.4 million kilograms) of dust every day,” said Carey Lisse, leader of NASA’s Comet ISON Observation Campaign and a senior research scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. “Previous observations made by NASA’s Hubble Space Telescope and the Swift Gamma-Ray Burst Mission and Deep Impact spacecraft gave us only upper limits for any gas emission from ISON. Thanks to Spitzer, we now know for sure the comet’s distant activity has been powered by gas.”
– NASA 07/23/2013

Comet ISON was emitting about 2.2 million pounds (1 million kilograms) of carbon dioxide gas per day in a tail about 186,400 miles (300,000 kilometers) long on June 13, 2013.

Here are some quick back-of-the-envelope numbers based on the information from June 13, 2013.

2.2 million pounds (1 million kilograms) equals 35,200,000 ounces.
As a comparison, a can of soda is roughly 12 ounces.

35200000/12 = 2,933,333.3 cans
3,074,186 = total population of Iowa estimated in 2012

(3074186*.235) = population under 18 years of age
3,074,186 - (3,074,186*.235) = 2,351,752 adults
2,933,333.3 - 2,351,752 = 581581.3
That’s the equivalent of 1 can of soda for every adult Iowan per day, with 581,581 having another serving.

As another example, an old vending machine can hold about 368 beverages.

2,933,333.3 /368 = 7971.01 vending machines
Keep in mind that the above example only uses the 2.2 million pounds (1 million kilograms) of carbon dioxide gas emitted by ISON. Comet ISON also ejects 120 million pounds (54.4 million kilograms) of dust.

The total mass of the Statue of Liberty is approximately 225 tons (or 450,000 pounds).

120 million pounds / 450,000 pounds = 266.66
That’s enough dust to weigh in the same as 266 Statues of Liberty per day.

Comet ISON is less than 3 miles (4.8 kilometers) in diameter, about the size of a small mountain, and weighs between 7 billion and 7 trillion pounds (3.2 billion and 3.2 trillion kilograms).

Let’s take the smaller number.

2.2 million pounds / 7 billion pounds * 100 = % of carbon dioxide emitted per day.
2.2 million pounds / 7 billion pounds * 100 = 0.0314%
Comet ISON emits less than 0.031% of its mass of carbon dioxide per day.

120 million pounds / 7 billion pounds * 100 = % of dust emitted per day
120 million pounds / 7 billion pounds * 100 = 1.714%
Comet ISON emits around 1.71% of its mass as dust per day, if we take the much smaller number.

If we take the much larger estimation of 7 trillion pounds, we get less than 0.000031% of its mass emitted as carbon dioxide per day.
If we also use the larger figure to compare the dust, we get less than 0.0017% of its mass emitted as dust per day.

Even though the larger estimation of 7 trillion pounds seems like a daunting amount, it’s fairly tiny in the scope of things here on Earth. In a May 2003 Outside magazine article, Roger Bilham, a geophysics professor at the University of Colorado, estimated that Mount Everest has a mass of around 357 trillion pounds.

Amazing!

Comet ISON is not a planet just a comet.

Let’s compare some sizes.

Size Comparisons

Saturn has a diameter of approximately 72,367 miles (116,464 km). That’s approximately 9.1 Earth diameters.
Jupiter has a diameter of approximately 86,881 miles (139,822 km). That’s approximately 10.9 Earth diameters.
Mars has a diameter of approximately 4,212 miles (6,779 km). That’s approximately 0.53 Earth diameters.

Earth has a diameter of approximately 7918 miles (12,742 km).
Earth’s moon has a diameter of approximately 2159.14 miles (3474.8 km). That’s larger than dwarf planet Pluto’s diameter of 1466.44 miles (2360 km) but smaller than Mercury’s diameter of 3032 miles (4879 km).


The diameter of Comet ISON is approximately 3 miles (4.8 km).

Based on observations by NASA’s Spitzer Space Telescope, the nucleus, or body of Comet ISON was approximately 3 miles (4.8 km) on June 13, 2013. To put this into perspective, the diameter of the nucleus, the main rocky and icy body of Comet ISON would be the same distance as a drive in downtown Dallas, Texas. You can use Google Earth to plot a measurement on any major city to get an idea of what 3 miles (4.8 km) looks like.

Here’s another interesting comparison.

According to the USGS, all the water on Earth would create a sphere of water with a diameter of 860 miles. If you only took the drinkable liquid fresh water, you would have a sphere about 169.6 miles in diameter. If you only took the amount of fresh water in lakes and rivers, you would have a sphere with a diameter of 34.9 miles.

Comet ISON’s diameter of 3 miles (4.8 km) is less than 1/10th of that last amount.

Comet ISON’s coma, which is a cloud of gas, was approximately 3,100 miles across, or 1.2 times the width of Australia, based on observations from NASA’s Hubble Space Telescope on April 10, 2013.
Since Comet ISON is a comet and not a planet or a moon, it has a dust tail.

On January 30, 2013, NASA’s Swift Telescope observed that Comet ISON’s dust tail extended more than 57,000 miles.

A few months later on June 13, 2013, NASA’s Spitzer Space Telescope observed that Comet ISON’s dust trail extended more than 186,400 miles (300,000 kilometers) long.

Comet ISON is expected to become visible to the eye in mid-November as it approaches the sun and reaches perihelion on November 28, 2013.

As a comparison, Comet Halley’s tail was approximately 13.67 million miles (22 million km) on February 22, 1986, thirteen days after it reached perihelion on February 9, 1986.

Comet ISON Wiki

Note: Comet ISON could potentially be brighter than full moon, but the caveat is that this is when ISON will be right next to the sun! Comet ISON will more than likely reach the brightness of Venus, which is around magnitude -4 as viewed from an urban area.

C/2012 S1 (ISON)

Comet ISON as captured by TRAPPIST on 15 November 2013

DISCOVERY
DISCOVERED BY Vitaly Nevsky and
Artyom Novichonok
at ISON-Kislovodsk, Russia
using a 0.4-m reflector (D00)[1]
DISCOVERY DATE 21 September 2012

ORBITAL CHARACTERISTICS A
EPOCH 14 December 2013
(JD 2456640.5)[2]
PERIHELION 0.01244 AU (q)[2]
ECCENTRICITY 0.9999947[2]
1.0002 (epoch 2050)[3]
ORBITAL PERIOD Ejection trajectory (epoch 2050)[3]
INCLINATION 62.4°[2]
LAST PERIHELION 28 November 2013[2]

Comet ISON, formally known as C/2012 S1, was a sungrazing comet discovered on 21 September 2012 by Vitali Nevski (Виталий Невский, Vitebsk, Belarus) and Artyom Novichonok (Артём Новичонок, Kondopoga, Russia).[4] The discovery was made using the 0.4-meter (16 in) reflector of the International Scientific Optical Network (ISON) near Kislovodsk, Russia.[1] Data processing was carried out by automated asteroid-discovery program CoLiTec.[5]Precovery images by the Mount Lemmon Survey from 28 December 2011 and by Pan-STARRS from 28 January 2012 were quickly located.[6] Follow-up observations were made on 22 September by a team from Remanzacco Observatory in Italy using the iTelescope network.[1][7] The discovery was announced by the Minor Planet Center on 24 September.[6] Observations by Swift in January 2013 suggested that Comet ISON’s nucleus was around 5 kilometers (3 mi) in diameter.[8] Later estimates were that the nucleus was only about 2 kilometers (1 mi) in diameter.[9]Mars Reconnaissance Orbiter (MRO) observations suggested the nucleus was smaller than 0.8 kilometers (0.5 mi) in diameter.[10]

Shortly after Comet ISON’s discovery, the media reported that it might become brighter than the full moon. However, as events transpired, it never became bright enough to be readily visible to the naked eye. Furthermore, it broke apart as it passed close to the Sun. Reports on 28 November 2013 (the day of perihelion passage)[11][12] indicated that Comet ISON had partially or completely disintegrated due to the Sun’s heat and tidal forces. However, later that day CIOC (NASA Comet ISON Observing Campaign) members discovered a coma-like feature, suggesting a small fragment of it may have survived perihelion.[11][12][13][14][15] On 29 November 2013, the coma dimmed to an apparent magnitude of 5.[16] By the end of 30 November 2013, the coma had further faded to below naked-eye visibility at magnitude 7.[17] On 1 December, the coma continued to fade even further as it finished traversing the Solar and Heliospheric Observatory’s view.[18][19] On 2 December 2013, the CIOC announced that Comet ISON had fully disintegrated.[20][21] The Hubble Space Telescope failed to detect fragments of ISON on 18 December 2013.[22]

Discovery

During routine observations on 21 November 2012, Vitali Nevski and Artyom Novichonok monitored areas of Gemini and Cancer after their observations were delayed by clouded weather for much of the night. The team used ISON’s 0.4-meter (16 in) reflector near Kislovodsk, Russia and CCD imaging to carry out their observations. Shortly after their session, Nevski processed data using CoLiTec, an automated asteroid discovery software program. In analysis he noted an unusually bright object with slow apparent movement, indicating a position outside the orbit of Jupiter based on the use of four 100-second CCD exposures.[23][24] At the time of discovery, the object’s apparent magnitude ranged from 19.1 to as bright as 18.8.[note 1][25]

The group reported their discovery to the Central Bureau for Astronomical Telegrams as an asteroidal object, which was subsequently forwarded to the Minor Planet Center. However, the group later reported that the object had a cometary appearance with a coma approximately 8 arcseconds across.[24] The object’s position and cometary appearance was confirmed by several other unaffiliated observers, and as such the comet was named ISON, after the international observational project and in accordance with International Astronomical Union naming guidelines.[23][24] Comet ISON was precovered in analysis of Mount Lemmon Observatory imagery by G.V. Williams and Pan-STARRS imagery in Haleakalā. Precovery images from Mount Lemmon were first taken on 28 December 2011 and indicated that the comet had an estimated apparent magnitude ranging from 19.5 to 19.9. Images from Pan-STARRS were taken on 28 January 2012 and in those images the comet had an estimated apparent magnitude ranging from 19.8 to 20.6.[23]

Orbit

Comet ISON came to perihelion (closest approach to the Sun) on 28 November 2013 at a distance of 0.0124 AU (1,860,000 km; 1,150,000 mi) from the center point of the Sun.[2] Accounting for the solar radius of 695,500 km (432,200 mi), Comet ISON passed approximately 1,165,000 km (724,000 mi) above the Sun’s surface.[26] Its trajectory appeared to be hyperbolic, which suggested that it was a dynamically new comet coming freshly from the Oort cloud.[27][28] Near perihelion, a generic heliocentric two-body solution to the orbit suggests that the orbital period was around 400,000 years.[29] But for objects at such high eccentricity, the Sun’s barycentric coordinates are more stable than heliocentric coordinates.[30] The orbit of a long-period comet is properly obtained when the osculating orbit is computed at an epoch after leaving the planetary region and is calculated with respect to the center of mass of the Solar System. Using JPL Horizons, the barycentric orbital elements for epoch 1 January 2050 generate a hyperbolic solution.[3] On its closest approach, Comet ISON passed about 0.07248 AU (10,843,000 km; 6,737,000 mi) from Mars on 1 October 2013, and the remnants of Comet ISON passed about 0.43 AU (64,000,000 km; 40,000,000 mi) from Earth on 26 December 2013.[31]

Shortly after its discovery, similarities between the orbital elements of Comet ISON and the Great Comet of 1680 led to speculation that there might be a connection between them.[32] Further observations of ISON, however, showed that the two comets are not related.[33]

Earth passed near the orbit of Comet ISON on 14–15 January 2014, at which time micron-sized dust particles blown by the Sun’s radiation may cause a meteor shower or noctilucent clouds;[34][35] however, both events are unlikely. Because Earth only passes near Comet ISON’s orbit, not through the tail, the chances that a meteor shower will occur are slim.[36] In addition, meteor showers from long-period comets that make just one pass into the inner solar system are very rare, if ever recorded.[37] The possibility that small particles left behind on the orbital path—almost one hundred days after the nucleus has passed—could form noctilucent clouds is also slim. No such events are known to have taken place in the past under similar circumstances.[37]


Position of comet remnants on 11 December 2013


Visualization of the orbit of comet ISON as it moved into the inner Solar System in 2013

Brightness, observations, and visibility

At the time of its discovery, Comet ISON’s apparent magnitude was approximately 18.8, far too dim to be seen with the naked eye, but bright enough to be imaged by amateurs with large telescopes.[38][39] It then followed the pattern of most comets and increased


The path of Comet ISON from December 2012 through October 2013 as it passed through Gemini, Cancer, and Leo

discovery, Comet ISON’s apparent magnitude was approximately 18.8, far too dim to be seen with the naked eye, but bright enough to be imaged by amateurs with large telescopes.[38][39] It then followed the pattern of most comets and increased gradually in brightness on approach to the Sun.

At least a dozen spacecraft imaged Comet ISON.[21] It was first imaged by the Swift and Deep Impact spacecraft in January and February 2013, and shown to be active with an extended tail. In April and May 2013 the Hubble Space Telescope (HST) measured Comet ISON’s size, and the color, extent, and polarization of its emitted dust. The Spitzer Space Telescope (SST) observed Comet ISON on 13 June and estimated carbon dioxide outgassing at about 1 million kilograms (2.2 million pounds) per day.[40] From 5 June to 29 August 2013, Comet ISON had an elongation less than 30 degrees from the Sun.[41] No obvious rotational variability was detected by either Deep Impact, HST, or Spitzer. Amateur astronomer Bruce Gary recovered it on 12 August 2013 when it was 6 degrees above the horizon and 19 degrees from the Sun.[42] Due to it brightening more slowly than predicted, Comet ISON only became visible through small telescopes during early October 2013.[43]


STEREO-B COR2 image of Comet ISON re-emerging ~7 hours after perihelion

On 1 October 2013, Comet ISON passed within 0.07 AU (10,000,000 km; 6,500,000 mi) of Mars. Between 29 September and 2 October, the Mars Reconnaissance Orbiter (MRO) detected Comet ISON.[44] The twin STEREO spacecraft began detecting Comet ISON in the second week of October.[45] October 2013 images of Comet ISON displayed a greenish tint, probably attributable to the release of cyanogen and diatomic carbon.[46] On 31 October 2013, Comet ISON was detected with 10×50 binoculars.[47]

On 14 November 2013, Comet ISON was reported to be visible to the naked eye by experienced observers located at dark sites.[48] It had an appearance similar to comet C/2013 R1 that is also visible to the naked eye. Comet ISON was not expected to reach the naked-eye magnitude of 6 until mid-November,[41][49] and was not expected to be observable by the general public until it brightened to about magnitude 4.[43] On 17–18 November, when Comet ISON was brighter and much closer to the morning twilight, it passed the bright star Spica in the constellation Virgo.[50] But due to the full moon and glow of twilight, Comet ISON had not become bright enough to be seen without optical aid by the general public. On 22 November, it started to drop below Mercury in the bright twilight.[51]SOHO started to view it on 27 November, first with the LASCO coronograph.[45][52] On 27 November ISON brightened to magnitude −2[note 2][53] and passed Delta Scorpii.[54] Around the time it reached perihelion on 28 November, it might have become extremely bright if it had remained fully intact. However, predicting the brightness of a comet is difficult, especially one that passes so close to the Sun and is affected by the forward scattering of light. Originally, media sources predicted that it might become brighter than the full moon,[27][28] but based on more recent observations, it was only expected to reach around apparent magnitude −3 to −5, about the same brightness as Venus.[49][55] In comparison, the brightest comet since 1935 was Comet Ikeya–Seki in 1965 at magnitude −10, which was much brighter than Venus.[56] On 29 November 2013, Comet ISON had dimmed to magnitude 5 in the LASCO images.[16] By the end of 30 November 2013, it had further faded to below naked-eye visibility at magnitude 7.[17]


Comet ISON, imaged by the Hubble Space Telescope on 10 April 2013—near Jupiter’s orbit;[57] also, enhanced (coma model ratio) version

In a February 2013 study, 1,897 observations were used to create a light curve. The resulting plot showed Comet ISON increasing its brightness relatively quickly at R+4.35.[Unit?][58] If this had continued to perihelion, it would have reached magnitude −17, brighter than the full moon. It had since exhibited a “slowdown event”, however, similar to the ones exhibited by many other Oort cloud comets, among them C/2011 L4. Therefore, Comet ISON’s brightness increased less quickly than predicted and it did not become as bright as expected. Further observations suggested that, even if it had remained intact, it might only brighten to about magnitude −6.[55] It had been determined that it was a “baby comet” (i.e. an object with a photometric age less than four comet years).[58] The temperature at perihelion had been calculated to reach 2,700 °C (4,890 °F), sufficient to melt iron. Additionally, it was within the Roche limit, meaning it might disintegrate due to the Sun’s gravity.

Comet ISON was expected to be brightest around the time it was closest to the Sun; but because it was less than 1° from the Sun at its closest, it would have been difficult to see against the Sun’s glare.[59] Comet ISON would have been well placed for observers in the northern hemisphere during mid to late December 2013.[60] If it had survived its perihelion passage fully intact, it might have remained visible to the naked eye until January 2014.[27][39] As Comet ISON moved north on the celestial sphere it would have passed within two degrees of Polaris on 8 January.[39]

Name


Comet ISON seen from the Mount Lemmon SkyCenter on 8 October 2013, as it passes through the constellation of Leo

For more details on this topic, see Naming of comets.
Comet ISON’s formal designation was C/2012 S1.[note 3][61] It was named “ISON” after the organization where its discovery was made, the Russia-based International Scientific Optical Network. The initial report of the object to the Central Bureau for Astronomical Telegrams identified the object as an asteroid, and it was listed on the Near Earth Objects Confirmation Page. Follow-up observations by independent teams were the first to report cometary features. Therefore, under the International Astronomical Union’s comet-naming guidelines, Comet ISON was named after the team that discovered it, rather than the individual discoverers.[62]

Media coverage

After it was discovered in 2012, some media sources called Comet ISON the “Comet of the Century” and speculated that it might outshine the full moon.[63] An Astronomy Now columnist wrote in September 2012 that “if predictions hold true then Comet ISON will certainly be one of the greatest comets in human history.”[27] As recently as October 2013, a Daily Mail columnist described Comet ISON as “the Comet of the Century” and said it was “hoped to be 15 times brighter than the Moon.”[64]

Astronomer Karl Battams criticized the media’s suggestion that Comet ISON would be ‘brighter than the full moon’, saying that members of the Comet ISON Observing Campaign did not foresee ISON becoming that bright.[65]

Comet ISON has been compared to Comet Kohoutek, seen in 1973–4, another highly-anticipated Oort cloud comet that peaked early and fizzled out.[66][67]

Notes

^ Astronomical magnitudes decrease as brightness increases, from large positive values, through zero, to negative values for very bright objects.
^ Astronomical magnitudes decrease as brightness increases, from large positive values, through zero, to negative values for very bright objects.
^ The “C” indicates that it was non-periodic, followed by the year of discovery. The “S” represents the half-month of discovery—in the case of C/2012 S1, the second half of September—and the number “1” shows that this was the first comet found in that half month.

References

^ a b c Guido, Ernesto; Sostero, Giovanni; Howes, Nick (24 September 2012). “New Comet: C/2012 S1 (ISON)”. Associazione Friulana di Astronomia e Meteorologia. Retrieved 24 September 2012.
^ a b c d e f “MPEC 2013-W16 : COMET C/2012 S1 (ISON)”. IAU Minor Planet Center. 26 November 2013. Retrieved 27 November 2013.
^ a b c Horizons output. “Barycentric Osculating Orbital Elements for Comet C/2012 S1 (ISON)”. NASA. Retrieved 25 November 2012. (Solution using the Solar System Barycenter and barycentric coordinates. Select Ephemeris Type:Elements and Center:@0)
^ Trigo-Rodríguez, J. M.; Meech, K. J.; Rodriguez, D.; Sánchez, A.; Lacruz, J.; Riesen, T. E. (2013). “Post-discovery Photometric Follow-up of Sungrazing Comet C/2012 S1 ISON”. 44th Lunar and Planetary Science Conference. 18–22 March 2013. The Woodlands, Texas. #1576.
^ “Open the Great Comet Comet C/2012 S1 (ISON)”. Neoastrasoft.com. 12 October 2012. Retrieved 11 September 2013.
^ a b “MPEC 2012-S63: Comet C/2012 S1 (ISON)”. IAU Minor Planet Center. 24 September 2012. CK12S010. Retrieved 24 September 2012.
^ Atkinson, Nancy (25 September 2012). “New ‘Sun-Skirting’ Comet Could Provide Dazzling Display in 2013”. Universe Today. Retrieved 28 September 2012.
^ Reddy, Francis (29 March 2013). “NASA’s Swift Sizes Up Comet ISON”. NASA.gov. Retrieved 23 April 2013.
^ Plait, Phil (21 November 2013). “12 Cool Facts about Comet ISON”. Slate (magazine). Retrieved 28 November 2013.
^ Fox, Karen C. (10 December 2013). “Fire vs. Ice: The Science of ISON at Perihelion”. NASA.gov. Retrieved 11 December 2013.
^ a b Battams, Karl (28 November 2013). “Schrödinger’s Comet”. CIOC. Retrieved 28 November 2013.
^ a b MacRobert, Alan (28 November 2013). “Latest Updates on Comet ISON”. Sky & Telescope. Retrieved 28 November 2013.
^ https://twitter.com/SungrazerComets/status/406252924705071104
^ Plait, Phil (28 November 2013). “ISON Update for 22:00 UTC Nov. 28”. Slate (magazine). Retrieved 28 November 2013.
^ Chang, Kenneth (29 November 2013). “Comet ISON, Presumed Dead, Shows New Life”. The New York Times. Retrieved 29 November 2013.
^ a b Battams, Karl (29 November 2013). “In ISON’s Wake, a Trail of Questions”. CIOC. Retrieved 30 November 2013.
^ a b https://twitter.com/SungrazerComets/status/406940937919533056
^ Comet ISON’s leftovers fade away, right before a satellite’s eyes. Boyle, Alan. NBC News. (November 30, 2013.) Retrieved December 9, 2013.
^ “comet ISON’s current status”. NASA Comet ISON Observing Campaign. 30 November 2013. Retrieved 1 December 2013.
^ Battams, Karl (2 December 2013). “In Memoriam”. CIOC. Retrieved 2 December 2013.
^ a b Fox, Karen C. (2 December 2013). “NASA Investigating the Life of Comet ISON”. NASA.gov. Retrieved 2 December 2013.
^ Levay, Zolt (20 December 2013). “BREAKING NEWS: Comet ISON Is Still Dead”. Hubblesite.org. Retrieved 21 December 2013.
^ a b c Kronk, Gary W. “C/2012 S1 (ISON)”. Cometography. Cometography.com. Retrieved 12 December 2013.
^ a b c Cometary Science Archive. “Comet C/2012 S1 (ISON)”. Cambridge, Massachusetts: Harvard University. Retrieved 12 December 2013.
^ Central Bureau for Astronomical Telegrams; International Astronomical Union (24 September 2012). “Electronic Telegram No. 3258” (TXT). Cambridge, Massachusetts: Harvard University. Retrieved 12 December 2013. Cite uses deprecated parameters (help)
^ Pickup, Alan (13 October 2013). “Starwatch: The brightening of ISON”. The Guardian. Retrieved 16 October 2013.
^ a b c d Grego, Peter (25 September 2012). “New comet might blaze brighter than the full Moon”. Astronomy Now. Retrieved 28 September 2012.
^ a b Hecht, Jeff (25 September 2012). “Newly spotted comet may outshine the full moon”. New Scientist. Retrieved 28 September 2012.
^ Plait, Phil (25 November 2013). “Is Comet ISON Heading for Interstellar Space?”. Slate (magazine). Retrieved 25 November 2013.
^ Kaib, Nathan A.; Becker, Andrew C.; Jones, R. Lynne; Puckett, Andrew W.; Bizyaev, Dmitry; Dilday, Benjamin; Frieman, Joshua A.; Oravetz, Daniel J.; Pan, Kaike; Quinn, Thomas; Schneider, Donald P.; Watters, Shannon (2009). “2006 SQ372: A Likely Long-Period Comet from the Inner Oort Cloud”. The Astrophysical Journal 695 (1): 268–275. arXiv:0901.1690. Bibcode:2009ApJ…695…268K. doi:10.1088/0004-637X/695/1/268. Cite uses deprecated parameters (help)
^ “JPL Close-Approach Data: C/2012 S1 (ISON)”. NASA.gov. 15 November 2012. Retrieved 25 November 2012.
^ Bortle, J. (24 September 2012). “Re: C/2012 S1 (ISON), Some Further Thoughts”. comets-ml mailing list. Yahoo! Groups. Retrieved 5 October 2012.
^ “Let History Be Our Guide?”. Comet ISON Observing Campaign. 22 July 2013. Retrieved 2 August 2013.
^ King, Bob (19 October 2012). “Wassup with comets Hergenrother, L4 PanSTARRS and S1 ISON”. Astro Bob. Areavoices.com. Retrieved 1 January 2013.
^ Phillips, Tony (19 April 2013). “Comet ISON Meteor Shower”. NASA.gov. Archived from the original on 21 September 2013. Retrieved 23 April 2013.
^ Sekhar, A.; Asher, D. J. (11 October 2013). “Meteor showers on Earth from sungrazing comets”. Monthly Notices of the Royal Astronomical Society. arXiv:1310.3171. Bibcode:2013arXiv1310.3171S.
^ a b “Comet ISON - Latest Updates, FAQ and Viewing Guide”. Nightskyinfo.com. Retrieved 22 July 2013.
^ Rao, Joe (25 September 2012). “Newfound Comet Could Look Spectacular in 2013”. Space.com. Retrieved 28 September 2012.
^ a b c Bakich, Michael E. (25 September 2012). “Comet ISON will light up the sky”. Astronomy. Retrieved 28 September 2012.
^ “NASA’s Spitzer Observes Gas Emission From Comet ISON”. NASA. 23 July 2013. Retrieved 1 August 2013.
^ a b “Elements and Ephemeris for C/2012 S1 (ISON)”. IAU Minor Planet Center. Retrieved 23 April 2013.
^ Gary, Bruce (12 August 2013). “Comet ISON Observations by an Amateur Observer: Recovery Observation”. BruceGary.net. Retrieved 4 October 2013.
^ a b Dickinson, David (23 September 2013). “Comet ISON: A Viewing Guide from Now to Perihelion”. Universe Today. Retrieved 4 October 2013.
^ Delamere, Alan; McEwen, Alfred. “First HiRISE Images of Comet ISON”. University of Arizona (HiRISE). Retrieved 2 October 2013.
^ a b “Anticipated STEREO observations of Comet ISON”. NASA STEREO Science Center. 27 February 2013. Retrieved 28 April 2013.
^ Atkinson, Nancy (24 October 2013). “Why Is Comet ISON Green?”. Universe Today. Retrieved 31 October 2013.
^ González, Juan José (31 October 2013). “C/2012 S1, C/2012 X1, C/2013 R1, 2P”. Yahoo! Groups: Comet Observations. Retrieved 31 October 2013. (just a few days ago we saw the first reports of ground-based observers being able to view ISON through binoculars)
^ Flanders, Tony (14 November 2013). “Comet ISON Comes to Life!”. Sky & Telescope. Retrieved 14 November 2013.
^ a b Bortle, John (13 June 2013). “Comet ISON approaches”. Sky & Telescope. Retrieved 14 June 2013.
^ Atkinson, Nancy (15 November 2013). “Whoa. Take a Look at Comet ISON Now”. Universe Today. Retrieved 18 November 2013.
^ King, Bob (11 November 2013). “Mercury enters early morning comet traffic jam”. Astro Bob. Areavoices.com. Retrieved 18 November 2013.
^ “LASCO image 27 November 16:08”.
^ Battams, Karl (27 November 2013). “Very quick update”. CIOC. Retrieved 28 November 2013.
^ Musgrave, Ian (28 November 2013). “Is Comet C/2012 S1 ISON on Track?”. Astroblog. Retrieved 28 November 2013.
^ a b “ISON Updates from the CIOC”. Sungrazing Comets. U.S. Navy. Retrieved 21 July 2013.
^ “Brightest comets seen since 1935”. International Comet Quarterly. Retrieved 28 September 2012.
^ “Hubble captures Comet ISON”. SpaceTelescope.org. 24 April 2013. Retrieved 30 April 2013.
^ a b Ferrín, Ignacio (2013). “Secular Light Curves of Comets C/2011 L4 Panstarrs and C/2012 S1 ISON Compared to 1P/Halley”. arXiv:1302.4621v1 [astro-ph].
^ Beatty, Kelly (27 September 2012). “A “Dream Comet” Heading Our Way?”. Sky & Telescope. Retrieved 28 September 2012.
^ Dickinson, David (25 September 2012). “Will we have a Christmas comet in 2013?”. Canada.com. Retrieved 30 September 2012.
^ “Comet ISON - Latest Updates, FAQ and Viewing Guide”. Nightskyinfo.com. Retrieved 18 June 2013.
^ “Comet C/2012 S1 (ISON)”. Harvard University Cometary Science Archive. 2012. Retrieved 22 November 2013.
^ Chang, Kenneth (27 November 2013). “Comet Nears Sun, Offering Planetary Clues”. The New York Times. Retrieved 27 November 2013.
^ Griffiths, Sarah (18 October 2013). “Dazzling ‘comet of the century’ is still intact! Icy ball 15 times brighter than the moon might be visible in December - IF it survives”. Daily Mail. Retrieved 20 October 2013.
^ Atkinson, Nancy (30 July 2013). “Rumors of Comet ISON ‘Fizzling’ May be Greatly Exaggerated”. Universe Today. Retrieved 20 October 2013.
^ Frederick N., Rasmussen (5 December 2013). “Back Story: Comet Kohoutek was another flameout”. Baltimore Sun. Retrieved 3 February 2014.
^ Powell, Cory S. (5 January 2014). “10 Lessons from the “Comet of the Century””. Discover Magazine blog. Retrieved 3 February 2014.

External links

 Wikimedia Commons has media related to C/2012 S1.
ISONCampaign.org, the NASA Comet ISON Observing Campaign
Comet ISON Toolkit at NASA Solar System Exploration
C/2012 S1 (ISON) at the IAU Minor Planet Center
C/2012 S1 (ISON) at the JPL Small-Body Database Browser
C/2012 S1 (ISON) at Aerith.net
C/2012 S1 (ISON) at Cometography.com
“Anticipated STEREO observations of Comet ISON” at NASA’s STEREO Science Center
“A Timeline Of Comet ISON’s Dangerous Journey” at NASA.gov
Media
Eyes on Comet ISON at NASA Solar System Exploration
ScienceCasts: Comet of the Century by Science@NASA at YouTube.com (story)
NASA’s Deep Impact Spacecraft Images Comet ISON by JPL News at YouTube.com
Path of Comet ISON through the SOHO/LASCO fields of view by Bill Thompson at Sungrazing Comets
Time-lapse image of C/2012 S1 (ISON) and main-belt asteroid 4417 Lecar by Erik Bryssinck at Astronomie.be
Minor Planet Electronic Circulars
MPEC 2013-W16 (2013 Nov 26 : 6120 obs : Epoch 2013 Dec 14 e=0.9999947 q=0.0124439 includes nongravitational parameters)
MPEC 2013-W13 (2013 Nov 25 : 5586 obs : Epoch 2013 Dec 14 e=1.0000019 q=0.0124479)
MPEC 2013-S75 (2013 Sep 30 : 4308 obs : Epoch 2013 Dec 14 e=1.0000020 q=0.0124441)
MPEC 2013-S08 (2013 Sep 16 : 3997 obs : Epoch 2013 Dec 14 e=1.0000019 q=0.0124442)
MPEC 2013-R59 (2013 Sep 6 : 3897 obs : Epoch 2013 Dec 14 e=1.0000019 q=0.0124441)
MPEC 2013-H38 (2013 Apr 23 : 3442 obs : Epoch 2013 Dec 14 e=1.0000020 q=0.0124437)
MPEC 2013-G31 (2013 Apr 9 : 3307 obs : Epoch 2013 Dec 14 e=1.0000021 q=0.0124435)
MPEC 2013-F47 (2013 Mar 25 : 3121 obs : Epoch 2013 Dec 14 e=1.0000022 q=0.0124434)
MPEC 2013-F20 (2013 Mar 18 : 3047 obs : Epoch 2013 Dec 14 e=1.0000022 q=0.0124434)
MPEC 2013-E40 (2013 Mar 9 : 2799 obs : Epoch 2013 Dec 14 e=1.0000022 q=0.0124437)
MPEC 2013-D50 (2013 Feb 23 : 2372 obs : Epoch 2013 Dec 14 e=1.0000020 q=0.0124436)
MPEC 2013-C52 (2013 Feb 12 : 1999 obs : Epoch 2013 Dec 14 e=1.0000019 q=0.0124439)
MPEC 2013-A85 (2013 Jan 14 : 1418 obs : Epoch 2013 Dec 14 e=1.0000016 q=0.0124445)
MPEC 2012-Y30 (2012 Dec 26 : 1000 obs : Epoch 2013 Dec 14 e=1.0000015 q=0.0124443)
MPEC 2012-X53 (2012 Dec 11 : 812 obs : Epoch 2013 Dec 14 e=1.0000014 q=0.0124453)
MPEC 2012-W54 (2012 Nov 27 : 706 obs : Epoch 2013 Dec 14 e=1.0000014 q=0.0124475)
MPEC 2012-V101 (2012 Nov 15 : 538 obs)
MPEC 2012-U109 (2012 Oct 26 : 418 obs : Epoch 2013 Dec 14 e=1.0000013 q=0.0124484)
MPEC 2012-T73 (2012 Oct 12 : 272 obs : Epoch 2013 Dec 14 e=1.0000008 q=0.0124472)
MPEC 2012-T08 (2012 Oct 3 : 163 obs : Epoch 2013 Dec 14 e=1.0000013 : (1/a)_orig = +0.00005808, (1/a)_fut = +0.00000785)

COMETS
FEATURES
Nucleus
Coma
Tails
Antitail
Dust trail
Dust
Comet C/1996 B2 (Hyakutake)
StardustTemple1.jpg
TYPES
Names
Antimatter
Encke-types
Exocomets
Extinct
Great
Halley-types
Interstellar
Lost
Main-belt
Sungrazing (Kreutz Sungrazers)
RELATED
Asteroid
Centaur
Comet discoverers
LINEAR
Extraterrestrial atmospheres
Oort cloud
Small Solar System body
See also
Comet vintages
Comets in fiction
Fictional comets
SPACE
MISSIONS
Comet Hopper
CONTOUR
CRAF
Deep Impact/EPOXI
Deep Space 1
Giotto
Hayabusa Mk2
ICE
Marco Polo
MarcoPolo-R
Rosetta
Philae
Sakigake
Stardust/NeXT
Suisei
Ulysses
Vega program
Vega 1
Vega 2
Vesta
LATEST
C/2013 R1 (Lovejoy)
C/2013 A1 (Siding Spring)
P/2013 P5 (PANSTARRS)
C/2012 S1 (ISON)
C/2012 K1 (PANSTARRS)
C/2012 F6 (Lemmon)
C/2012 E2 (SWAN)
276P/Vorobjov
2P/Encke

LISTS (MORE)
PERIODIC
COMETS
NUMBER
1P/Halley
2P/Encke
3D/Biela
4P/Faye
5D/Brorsen
6P/d’Arrest
7P/Pons–Winnecke
8P/Tuttle
9P/Tempel
10P/Tempel
11P/Tempel–Swift–LINEAR
12P/Pons–Brooks
13P/Olbers
14P/Wolf
15P/Finlay
16P/Brooks
17P/Holmes
18D/Perrine–Mrkos
19P/Borrelly
20D/Westphal
21P/Giacobini–Zinner
22P/Kopff
23P/Brorsen–Metcalf
24P/Schaumasse
25D/Neujmin
26P/Grigg–Skjellerup
27P/Crommelin
28P/Neujmin
29P/Schwassmann–Wachmann
30P/Reinmuth
31P/Schwassmann–Wachmann
32P/Comas Solá
33P/Daniel
34D/Gale
35P/Herschel–Rigollet
36P/Whipple
37P/Forbes
38P/Stephan–Oterma
39P/Oterma
40P/Väisälä
41P/Tuttle–Giacobini–Kresák
42P/Neujmin
43P/Wolf–Harrington
44P/Reinmuth
45P/Honda–Mrkos–Pajdušáková
46P/Wirtanen
47P/Ashbrook–Jackson
48P/Johnson
49P/Arend–Rigaux
50P/Arend
51P/Harrington
52P/Harrington–Abell
53P/Van Biesbroeck
54P/de Vico–Swift–NEAT
55P/Tempel–Tuttle
56P/Slaughter–Burnham
57P/du Toit–Neujmin–Delporte
60P/Tsuchinshan
62P/Tsuchinshan
65P/Gunn
67P/Churyumov–Gerasimenko
68P/Klemola
69P/Taylor
71P/Clark
73P/Schwassmann–Wachmann
74P/Smirnova–Chernykh
75D/Kohoutek
77P/Longmore
78P/Gehrels
81P/Wild
82P/Gehrels
84P/Giclas
85P/Boethin
87P/Bus
88P/Howell
91P/Russell
94P/Russell
95P/Chiron
96P/Machholz
98P/Takamizawa
99P/Kowal
100P/Hartley
101P/Chernykh
102P/Shoemaker
103P/Hartley
105P/Singer Brewster
107P/Wilson–Harrington
109P/Swift–Tuttle
111P/Helin–Roman–Crockett
112P/Urata–Niijima
113P/Spitaler
114P/Wiseman–Skiff
115P/Maury
116P/Wild
117P/Helin–Roman–Alu
118P/Shoemaker–Levy
119P/Parker–Hartley
120P/Mueller
121P/Shoemaker–Holt
122P/de Vico
125P/Spacewatch
128P/Shoemaker–Holt
129P/Shoemaker–Levy
130P/McNaught–Hughes
132P/Helin–Roman–Alu
133P/Elst–Pizarro
136P/Mueller
138P/Shoemaker–Levy
139P/Väisälä–Oterma
143P/Kowal–Mrkos
144P/Kushida
147P/Kushida–Muramatsu
152P/Helin–Lawrence
153P/Ikeya–Zhang
157P/Tritton
158P/Kowal–LINEAR
159P/LONEOS
160P/LINEAR
161P/Hartley–IRAS
163P/NEAT
164P/Christensen
165P/LINEAR
166P/NEAT
167P/CINEOS
168P/Hergenrother
169P/NEAT
170P/Christensen
171P/Spahr
172P/Yeung
173P/Mueller
174P/Echeclus
176P/LINEAR
177P/Barnard
178P/Hug–Bell
206P/Barnard–Boattini
208P/McMillan
209P/LINEAR
238P/Read
246P/NEAT
255P/Levy
271P/van Houten–Lemmon
276P/Vorobjov
289P/Blanpain
NONE
D/1770 L1
D/1993 F2
P/1997 B1
P/2007 R5
P/2010 A2
P/2010 B2
P/2010 V1
P/2011 NO1
P/2013 P5
NON
PERIODIC
COMETS
BEFORE
1910
C/-43 K1 (Comet Caesar)
X/1106 C1
C/1577 V1 (Great Comet of 1577)
C/1652 Y1
C/1680 V1 (Kirsch’s Comet)
C/1702 H1
C/1729 P1 (Comet of 1729)
C/1743 X1
C/1760 A1
C/1769 P1
C/1811 F1 (Great Comet of 1811)
C/1823 Y1 (Great Comet of 1823)
C/1843 D1 (Great March Comet of 1843)
C/1847 T1 (Miss Mitchell’s Comet)
C/1858 L1 (Comet Donati)
C/1861 G1 (Comet Thatcher)
C/1861 J1 (Great Comet of 1861)
X/1872 X1
C/1882 R1 (Great September Comet of 1882)
C/1887 B1 (Great Southern Comet of 1887)
C/1910 A1 (Great January Comet of 1910)
AFTER
1910
C/1911 O1 (Brooks)
C/1911 S3 (Beljawsky)
C/1927 X1 (Skjellerup–Maristany)
C/1948 V1 (Eclipse)
C/1956 R1 (Arend–Roland)
C/1961 R1 (Humason)
C/1965 S1 (Ikeya-Seki)
C/1969 Y1 (Bennett)
C/1970 K1 (White–Ortiz–Bolelli)
C/1973 E1 (Kohoutek)
C/1975 V1 (West)
C/1980 E1 (Bowell)
C/1989 X1 (Austin)
C/1989 Y1 (Skorichenko–George)
C/1993 Y1 (McNaught–Russell)
C/1995 O1 (Hale–Bopp)
C/1996 B2 (Hyakutake)
C/1997 L1 (Zhu–Balam)
C/1998 H1 (Stonehouse)
C/1999 F1 (Catalina)
C/1999 S4 (LINEAR)
C/2000 U5 (LINEAR)
C/2000 W1 (Utsunomiya-Jones)
C/2001 Q4 (NEAT)
C/2002 T7 (LINEAR)
C/2004 Q2 (Machholz)
C/2006 A1 (Pojmański)
C/2006 M4 (SWAN)
C/2006 P1 (McNaught)
C/2007 F1 (LONEOS)
C/2007 N3 (Lulin)
C/2007 Q3 (Siding Spring)
C/2007 W1 (Boattini)
C/2009 F6 (Yi–SWAN)
C/2009 R1 (McNaught)
C/2010 X1 (Elenin)
C/2011 L4 (PANSTARRS)
C/2011 W3 (Lovejoy)
C/2012 E2 (SWAN)
C/2012 F6 (Lemmon)
C/2012 K1 (PANSTARRS)
C/2012 S1 (ISON)
C/2013 A1 (Siding Spring)
P/2013 P5 (PANSTARRS)
C/2013 R1 (Lovejoy)
AFTER
1910
(ABC’S)
Arend–Roland
Austin
Beljawsky
Bennett
Boattini
Bowell
Brooks
Catalina
Eclipse
Elenin
Hale-Bopp
Humason
Hyakutake
Ikeya-Seki
ISON
Kohoutek
Lemmon
LINEAR
C/1999 S4
C/2000 U5
C/2002 T7
LONEOS
Lovejoy
C/2011 W3
C/2013 R1
Lulin
Machholz
McNaught
C/2006 P1
C/2009 R1
McNaught–Russell
NEAT
Pan-STARRS
C/2011 L4
C/2012 K1
P/2013 P5
Pojmański
Siding Spring
C/2007 Q3
C/2013 A1
Skjellerup–Maristany
Skorichenko–George
Stonehouse
SWAN
C/2006 M4
C/2012 E2
Utsunomiya–Jones
West
White–Ortiz–Bolelli
Yi–SWAN
Zhu–Balam
MISC
ASTEROID
COMETS
2060 Chiron (95P/Chiron)
7968 Elst–Pizarro (133P/Elst–Pizarro)
4015 Wilson–Harrington (107P/Wilson-Harrington)
60558 Echeclus (174P/Echeclus)
118401 LINEAR (176P/LINEAR, prev 1999 RE70)
LOST
COMETS
11P/Tempel–Swift–LINEAR
15P/Finlay
17P/Holmes
27P/Crommelin
54P/de Vico–Swift–NEAT
69P/Taylor
73P/Schwassmann–Wachmann
113P/Spitaler
107P/Wilson–Harrington
177P/Barnard
206P/Barnard–Boattini
D/1770 L1 (Lexell)
3D/Biela
5D/Brorsen
18D/Perrine–Mrkos
20D/Westphal
25D/Neujmin
34D/Gale
UNKNOWN
ORBITS
X/1106 C1
X/1872 X1
V
T
E
2013 IN SPACE
LAUNCHES
SPACE PROBES
LADEE
Mars Orbiter Mission
MAVEN
Chang’e 3
Yutu
SPACE OBSERVATORIES
IRIS
Hisaki
Gaia
LADEE fires small engines.jpg
IMPACT EVENTS
Chelyabinsk meteor
Chelyabinsk meteorite
Montreal bolide
NOVAE
Nova Centauri 2013
Nova Delphini 2013
COMETS
C/2011 L4 (PANSTARRS)
C/2012 F6 (Lemmon)
C/2012 K1 (PANSTARRS)
C/2012 S1 (ISON)
C/2013 A1 (Siding Spring)
P/2013 P5 (PANSTARRS)
C/2013 R1 (Lovejoy)
NEOS
(7888) 1993 UC
(52760) 1998 ML14
(285263) 1998 QE2
(163364) 2002 OD20
(277475) 2005 WK4
2006 BL8
2011 BT15
2012 YQ1
2013 EC
2013 ET
2013 MZ5
2013 PJ10
2013 TV135
2013 XY8
2013 YP139
367943 Duende
3361 Orpheus
EXOPLANETS
GJ 504b
Gliese 667 Cd
Gliese 667 Ce
Gliese 667 Cf
Gliese 667 Cg
Gliese 667 Ch
HD 106906 b
HD 95086 b
Kepler-37b
Kepler-37c
Kepler-37d
Kepler-61b
Kepler-62b
Kepler-62c
Kepler-62d
Kepler-62e
Kepler-62f
Kepler-65
Kepler-66
Kepler-67
Kepler-68b
Kepler-68c
Kepler-68d
Kepler-69b
Kepler-69c
Kepler-76b
Kepler-78b
Kepler-87c
PSO J318.5-22
ROXs 42Bb
OTHERS
Asteroid close approaches
GRB 130427A
Hercules–Corona Borealis Great Wall
Herschel Space Observatory retiring
The Day the Earth Smiled
CategoryCategory:2013 in space
Source: http://en.wikipedia.org/wiki/C/2012_S1
Here’s an Easter egg. For fun.

Nostradamus writes:

Century 2 Quatrain 41

The great star will burn for seven days,
The cloud will cause two suns to appear:
The big mastiff will howl all night
When the great pontiff will change country.

The pontiff will “flee Rome in December when the great comet is seen in the daytime.”

La grand étoile par sept jours brulera,
Nuée fera deux soleils apparoir:
Le gros mâ(s)tin toute bute hurlera
Quand grand pontife changera de terroir.

Astronomical Unit Wiki

astronomical unit
Unit system Astronomical system of units
(Accepted for use with the SI)
Unit of length
Symbol au
Unit conversions
1 au in… is equal to…
SI units 1.496×1011 m
imperial & US units 9.2956×107 mi
other astronomical 4.8481×10−6 pc
units 1.5813×10−5 ly

An astronomical unit (abbreviated as au;[1] other abbreviations that are sometimes used include ㍳[citation needed], a.u.[citation needed] and ua[2]) is a unit of length now defined as exactly 149,597,870,700 m (about 93 million miles),[3] or roughly the mean Earth–Sun distance.

Definition

See also: Earth’s orbit
The astronomical unit was originally defined as the length of the semi-major axis of the Earth’s elliptical orbit around the Sun.

In 1976 for greater precision, the International Astronomical Union (IAU) formally adopted the definition that “the astronomical unit of length is that length (A) for which the Gaussian gravitational constant (k) takes the value 0.01720209895 when the units of measurement are the astronomical units of length, mass and time”.[4][5][6] An equivalent definition is the radius of an unperturbed circular Newtonian orbit about the sun of a particle having infinitesimal mass, moving with an angular frequency of 0.01720209895 radians per day;[7] or that length such that, when used to describe the positions of the objects in the Solar System, the heliocentric gravitational constant (the product GM☉) is equal to (0.01720209895)2 au3/d2.

In the IERS numerical standards, the speed of light in a vacuum is defined as c0 = 299792458 m/s, in accordance with the SI units. The time to traverse an au is found to be τA = 499.0047838061±1.0E-8 s, resulting in the astronomical unit in metres as c0τA = 149597870700 ±3 m.[8] It is approximately equal to the distance from the Earth to the Sun.

The 1976 value of the astronomical unit was indirectly derived from physical analysis of the motion of the Earth around the Sun, while it had since become possible to measure the distance to celestial bodies directly.[9][10] Furthermore, it was subject to relativity and thus was not constant for all observers. Therefore, in 2012 the IAU redefined the astronomical unit as a conventional unit of length directly tied to the meter, with a length of exactly 149597870700 m and the official abbreviation of au.[9][11]

1 astronomical unit = 149597870700 metres (exactly)
≈ 92.955807 million miles
≈ 4.8481368 millionths of a light-year
≈ 15.812507 millionths of a parsec

Modern determinations

Precise measurements of the relative positions of the inner planets can be made by radar and by telemetry from space probes. As with all radar measurements, these rely on measuring the time taken for photons to be reflected from an object. These measured positions are then compared with those calculated by the laws of celestial mechanics: an assembly of calculated positions is often referred to as an ephemeris, in which distances are commonly calculated in astronomical units. One of several ephemeris computation services is provided by the Jet Propulsion Laboratory.[12]

The comparison of the ephemeris with the measured positions leads to a value for the speed of light in astronomical units, which is 173.144 632 6847(69) au/d (TDB).[13] As the speed of light in meters per second (c0) is fixed in the International System of Units, this measurement of the speed of light in au/d (cAU) also determines the value of the astronomical unit in meters (A):


The best current (2009) estimate of the International Astronomical Union (IAU) for the value of the astronomical unit in meters is A = 149 597 870 700(3) m, based on a comparison of JPL and IAA–RAS ephemerides.[14][15][16]

Usage

With the definitions used before 2012, the astronomical unit was dependent on the heliocentric gravitational constant, that is the product of the gravitational constant G and the solar mass M☉. Neither G nor M☉ can be measured to high accuracy in SI units, but the value of their product is known very precisely from observing the relative positions of planets (Kepler’s Third Law expressed in terms of Newtonian gravitation). Only the product is required to calculate planetary positions for an ephemeris, which explains why ephemerides are calculated in astronomical units and not in SI units.

The calculation of ephemerides also requires a consideration of the effects of general relativity. In particular, time intervals measured on the surface of the Earth (terrestrial time, TT) are not constant when compared to the motions of the planets: the terrestrial second (TT) appears to be longer in Northern Hemisphere winter and shorter in Northern Hemisphere summer when compared to the “planetary second” (conventionally measured in barycentric dynamical time, TDB). This is because the distance between the Earth and the Sun is not fixed (it varies between 0.9832898912 and 1.0167103335 au) and, when the Earth is closer to the Sun (perihelion), the Sun’s gravitational field is stronger and the Earth is moving faster along its orbital path. As the meter is defined in terms of the second, and the speed of light is constant for all observers, the terrestrial meter appears to change in length compared to the “planetary meter” on a periodic basis.

The meter is defined to be a unit of proper length, but the SI definition does not specify the metric tensor to be used in determining it. Indeed, the International Committee for Weights and Measures (CIPM) notes that “its definition applies only within a spatial extent sufficiently small that the effects of the non-uniformity of the gravitational field can be ignored.”[17] As such, the meter is undefined for the purposes of measuring distances within the Solar System. The 1976 definition of the astronomical unit was incomplete, in particular because it does not specify the frame of reference in which time is to be measured, but proved practical for the calculation of ephemerides: a fuller definition that is consistent with general relativity was proposed,[18] and “vigorous debate” ensued [19] until in August 2012 the International Astronomical Union adopted the current definition of 1 astronomical unit = 149597870700 meters.

The au is too small for interstellar distances, where the parsec is commonly used. See the article cosmic distance ladder. The light year is often used in popular works, but is not an approved non-SI unit.[20]

History

According to Archimedes in the Sandreckoner (2.1), Aristarchus of Samos estimated the distance to the Sun to be 10000 times the Earth’s radius (the true value is about 23000).[21] However, the book On the Sizes and Distances of the Sun and Moon, which has long been ascribed to Aristarchus, says that he calculated the distance to the sun to be between 18 and 20 times the distance to the Moon, whereas the true ratio is about 389.174. The latter estimate was based on the angle between the half moon and the Sun, which he estimated as 87° (the true value being close to 89.853°). Depending on the distance Van Helden assumes Aristarchus used for the distance to the Moon, his calculated distance to the Sun would fall between 380 and 1520 Earth radii.[22]

According to Eusebius of Caesarea in the Praeparatio Evangelica (Book XV, Chapter 53), Eratosthenes found the distance to the sun to be “σταδιων μυριαδας τετρακοσιας και οκτωκισμυριας” (literally “of stadia myriads 400 and 80000” but with the additional note that in the Greek text the grammatical agreement is between myriads (not stadia) on the one hand and both 400 and 80000 on the other, as in Greek, unlike English, all three or all four if one were to include stadia, words are inflected). This has been translated either as 4080000 stadia (1903 translation by Edwin Hamilton Gifford), or as 804000000 stadia (edition of Édouard des Places, dated 1974–1991). Using the Greek stadium of 185 to 190 meters,[23][24] the former translation comes to a far too low 755000 km whereas the second translation comes to 148.7 to 152.8 million kilometres (accurate within 2%).[25]Hipparchus also gave an estimate of the distance of the Sun from the Earth, quoted by Pappus as equal to 490 Earth radii. According to the conjectural reconstructions of Noel Swerdlow and G. J. Toomer, this was derived from his assumption of a “least perceptible” solar parallax of 7 arc minutes.[26]

A Chinese mathematical treatise, the Zhoubi suanjing (c. 1st century BCE), shows how the distance to the Sun can be computed geometrically, using the different lengths of the noontime shadows observed at three places 1000 li apart and the assumption that the Earth is flat.[27]

SOLAR
    

PARALLAX EARTH
RADII
Archimedes in Sandreckoner
(3rd century BC) 40″ 10000
Aristarchus in On Sizes (3rd century BC) 380-1520
Hipparchus (2nd century BC) 7′  490
Posidonius (1st century BC) quoted in Cleomedes (1st century) 10,000
Ptolemy (2nd century) 2′ 50″ 1210
Godefroy Wendelin (1635) 15″ 14000
Jeremiah Horrocks (1639) 15″ 14000
Christiaan Huygens (1659) 8.6″ 24000
Cassini & Richer (1672) 9 1⁄2″ 21700
Jérôme Lalande (1771) 8.6″ 24000
Simon Newcomb (1895) 8.80″ 23440
Arthur Hinks (1909) 8.807″ 23420
H. Spencer Jones (1941) 8.790″ 23466
modern 8.794143″ 23455
In the 2nd century CE, Ptolemy estimated the mean distance of the Sun as 1210 times the Earth radius.[28][29] To determine this value, Ptolemy started by measuring the Moon’s parallax, finding what amounted to a horizontal lunar parallax of 1° 26′, which was much too large. He then derived a maximum lunar distance of 64 1⁄6 Earth radii. Because of cancelling errors in his parallax figure, his theory of the Moon’s orbit, and other factors, this figure was approximately correct.[30][31] He then measured the apparent sizes of the Sun and the Moon and concluded that the apparent diameter of the Sun was equal to the apparent diameter of the Moon at the Moon’s greatest distance, and from records of lunar eclipses, he estimated this apparent diameter, as well as the apparent diameter of the shadow cone of the Earth traversed by the Moon during a lunar eclipse. Given these data, the distance of the Sun from the Earth can be trigonometrically computed to be 1210 Earth radii. This gives a ratio of solar to lunar distance of approximately 19, matching Aristarchus’s figure. Although Ptolemy’s procedure is theoretically workable, it is very sensitive to small changes in the data, so much so that changing a measurement by a few percent can make the solar distance infinite.[30]

After Greek astronomy was transmitted to the medieval Islamic world, astronomers made some changes to Ptolemy’s cosmological model, but did not greatly change his estimate of the Earth–Sun distance. For example, in his introduction to Ptolemaic astronomy, al-Farghānī gave a mean solar distance of 1170 Earth radii, while in his zij, al-Battānī used a mean solar distance of 1108 Earth radii. Subsequent astronomers, such as al-Bīrūnī, used similar values.[32] Later in Europe, Copernicus and Tycho Brahe also used comparable figures (1142 and 1150 Earth radii), and so Ptolemy’s approximate Earth–Sun distance survived through the 16th century.[33]

Johannes Kepler was the first to realize that Ptolemy’s estimate must be significantly too low (according to Kepler, at least by a factor of three) in his Rudolphine Tables (1627). Kepler’s laws of planetary motion allowed astronomers to calculate the relative distances of the planets from the Sun, and rekindled interest in measuring the absolute value for the Earth (which could then be applied to the other planets). The invention of the telescope allowed far more accurate measurements of angles than is possible with the naked eye. Flemish astronomer Godefroy Wendelin repeated Aristarchus’ measurements in 1635, and found that Ptolemy’s value was too low by a factor of at least eleven.

A somewhat more accurate estimate can be obtained by observing the transit of Venus.[34] By measuring the transit in two different locations, one can accurately calculate the parallax of Venus and from the relative distance of the Earth and Venus from the Sun, the solar parallax α (which cannot be measured directly[35]). Jeremiah Horrocks had attempted to produce an estimate based on his observation of the 1639 transit (published in 1662), giving a solar parallax of 15 arcseconds, similar to Wendelin’s figure. The solar parallax is related to the Earth–Sun distance as measured in Earth radii by


The smaller the solar parallax, the greater the distance between the Sun and the Earth: a solar parallax of 15" is equivalent to an Earth–Sun distance of 13,750 Earth radii.

Christiaan Huygens believed the distance was even greater: by comparing the apparent sizes of Venus and Mars, he estimated a value of about 24,000 Earth radii,[36] equivalent to a solar parallax of 8.6". Although Huygens’ estimate is remarkably close to modern values, it is often discounted by historians of astronomy because of the many unproven (and incorrect) assumptions he had to make for his method to work; the accuracy of his value seems to be based more on luck than good measurement, with his various errors cancelling each other out.



Transits of Venus across the face of the Sun were, for a long time, the best method of measuring the astronomical unit, despite the difficulties (here, the so-called “black drop effect”) and the rarity of observations.
Jean Richer and Giovanni Domenico Cassini measured the parallax of Mars between Paris and Cayenne in French Guiana when Mars was at its closest to Earth in 1672. They arrived at a figure for the solar parallax of 9 1⁄2", equivalent to an Earth–Sun distance of about 22±0 Earth radii. They were also the first astronomers to have access to an accurate and reliable value for the radius of the Earth, which had been measured by their colleague Jean Picard in 1669 as 3269 thousand toises. Another colleague, Ole Rømer, discovered the finite speed of light in 1676: the speed was so great that it was usually quoted as the time required for light to travel from the Sun to the Earth, or “light time per unit distance”, a convention that is still followed by astronomers today.

A better method for observing Venus transits was devised by James Gregory and published in his Optica Promata (1663). It was strongly advocated by Edmond Halley[37] and was applied to the transits of Venus observed in 1761 and 1769, and then again in 1874 and 1882. Transits of Venus occur in pairs, but less than one pair every century, and observing the transits in 1761 and 1769 was an unprecedented international scientific operation. Despite the Seven Years’ War, dozens of astronomers were dispatched to observing points around the world at great expense and personal danger: several of them died in the endeavour.[38] The various results were collated by Jérôme Lalande to give a figure for the solar parallax of 8.6″.

DATE METHOD A/GM UNCERTAINTY
1895 aberration 149.25 0.12
1941 parallax 149.674 0.016
1964 radar 149.5981 0.001
1976 telemetry 149.597 870 0.000 001
2009 telemetry 149.597 870 700 0.000 000 003
Another method involved determining the constant of aberration, and Simon Newcomb gave great weight to this method when deriving his widely accepted value of 8.80″ for the solar parallax (close to the modern value of 8.794143″), although Newcomb also used data from the transits of Venus. Newcomb also collaborated with A. A. Michelson to measure the speed of light with Earth-based equipment; combined with the constant of aberration (which is related to the light time per unit distance) this gave the first direct measurement of the Earth–Sun distance in kilometers. Newcomb’s value for the solar parallax (and for the constant of aberration and the Gaussian gravitational constant) were incorporated into the first international system of astronomical constants in 1896,[39] which remained in place for the calculation of ephemerides until 1964.[40] The name “astronomical unit” appears first to have been used in 1903.[41]

The discovery of the near-Earth asteroid 433 Eros and its passage near the Earth in 1900–1901 allowed a considerable improvement in parallax measurement.[42] Another international project to measure the parallax of 433 Eros was undertaken in 1930–1931.[35][43]

Direct radar measurements of the distances to Venus and Mars became available in the early 1960s. Along with improved measurements of the speed of light, these showed that Newcomb’s values for the solar parallax and the constant of aberration were inconsistent with one another.[44]

Developments



The astronomical distance unit parsec uses the au as a baseline and an angle of one arcsecond for parallax. 1 au and 1 pc not to scale. (See also stellar parallax)
The unit distance A (the value of the astronomical unit in meters) can be expressed in terms of other astronomical constants:


where G is the Newtonian gravitational constant, M☉ is the solar mass, k is the numerical value of Gaussian gravitational constant and D is the time period of one day. The Sun is constantly losing mass by radiating away energy,[45] so the orbits of the planets are steadily expanding outward from the Sun. This has led to calls to abandon the astronomical unit as a unit of measurement.[46] There have also been calls to redefine the astronomical unit in terms of a fixed number of meters.[47]

As the speed of light has an exact defined value in SI units and the Gaussian gravitational constant k is fixed in the astronomical system of units, measuring the light time per unit distance is exactly equivalent to measuring the product GM☉ in SI units. Hence, it is possible to construct ephemerides entirely in SI units, which is increasingly becoming the norm.

A 2004 analysis of radiometric measurements in the inner Solar System suggested that the secular increase in the unit distance was much larger than can be accounted for by solar radiation, +15±4 meters per century.[48][49]

The measurements of the secular variations of the astronomical unit are not confirmed by other authors and are quite controversial. Furthermore, since 2010, the astronomical unit is not yet estimated by the planetary ephemerides.[50]

Examples

The distances are approximate mean distances. It has to be taken into consideration that the distances between celestial bodies change in time due to their orbits and other factors.

The Moon is 0.0026 ± 0.0001 au from the Earth.
Mercury is 0.39 ± 0.09 au from the Sun.
Venus is 0.72 ± 0.01 au from the Sun.
The Earth is 1.00 ± 0.02 au from the Sun.
Mars is 1.52 ± 0.14 au from the Sun.
Ceres is 2.77 ± 0.22 au from the Sun.
Jupiter is 5.20 ± 0.25 au from the Sun.
The mean diameter of Betelgeuse is 5.5 au.
NML Cygni, the largest known star, has a radius of 7.67 au.
Saturn is 9.58 ± 0.53 au from the Sun.
Uranus is 19.23 ± 0.85 au from the Sun.
The New Horizons spacecraft is about 27.15 au from the Sun (as of August 2013[update]), as it makes its way to Pluto for a flyby.
Neptune is 30.10 ± 0.34 au from the Sun.
The Kuiper belt begins at roughly 30 au.[51]
Pluto is 39.3 ± 9.6 au from the Sun.
Beginning of the scattered disk at 45 au (10 au overlap with Kuiper Belt)
Ending of Kuiper belt at 50–55 au.
Eris is 68.01 ± 29.64 au from the Sun.
90377 Sedna’s orbit ranges between 76 and 942 au from the Sun; Sedna is currently (as of 2012[update]) about 87 au from the Sun.[52]
The termination shock between solar winds/interstellar winds/interstellar medium occurs at 94 au.
The distance of dwarf planet Eris from the Sun is 96.7 au, as of 2009[update]. Eris and its moon are currently the most distant known objects in the Solar System apart from long-period comets and space probes.[53]
100 au: heliosheath
125 au: as of August 2013[update], Voyager 1 is the furthest human-made object from the Sun; it is currently traveling at about 3½ au/yr.[54]
100–1000 au: mostly populated by objects from the scattered disc
1000–3000 au: beginning of Hills cloud/inner Oort cloud
20000 au: ending of Hills cloud/inner Oort cloud, beginning of outer Oort cloud
50000 au: possible closest estimate of the outer Oort cloud limits
63241.077 au: a light-year, the distance light travels in 1 year
100000 au: possible farthest estimate of the outer Oort cloud limits (1.6 ly)
206264.81 au: one parsec
230000 au: maximum extent of influence of the Sun’s gravitational field (Hill/Roche sphere)[55]—beyond this is true interstellar medium. This distance is 1.1 parsecs (3.6 light-years).[55]
Proxima Centauri (the nearest star to Earth, excluding the Sun) is ~268 000 au from the Sun
The distance from the Sun to the center of the Milky Way is approximately 1.7×109 au

Other views

In 2006 the BIPM defined the astronomical unit as (1.49597870691(6))×1011 m, and recommended “ua” as the symbol for the unit.[2]

See also

 Time portal
Orders of magnitude (length)

Notes and references

^ “RESOLUTION B2 on the re-definition of the astronomical unit of length”, RESOLUTION B2, Beijing, Kina: International Astronomical Union, 31 August 2012, retrieved 2013-05-11, “The XXVIII General Assembly of International Astronomical Union recommends [adopted] … that the unique symbol “au” be used for the astronomical unit.”
^ a b Bureau International des Poids et Mesures (2006), The International System of Units (SI) (8th ed.), Organisation Intergouvernementale de la Convention du Mètre, p. 126
^ “RESOLUTION B2 on the re-definition of the astronomical unit of length”, RESOLUTION B2, Beijing, Kina: International Astronomical Union, 31 August 2012, retrieved 2012-09-19, “The XXVIII General Assembly of International Astronomical Union recommends [adopted] that the astronomical unit be re-defined to be a conventional unit of length equal to exactly 149,597,870,700 meters, in agreement with the value adopted in IAU 2009 Resolution B2”
^ Resolution No. 10 of the XVIth General Assembly of the International Astronomical Union, Grenoble, 1976
^ H. Hussmann, F. Sohl, J. Oberst (2009), “§4.2.2.1.3: Astronomical units”, in Joachim E Trümper, Astronomy, astrophysics, and cosmology — Volume VI/4B Solar System, Springer, p. 4, ISBN 3-540-88054-2
^ Gareth V Williams (1997), “Astronomical unit”, in James H. Shirley, Rhodes Whitmore Fairbridge, Encyclopedia of planetary sciences, Springer, p. 48, ISBN 0-412-06951-2
^ International Bureau of Weights and Measures (2006), The International System of Units (SI) (8th ed.), p. 126, ISBN 92-822-2213-6
^ Gérard Petit and Brian Luzum, eds. [clarification needed] (2010). “Table 1.1: IERS numerical standards”. IERS technical note no. 36: General definitions and numerical standards. International Earth Rotation and Reference Systems Service. For complete document see Gérard Petit and Brian Luzum, eds. [clarification needed] (2010). IERS Conventions (2010): IERS technical note no. 36. International Earth Rotation and Reference Systems Service. ISBN 978-3-89888-989-6.
^ a b Capitaine, Nicole; Klioner, Sergei; McCarthy, Dennis (2012), “The re-definition of the astronomical unit of length:reasons and consequences”, IAU Joint Discussion 7: Space-Time Reference Systems for Future Research, Beijing, China, Bibcode:2012IAUJD…7E…40C, retrieved 16 May 2013
^ “Table 6: Units outside the SI that are accepted for use with the SI”. The NIST reference on constants, units, and uncertainty: International system of units (SI). NIST, USA. Retrieved 2012-01-16.
^ Geoff Brumfiel (14 September 2012). “The astronomical unit gets fixed: Earth–Sun distance changes from slippery equation to single number.”. Retrieved 14 September 2012.
^ “HORIZONS System”. Solar system dynamics. NASA: Jet Propulsion Laboratory. 4 January 2005. Retrieved 2012-01-16.
^ “2009 Selected Astronomical Constants” in The Astronomical Almanac Online, USNO–UKHO
^ IAU WG on NSFA Current Best Estimates, retrieved 25 September 2009
^ Pitjeva, E. V.; Standish, E. M. (2009), “Proposals for the masses of the three largest asteroids, the Moon-Earth mass ratio and the Astronomical Unit”, Celest. Mech. Dynam. Astron. 103 (4): 365–72, Bibcode:2009CeMDA.103…365P, doi:10.1007/s10569-009-9203-8
^ “The Final Session of the General Assembly”, Estrella d’Alva, 14 August 2009: 1
^ International Bureau of Weights and Measures (2006), The International System of Units (SI) (8th ed.), pp. 166–67, ISBN 92-822-2213-6
^ Huang, T.-Y.; Han, C.-H.; Yi, Z.-H.; Xu, B.-X.; Han; Yi; Xu (1995), “What is the astronomical unit of length?”, Astron. Astrophys. 298: 629–33, Bibcode:1995A&A…298…629H
^ Richard Dodd (2011). “§6.2.3: Astronomical unit: Definition of the astronomical unit, future versions”. Using SI Units in Astronomy. Cambridge University Press. p. 76. ISBN 0-521-76917-5. and also p. 91, Summary and recommendations.
^ See, for example, the work cited above: Richard Dodd. “§6.2.8: Light year”. Using SI Units in Astronomy. p. 82. ISBN 0-521-76917-5.
^ Gomez, A. G. (2013) Aristarchos of Samos, the Polymath AuthorHouse, ISBN 9781481789493.
^ Van Helden, Albert (1985), Measuring the Universe: Cosmic Dimensions from Aristarchus to Halley, Chicago: University of Chicago Press, pp. 5–9, ISBN 0-226-84882-5
^ Engels, Donald (1985), “The Length of Eratosthenes’ Stade”, Am. J. Philol. (The Johns Hopkins University Press) 106 (3): 298–311, doi:10.2307/295030, JSTOR 295030
^ Gulbekian, Edward (1987), “The origin and value of the stadion unit used by Eratosthenes in the third century B.C.”, Archive for History of Exact Sciences 37 (4): 359–63, doi:10.1007/BF00417008 (inactive 2010-01-09)
^ Rawlins, D. (2008), “Eratothenes’ large earth and tiny universe”, DIO 14: 3–12
^ Toomer, G. J. (1974). “Hipparchus on the distances of the sun and moon”. Archive for History of Exact Sciences 14 (2): 126–142. doi:10.1007/BF00329826. edit
^ Lloyd, G. E. R. (1996), Adversaries and Authorities: Investigations into Ancient Greek and Chinese Science, Cambridge University Press, pp. 59–60, ISBN 0-521-55695-3
^ Goldstein, Bernard R. (1967), “The Arabic Version of Ptolemy’s Planetary Hypotheses”, Trans. Am. Phil. Soc. 57 (4): 9–12
^ van Helden, Albert (1985), Measuring the Universe: Cosmic Dimensions from Aristarchus to Halley, Chicago: University of Chicago Press, pp. 15–27, ISBN 0-226-84882-5
^ a b pp. 16–19, van Helden 1985
^ p. 251, Ptolemy’s Almagest, translated and annotated by G. J. Toomer, London: Duckworth, 1984, ISBN 0-7156-1588-2
^ pp. 29–33, van Helden 1985
^ pp. 41–53, van Helden 1985
^ An extended historical discussion of this method is provided by Trudy E Bell. “Quest for the astronomical unit”. The Bent of Tau Beta Pi, Summer 2004, p. 20. Retrieved 2012-01-16.
^ a b Weaver, Harold F. (1943), “The Solar Parallax”, Astronomical Society of the Pacific Leaflets 4: 144–51, Bibcode:1943ASPL…4…144W
^ Goldstein, S. J., Jr. (1985), “Christiaan Huygens’ Measurement of the Distance to the Sun”, Observatory 105: 32–33, Bibcode:1985Obs…105…32G
^ Halley, E. (1716), “A new Method of determining the Parallax of the Sun, or his Distance from the Earth”, Philosophical Transactions of the Royal Society 29: 454–64
^ Pogge, Richard (May 2004), How Far to the Sun? The Venus Transits of 1761 & 1769, Ohio State University, retrieved 15 November 2009
^ Conférence internationale des étoiles fondamentales, Paris, 18–21 May 1896
^ Resolution No. 4 of the XIIth General Assembly of the International Astronomical Union, Hamburg, 1964
^ astronomical unit Merriam-Webster’s Online Dictionary
^ Hinks, Arthur R. (1909), “Solar Parallax Papers No. 7: The General Solution from the Photographic Right Ascensions of Eros, at the Opposition of 1900”, Month. Not. R. Astron. Soc. 69 (7): 544–67, Bibcode:1909MNRAS…69…544H
^ Spencer Jones, H. (1941), “The Solar Parallax and the Mass of the Moon from Observations of Eros at the Opposition of 1931”, Mem. R. Astron. Soc. 66: 11–66
^ Mikhailov, A. A. (1964), “The Constant of Aberration and the Solar Parallax”, Sov. Astron. 7 (6): 737–39, Bibcode:1964SvA…7…737M
^ Noerdlinger, Peter D. (2008), “Solar Mass Loss, the Astronomical Unit, and the Scale of the Solar System”, Celest. Mech. Dynam. Astron. 0801: 3807, arXiv:0801.3807, Bibcode:2008arXiv0801.3807N
^ “AU may need to be redefined”, New Scientist, 6 February 2008
^ Capitaine, N; Guinot, B (2008). “The astronomical units”. arXiv:0812.2970v1 [astro-ph].
^ Krasinsky, G. A.; Brumberg, V. A. (2004), “Secular increase of astronomical unit from analysis of the major planet motions, and its interpretation”, Celest. Mech. Dynam. Astron. 90 (3–4): 363, arXiv:1108.5546, Bibcode:2011CeMDA.111…363F, doi:10.1007/s10569-011-9377-8
^ John D. Anderson and Michael Martin Nieto (2009), “Astrometric Solar-System Anomalies;§2: Increase in the astronomical unit”, American Astronomical Society 261: 0702, arXiv:0907.2469, Bibcode:2009IAU…261.0702A.
^ Fienga, A.et al. (2011), “The INPOP10a planetary ephemeris and its applications in fundamental physics”, Celest. Mech. Dynam. Astron. 111 (3): 363, arXiv:1108.5546, Bibcode:2011CeMDA.111…363F, doi:10.1007/s10569-011-9377-8
^ Alan Stern; Colwell, Joshua E. (1997), “Collisional Erosion in the Primordial Edgeworth-Kuiper Belt and the Generation of the 30–50 au Kuiper Gap”, The Astrophysical Journal 490 (2): 879–882, Bibcode:1997ApJ…490…879S, doi:10.1086/304912.
^ “AstDys (90377) Sedna Ephemerides”. Department of Mathematics, University of Pisa, Italy. Retrieved 2011-05-05.
^ Chris Peat, Spacecraft escaping the Solar System, Heavens-Above, retrieved 25 January 2008
^ Voyager 1, Where are the Voyagers – NASA Voyager 1
^ a b Chebotarev, G.A. (1964), “Gravitational Spheres of the Major Planets, Moon and Sun”, Soviet Astronomy 7 (5): 618–622, Bibcode:1964SvA…7…618C

Bibliography

Williams, D.; Davies, R. D. (1968), “A radio method for determining the astronomical unit”, Monthly Notices of the Royal Astronomical Society 140: 537, Bibcode:1968MNRAS.140…537W

External links

The IAU and astronomical units
Recommendations concerning Units (HTML version of the IAU Style Manual)
Chasing Venus, Observing the Transits of Venus
Transit of Venus
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What is a comet?

A comet is an icy small Solar System body that, when passing close to the Sun, heats up and begins to outgas, displaying a visible atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of solar radiation and the solar wind upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of loose collections of ice, dust, and small rocky particles. The coma and tail are much larger, and if sufficiently bright may be seen from the Earth without the aid of a telescope. Comets have been observed and recorded since ancient times by many different cultures.

Comets have a wide range of orbital periods, ranging from several years to several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune. Longer-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper Belt to halfway to the next nearest star. Long-period comets are directed towards the Sun from the Oort cloud by gravitational perturbations caused by passing stars and the galactic tide. Hyperbolic comets may pass once through the inner Solar System before being flung out to interstellar space along hyperbolic trajectories.

Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus. This atmosphere has parts termed the coma (the central atmosphere immediately surrounding the nucleus) and the tail (a typically linear section consisting of dust or gas blown out from the coma by the Sun’s light pressure or outstreaming solar wind plasma). However, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids.[1] Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System.[2][3] The discovery of main-belt comets and active centaurs has blurred the distinction between asteroids and comets.

As of July 2013[update] there were 4,894 known comets,[4] and this number is steadily increasing. However, this represents only a tiny fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System may number one trillion.[5] Roughly one comet per year is visible to the naked eye, though many of these are faint and unspectacular.[6] Particularly bright examples are called “Great Comets”.

On 22 January 2014, ESA scientists reported the detection, for the first definitive time, of water vapor on the dwarf planet, Ceres, largest object in the asteroid belt.[7] The detection was made by using the far-infrared abilities of the Herschel Space Observatory.[8] The finding is unexpected because comets, not asteroids, are typically considered to “sprout jets and plumes”. According to one of the scientists, “The lines are becoming more and more blurred between comets and asteroids.”[8]

Etymology

The word comet derives from the Old English cometa from the Latin comēta or comētēs. That, in turn, is a latinisation of the Greek κομήτης (“wearing long hair”), and the Oxford English Dictionary notes that the term (ἀστὴρ) κομήτης already meant “long-haired star, comet” in Greek. Κομήτης was derived from κομᾶν (“to wear the hair long”), which was itself derived from κόμη (“the hair of the head”) and was used to mean “the tail of a comet”.[9][10]

The astronomical symbol for comets is (☄), consisting of a small disc with three hairlike extensions.[11]

Physical characteristics

Nucleus

Main article: Comet nucleus

Nucleus of Comet 103P/Hartley as imaged during a spacecraft flyby. The nucleus is about 2 km in length.

C/2011 W3 (Lovejoy) heads towards the Sun


Comet Borrelly exhibits jets, but has no surface ice.


Comet Wild 2 exhibits jets on light side and dark side, stark relief, and is dry.
The solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen gases such as carbon dioxide, carbon monoxide, methane, and ammonia.[12] As such, they are popularly described as “dirty snowballs” after Fred Whipple’s model.[13] However, some comets may have a higher dust content, leading them to be called “icy dirtballs”.[14]

The surface of the nucleus is generally dry, dusty or rocky, suggesting that the ices are hidden beneath a surface crust several metres thick. In addition to the gases already mentioned, the nuclei contain a variety of organic compounds, which may include methanol, hydrogen cyanide, formaldehyde, ethanol, and ethane and perhaps more complex molecules such as long-chain hydrocarbons and amino acids.[15][16] In 2009, it was confirmed that the amino acid glycine had been found in the comet dust recovered by NASA’s Stardust mission.[17] In August 2011, a report, based on NASA studies of meteorites found on Earth, was published suggesting DNA and RNA components (adenine, guanine, and related organic molecules) may have been formed on asteroids and comets.[18][19]

The outer surfaces of cometary nuclei have a very low albedo, making them among the least reflective objects found in the Solar System. The Giotto space probe found that the nucleus of Halley’s Comet reflects about four percent of the light that falls on it,[20] and Deep Space 1 discovered that Comet Borrelly’s surface reflects less than 3.0% of the light that falls on it;[20] by comparison, asphalt reflects seven percent of the light that falls on it. The dark surface material of the nucleus may consist of complex organic compounds. Solar heating drives off lighter volatile compounds, leaving behind larger organic compounds that tend to be very dark, like tar or crude oil. The low reflectivity of cometary surfaces enables them to absorb the heat necessary to drive their outgassing processes.[21]

Properties of some comets
NAME DIMENSIONS
KM DENSITY
G/CM3 MASS
KG[22]
Halley’s Comet 15 × 8 × 8[23] 0.6[24] 3×1014
Tempel 1 7.6 × 4.9[25] 0.62[26] 7.9×1013
19P/Borrelly 8 × 4×4 0.3[26] 2×1013
81P/Wild 5.5 × 4.0 × 3.3[27] 0.6[26] 2.3×1013
Comet nuclei with radii of up to 30 kilometres (19 mi) have been observed,[28] but ascertaining their exact size is difficult.[29] The nucleus of P/2007 R5 is probably only 100–200 metres in diameter.[30] A lack of smaller comets being detected despite the increased sensitivity of instruments has led some to suggest that there is a real lack of comets smaller than 100 metres (330 ft) across.[31] Known comets have been estimated to have an average density of 0.6 g/cm3.[26] Because of their low mass, comet nuclei do not become spherical under their own gravity and therefore have irregular shapes.[32]

Roughly six percent of the near-Earth asteroids are thought to be extinct nuclei of comets that no longer experience outgassing,[33] including 14827 Hypnos and 3552 Don Quixote.

Coma

Main article: Coma (cometary)


Hubble image of comet ISON shortly before perihelion.[34]
The streams of dust and gas thus released form a huge and extremely thin atmosphere around the comet called the “coma”, and the force exerted on the coma by the Sun’s radiation pressure and solar wind cause an enormous “tail” to form pointing away from the Sun.[35]

The coma is generally made of  H2O and dust, with water making up to 90% of the volatiles that outflow from the nucleus when the comet is within 3 to 4 astronomical units (450,000,000 to 600,000,000 km; 280,000,000 to 370,000,000 mi) of the Sun.[36] The  H2O parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization, with the solar wind playing a minor role in the destruction of water compared to photochemistry.[36] Larger dust particles are left along the comet’s orbital path whereas smaller particles are pushed away from the Sun into the comet’s tail by light pressure.[37]

Although the solid nucleus of comets is generally less than 60 kilometres (37 mi) across, the coma may be thousands or millions of kilometres across, sometimes becoming larger than the Sun.[38] For example, about a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun.[39] The Great Comet of 1811 also had a coma roughly the diameter of the Sun.[40] Even though the coma can become quite large, its size can actually decrease about the time it crosses the orbit of Mars around 1.5 astronomical units (220,000,000 km; 140,000,000 mi) from the Sun.[40] At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, enlarging the tail.[40] Ion tails have been observed to extend one astronomical unit (150 million km) or more.[39]

Both the coma and tail are illuminated by the Sun and may become visible when a comet passes through the inner Solar System, the dust reflecting sunlight directly and the gases glowing from ionisation.[41] Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye.[42] Occasionally a comet may experience a huge and sudden outburst of gas and dust, during which the size of the coma greatly increases for a period of time. This happened in 2007 to Comet Holmes.[39]

In 1996, comets were found to emit X-rays.[43] This greatly surprised astronomers because X-ray emission is usually associated with very high-temperature bodies. The X-rays are generated by the interaction between comets and the solar wind: when highly charged solar wind ions fly through a cometary atmosphere, they collide with cometary atoms and molecules, “stealing” one or more electrons from the atom in a process called “charge exchange”. This exchange or transfer of an electron to the solar wind ion is followed by its de-excitation into the ground state of the ion, leading to the emission of X-rays and far ultraviolet photons.[44]

Tails

Main article: Comet tail


Diagram of a comet showing the dust trail, the dust tail (or antitail) and the ion gas tail, which is formed by the solar wind flow.
In the outer Solar System, comets remain frozen and inactive and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from observations by the Hubble Space Telescope[45][46] but these detections have been questioned.[47][48] As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them.

The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet’s orbit in such a manner that it often forms a curved tail called the type II or dust tail.[41] At the same time, the ion or type I tail, made of gases, always points directly away from the Sun because this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory.[49] On occasions - such as when the Earth passes through a comet’s orbital plane, and we see the track of the comet edge-on, a tail pointing in the opposite direction to the ion and dust tails may be seen – the antitail. [50] (The dust tail of the comet prior to its rounding of the sun, is collinear with the dust tail post the rounding of the sun).

The observation of antitails contributed significantly to the discovery of solar wind.[51] The ion tail is formed as a result of the ionisation by solar ultra-violet radiation of particles in the coma. Once the particles have been ionized, they attain a net positive electrical charge, which in turn gives rise to an “induced magnetosphere” around the comet. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles. Because the relative orbital speed of the comet and the solar wind is supersonic, a bow shock is formed upstream of the comet in the flow direction of the solar wind. In this bow shock, large concentrations of cometary ions (called “pick-up ions”) congregate and act to “load” the solar magnetic field with plasma, such that the field lines “drape” around the comet forming the ion tail.[52]



Encke’s Comet loses its tail
If the ion tail loading is sufficient, then the magnetic field lines are squeezed together to the point where, at some distance along the ion tail, magnetic reconnection occurs. This leads to a “tail disconnection event”.[52] This has been observed on a number of occasions, one notable event being recorded on April 20, 2007, when the ion tail of Encke’s Comet was completely severed while the comet passed through a coronal mass ejection. This event was observed by the STEREO space probe.[53]

In 2013 ESA scientists reported that the ionosphere of the planet Venus streams outwards in a manner similar to the ion tail seen streaming from a comet under similar conditions."[54][55]

Jets



Gas and snow jets on Comet Hartley 2
 This section requires expansion. (December 2013)
Uneven heating can cause newly generated gases to break out of a weak spot on the surface of comet’s nucleus, like a geyser.[56] These streams of gas and dust can cause the nucleus to spin, and even split apart.[56] In 2010 it was revealed dry ice (frozen carbon dioxide) can power jets of material flowing out of a comet nucleus.[57] This is known because a spacecraft got so close that it could see where the jets where coming out, then measure the infrared spectrum at that point which shows what some of the materials are.[58]

Orbital characteristics

Most comets are small Solar System bodies with elongated elliptical orbits that take them close to the Sun for a part of their orbit and then out into the further reaches of the Solar System for the remainder.[59] Comets are often classified according to the length of their orbital periods: The longer the period the more elongated the ellipse.

Short period

Periodic comets or short-period comets are generally defined as having orbital periods of less than 200 years.[60] They usually orbit more-or-less in the ecliptic plane in the same direction as the planets.[61] Their orbits typically take them out to the region of the outer planets (Jupiter and beyond) at aphelion; for example, the aphelion of Halley’s Comet is a little beyond the orbit of Neptune. Comets whose aphelia are near a major planet’s orbit are called its “family”.[62] Such families are thought to arise from the planet capturing formerly long-period comets into shorter orbits.[63]

At the shorter extreme, Encke’s Comet has an orbit that does not reach the orbit of Jupiter, and is known as an Encke-type comet. Short-period comets with orbital periods shorter than 20 years and low inclinations (up to 30 degrees) are called “Jupiter-family comets”.[64][65] Those like Halley, with orbital periods of between 20 and 200 years and inclinations extending from zero to more than 90 degrees, are called “Halley-type comets”.[66][67] As of 2013[update], only 72 Halley-type comets have been observed, compared with 470 identified Jupiter-family comets.[68]

Recently discovered main-belt comets form a distinct class, orbiting in more circular orbits within the asteroid belt.[69]

Because their elliptical orbits frequently take them close to the giant planets, comets are subject to further gravitational perturbations.[70] Short-period comets display a tendency for their aphelia to coincide with a gas giant’s orbital radius, with the Jupiter family of comets being the largest.[65] It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined. These perturbations can deflect long-period comets into shorter orbital periods.[71][72]

Based on their orbital characteristics, short-period comets are thought to originate from the centaurs and the Kuiper belt/scattered disc[73] —a disk of objects in the trans-Neptunian region—whereas the source of long-period comets is thought to be the far more distant spherical Oort cloud (after the Dutch astronomer Jan Hendrik Oort who hypothesised its existence).[74] Vast swarms of comet-like bodies are believed to orbit the Sun in these distant regions in roughly circular orbits. Occasionally the gravitational influence of the outer planets (in the case of Kuiper belt objects) or nearby stars (in the case of Oort cloud objects) may throw one of these bodies into an elliptical orbit that takes it inwards toward the Sun to form a visible comet. Unlike the return of periodic comets, whose orbits have been established by previous observations, the appearance of new comets by this mechanism is unpredictable.[75]

Long period



Orbits of the Kohoutek Comet (red) and the Earth (blue), illustrating the high eccentricity of its orbit and its rapid motion when close to the Sun.
Hyperbolic
comet discoveries[76]
YEAR #
2013 8
2012 10
2011 12
2010 4
2009 8
2008 7
2007 12
Long-period comets have highly eccentric orbits and periods ranging from 200 years to thousands of years.[77] An eccentricity greater than 1 when near perihelion does not necessarily mean that a comet will leave the Solar System.[78] For example, Comet McNaught had a heliocentric osculating eccentricity of 1.000019 near its perihelion passage epoch in January 2007 but is bound to the Sun with roughly a 92,600-year orbit because the eccentricity drops below 1 as it moves further from the Sun. The future orbit of a long-period comet is properly obtained when the osculating orbit is computed at an epoch after leaving the planetary region and is calculated with respect to the center of mass of the Solar System. By definition long-period comets remain gravitationally bound to the Sun; those comets that are ejected from the Solar System due to close passes by major planets are no longer properly considered as having “periods”. The orbits of long-period comets take them far beyond the outer planets at aphelia, and the plane of their orbits need not lie near the ecliptic. Long-period comets such as Comet West and C/1999 F1 can have apoapsis distances of nearly 70,000 AU with orbital periods estimated around 6 million years.



Comets C/2012 F6 (Lemmon) (top) and C/2011 L4 (PANSTARRS) (bottom)
Single-apparition or non-periodic comets are similar to long-period comets because they also have parabolic or slightly hyperbolic trajectories[77] when near perihelion in the inner Solar System. However, gravitational perturbations from giant planets cause their orbits to change. Single-apparition or comets are those with a hyperbolic or parabolic osculating, which makes them permanently exit the Solar System after a single pass of the Sun.[79] The Sun’s Hill sphere has an unstable maximum boundary of 230,000 AU (1.1 parsecs (3.6 light-years)).[80] Only a few hundred comets have been seen to achieve a hyperbolic orbit (e > 1) when near perihelion[81] that using a heliocentric unperturbed two-body best-fit suggests they may escape the Solar System.

No comets with an eccentricity significantly greater than one have been observed,[81] so there are no confirmed observations of comets that are likely to have originated outside the Solar System. Comet C/1980 E1 had an orbital period of roughly 7.1 million years before the 1982 perihelion passage, but a 1980 encounter with Jupiter accelerated the comet giving it the largest eccentricity (1.057) of any known hyperbolic comet.[82] Comets not expected to return to the inner Solar System include C/1980 E1, C/2000 U5, C/2001 Q4 (NEAT), C/2009 R1, C/1956 R1, and C/2007 F1 (LONEOS).

Some authorities use the term “periodic comet” to refer to any comet with a periodic orbit (that is, all short-period comets plus all long-period comets),[83] whereas others use it to mean exclusively short-period comets.[77] Similarly, although the literal meaning of “non-periodic comet” is the same as “single-apparition comet”, some use it to mean all comets that are not “periodic” in the second sense (that is, to also include all comets with a period greater than 200 years).

Early observations have revealed a few genuinely hyperbolic (i.e. non-periodic) trajectories, but no more than could be accounted for by perturbations from Jupiter. If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of kms per second). If such objects entered the Solar System, they would have positive specific orbital energy and would be observed to have genuinely hyperbolic trajectories. A rough calculation shows that there might be four hyperbolic comets per century within Jupiter’s orbit, give or take one and perhaps two orders of magnitude.[84]

Oort Cloud and Hills cloud



The Oort cloud is a vast cloud of comets that is thought to surround the Solar System.
Main article: Oort cloud
The Oort cloud is thought to occupy a vast space from somewhere between 2,000 and 5,000 AU (0.03 and 0.08 ly)[85] to as far as 50,000 AU (0.79 ly)[66] from the Sun. Some estimates place the outer edge at between 100,000 and 200,000 AU (1.58 and 3.16 ly).[85] The region can be subdivided into a spherical outer Oort cloud of 20,000–50,000 AU (0.32–0.79 ly), and a doughnut-shaped inner Oort cloud of 2,000–20,000 AU (0.03–0.32 ly). The outer cloud is only weakly bound to the Sun and supplies the long-period (and possibly Halley-type) comets to inside the orbit of Neptune.[66] The inner Oort cloud is also known as the Hills cloud, named after J. G. Hills, who proposed its existence in 1981.[86] Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo;[86][87][88] it is seen as a possible source of new comets to resupply the relatively tenuous outer cloud as the latter’s numbers are gradually depleted. The Hills cloud explains the continued existence of the Oort cloud after billions of years.[89]

Exocomets

Main article: Exocomet
Exocomets beyond our Solar System have also been detected and may be common in the Milky Way Galaxy.[90] The first exocomet system detected was around Beta Pictoris, a very young type A V star, in 1987.[91][92] A total of 10 such exocomet systems have been identified as of 2013[update], using the absorption spectrum caused by the large clouds of gas emitted by comets when passing close to their star.[90][91]

Effects of comets

Connection to meteor showers



Diagram of Perseids meteors
As a result of outgassing, comets leave in their wake a trail of solid debris too large to be swept away by radiation pressure and the solar wind.[93] If the comet’s path crosses the path the Earth follows in orbit around the Sun, then at that point there are likely to be meteor showers as Earth passes through the trail of debris. The Perseid meteor shower, for example, occurs every year between August 9 and August 13, when Earth passes through the orbit of Comet Swift–Tuttle.[94]Halley’s comet is the source of the Orionid shower in October.[94]

Comets and impact on life

Many comets and asteroids collided into Earth in its early stages. Many scientists believe that comets bombarding the young Earth about 4 billion years ago brought the vast quantities of water that now fill the Earth’s oceans, or at least a significant portion of it. Other researchers have cast doubt on this theory.[95] The detection of organic molecules in significant quantities in comets has led some to speculate that comets or meteorites may have brought the precursors of life—or even life itself—to Earth.[96] In 2013 it was suggested that impacts between rocky and icy surfaces, such as comets, had the potential to create the amino acids that make up proteins through shock synthesis.[97]

It is suspected that comet impacts have, over long timescales, also delivered significant quantities of water to the Earth’s Moon, some of which may have survived as lunar ice.[98] Comet and meteoroid impacts are also believed responsible for the existence of tektites and australites.[99]

Fate of comets

Departure (ejection) from Solar System

If a comet is traveling fast enough, it may leave the Solar System; such is the case for hyperbolic comets. To date, comets are only known to be ejected by interacting with another object in the Solar System, such as Jupiter.[100]

Volatiles exhausted

Main article: Extinct comet
Jupiter-family comets and long-period comets appear to follow very different fading laws. The JFCs are active over a lifetime of about 10,000 years or ~1,000 orbits whereas long-period comets fade much faster. Only 10% of the long-period comets survive more than 50 passages to small perihelion and only 1% of them survive more than 2,000 passages.[33] Eventually most of the volatile material contained in a comet nucleus evaporates away, and the comet becomes a small, dark, inert lump of rock or rubble that can resemble an asteroid.[101] Some asteroids in elliptical orbits are now identified as extinct comets.[102] Roughly six percent of the near-Earth asteroids are thought to be extinct nuclei of comets that no longer emit gas.[33]

Breakup



Breaking up of 73P/Schwassmann–Wachmann in 1995. This animation covers three days.
The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart.[103] A significant cometary disruption was that of Comet Shoemaker–Levy 9, which was discovered in 1993. A close encounter in July 1992 had broken it into pieces, and over a period of six days in July 1994, these pieces fell into Jupiter’s atmosphere—the first time astronomers had observed a collision between two objects in the Solar System.[104][105] Other splitting comets include 3D/Biela in 1846 and 73P/Schwassmann–Wachmann from 1995 to 2006.[106] Greek historian Ephorus reported that a comet split apart as far back as the winter of 372–373 BC.[107] Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.[108]

Comets 42P/Neujmin and 53P/Van Biesbroeck appear to be fragments of a parent comet. Numerical integrations have shown that both comets had a rather close approach to Jupiter in January 1850, and that, before 1850, the two orbits were nearly identical.[109]

Some comets have been observed to break up during their perihelion passage, including great comets West and Ikeya–Seki. Biela’s Comet was one significant example, when it broke into two pieces during its passage through the perihelion in 1846. These two comets were seen separately in 1852, but never again afterward. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A lesser meteor shower, the Andromedids, occurs annually in November, and it is caused when the Earth crosses the orbit of Biela’s Comet.[110]

Collisions



Brown spots mark impact sites of Comet Shoemaker–Levy on Jupiter
Some comets meet a more spectacular end – either falling into the Sun[111] or smashing into a planet or other body. Collisions between comets and planets or moons were common in the early Solar System: some of the many craters on the Moon, for example, may have been caused by comets. A recent collision of a comet with a planet occurred in July 1994 when Comet Shoemaker–Levy 9 broke up into pieces and collided with Jupiter.[112]

Nomenclature

Main article: Naming of comets


Halley’s comet, named after the astronomer Edmund Halley for successfully calculating its orbit. 1910 photo.
The names given to comets have followed several different conventions over the past two centuries. Prior to the early 20th century, most comets were simply referred to by the year when they appeared, sometimes with additional adjectives for particularly bright comets; thus, the “Great Comet of 1680”, the “Great Comet of 1882”, and the “Great January comet of 1910”.

After Edmund Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759, that comet became known as Halley’s comet.[113] Similarly, the second and third known periodic comets, Encke’s Comet[114] and Biela’s Comet,[115] were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their apparition.[116]

In the early 20th century, the convention of naming comets after their discoverers became common, and this remains so today. A comet can be named after its discoverers, or an instrument or program that helped to find it.[116]

History of study

Main article: Observational history of comets

Early observations and thought



Halley’s Comet appeared at the Battle of Hastings in 1066 (Bayeux Tapestry).
From ancient sources, such as Chinese oracle bones, it is known that their appearances have been noticed by humans for millennia.[117] Until the sixteenth century, comets were usually considered bad omens of deaths of kings or noble men, or coming catastrophes, or even interpreted as attacks by heavenly beings against terrestrial inhabitants.[118][119]

Aristotle believed that comets were atmospheric phenomena, due to the fact that they could appear outside of the Zodiac and vary in brightness over the course of a few days.[120]Pliny the Elder believed that comets were connected with political unrest and death.[121]

In the 16th century Tycho Brahe demonstrated that comets must exist outside the Earth’s atmosphere by measuring the parallax of the Great Comet of 1577 from observations collected by geographically separated observers. Within the precision of the measurements, this implied the comet must be at least four times more distant than from the Earth to the Moon.[122][123]

Orbital studies



The orbit of the comet of 1680, fitted to a parabola, as shown in Isaac Newton’s Principia
Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of his inverse square law of universal gravitation must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet’s path through the sky to a parabolic orbit, using the comet of 1680 as an example.[124]

In 1705, Edmond Halley (1656–1742) applied Newton’s method to twenty-three cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758–9.[125] Halley’s predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet’s 1759 perihelion to within one month’s accuracy.[126] When the comet returned as predicted, it became known as Halley’s Comet (with the latter-day designation of 1P/Halley). It will next appear in 2061.[127]

Studies of physical characteristics

“From his huge vapouring train perhaps to shake Reviving moisture on the numerous orbs, Thro’ which his long ellipsis winds; perhaps To lend new fuel to declining suns, To light up worlds, and feed th’ ethereal fire.”

James Thomson The Seasons (1730; 1748)[128]

Isaac Newton described comets as compact and durable solid bodies moving in oblique orbit and their tails as thin streams of vapor emitted by their nuclei, ignited or heated by the Sun. Newton suspected that comets were the origin of the life-supporting component of air.[129]

As early as the 18th century, some scientists had made correct hypotheses as to comets’ physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion.[130] In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor during the appearance of Halley’s Comet in 1835, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet’s orbit, and he argued that the non-gravitational movements of Encke’s Comet resulted from this phenomenon.[131]

In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[132] This “dirty snowball” model soon became accepted and appeared to be supported by the observations of an armada of spacecraft (including the European Space Agency’s Giotto probe and the Soviet Union’s Vega 1 and Vega 2) that flew through the coma of Halley’s Comet in 1986, photographed the nucleus, and observed jets of evaporating material.[133]

Spacecraft missions

See also: List of comets visited by spacecraft


View from the impactor in its last moments before hitting the comet in the Deep Impact mission


NASA is developing a comet harpoon for returning samples to Earth.
Debate continues about how much ice is in a comet. In 2001, the Deep Space 1 spacecraft obtained high-resolution images of the surface of Comet Borrelly. It was found that the surface of comet Borrelly is hot and dry, with a temperature of between 26 to 71 °C (79 to 160 °F), and extremely dark, suggesting that the ice has been removed by solar heating and maturation, or is hidden by the soot-like material that covers Borrelly’s.[134]

In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. The mission yielded results suggesting that the majority of a comet’s water ice is below the surface and that these reservoirs feed the jets of vaporised water that form the coma of Tempel 1.[135] Renamed EPOXI, it made a flyby of Comet Hartley 2 on November 4, 2010.

Data from the Stardust mission show that materials retrieved from the tail of Wild 2 were crystalline and could only have been “born in fire,” at extremely high temperatures of over 1,000 °C (1,830 °F).[136][137] Although comets formed in the outer Solar System, radial mixing of material during the early formation of the Solar System is thought to have redistributed material throughout the proto-planetary disk,[138] so comets also contain crystalline grains that formed in the hot inner Solar System. This is seen in comet spectra as well as in sample return missions. More recent still, the materials retrieved demonstrate that the “comet dust resembles asteroid materials”.[139] These new results have forced scientists to rethink the nature of comets and their distinction from asteroids.[140]

The Rosetta probe is presently en route to Comet Churyumov–Gerasimenko; in 2014 it will go into orbit around the comet and place a small lander on its surface.[141]

Examples


Comet C/2006 P1 (McNaught) taken from Victoria, Australia 2007


The Great Comet of 1882 is a member of the Kreutz group


Great Comet 1861

Great comets

Main article: Great Comet
Approximately once a decade, a comet becomes bright enough to be noticed by a casual observer, leading such comets to be designated as Great Comets.[107]



Woodcut of the Great Comet of 1577
Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet’s brightness to depart drastically from predictions.[142] Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it has a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular but failed to do so.[143]Comet West, which appeared three years later, had much lower expectations but became an extremely impressive comet.[144]

The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick succession—Comet Hyakutake in 1996, followed by Hale–Bopp, which reached maximum brightness in 1997 having been discovered two years earlier. The first great comet of the 21st century was C/2006 P1 (McNaught), which became visible to naked eye observers in January 2007. It was the brightest in over 40 years.[145]

Sungrazing comets

Main article: Sungrazing comet


SOHO spots a Kreutz Sungrazer with a prominent tail, plunging towards the Sun
A sungrazing comet is a comet that passes extremely close to the Sun at perihelion, generally within a few million kilometres.[146] Although small sungrazers can be completely evaporated during such a close approach to the Sun, larger sungrazers can survive many perihelion passages. However, the strong tidal forces they experience often lead to their fragmentation.[147]

About 90% of the sungrazers observed with SOHO are members of the Kreutz group, which all originate from one giant comet that broke up into many smaller comets during its first passage through the inner Solar System.[148] The remainder contains some sporadic sungrazers, but four other related groups of comets have been identified among them: the Kracht, Kracht 2a, Marsden, and Meyer groups. The Marsden and Kracht groups both appear to be related to Comet 96P/Machholz, which is also the parent of two meteor streams, the Quadrantids and the Arietids.[149]

Unusual comets



“Active asteroid” P/2013 P5 with several tails. [150]
Of the thousands of known comets, some exhibit unusual properties. Encke’s Comet orbits from outside the asteroid belt to just inside the orbit of the planet Mercury whereas the Comet 29P/Schwassmann–Wachmann currently travels in a nearly circular orbit entirely between the orbits of Jupiter and Saturn.[151]2060 Chiron, whose unstable orbit is between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[152] Similarly, Comet Shoemaker–Levy 2 was originally designated asteroid 1990 UL3.[153]

See also Fate of comets.

Observation



X-ray emission from Hyakutake, as seen by the ROSAT satellite.
A comet may be discovered photographically using a wide-field telescope or visually with binoculars. However, even without access to optical equipment, it is still possible for the amateur astronomer to discover a sungrazing comet online by downloading images accumulated by some satellite observatories such as SOHO.[30] SOHO’s 2000th comet was discovered by Polish amateur astronomer Michał Kusiak on 26 December 2010.[154]

Lost

Main article: Lost comet
A number of periodic comets discovered in earlier decades or previous centuries are now lost comets. Their orbits were never known well enough to predict future appearances or the comets have disintegrated. However, occasionally a “new” comet is discovered, and calculation of its orbit shows it to be an old “lost” comet. An example is Comet 11P/Tempel–Swift–LINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001.[155]

Comets & culture

See also: Comets in fiction and Category:Impact events in fiction


Comet Hale-Bopp, as seen in Pazin, Croatia 1997.
The depiction of comets in popular culture is firmly rooted in the long Western tradition of seeing comets as harbingers of doom and as omens of world-altering change.[156] Halley’s Comet alone has caused a slew of sensationalist publications of all sorts at each of its reappearances. It was especially noted that the birth and death of some notable persons coincided with separate appearances of the comet, such as with writers Mark Twain (who correctly speculated that he’d “go out with the comet” in 1910)[156] and Eudora Welty, to whose life Mary Chapin Carpenter dedicated the song Halley Came to Jackson.[156]

In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley’s Comet in 1910, the Earth passed through the comet’s tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions,[157] whereas the appearance of Comet Hale–Bopp in 1997 triggered the mass suicide of the Heaven’s Gate cult.[158]

In science fiction, the impact of comets has been depicted as a threat overcome by technology and heroism (Deep Impact, 1998), or as a trigger of global apocalypse (Lucifer’s Hammer, 1979) or of waves of zombies (Night of the Comet, 1984).[156] In Jules Verne’s Off on a Comet a group of people are stranded on a comet orbiting the Sun, while a large manned space expedition visits Halley’s Comet in Sir Arthur C. Clarke’s novel 2061: Odyssey Three.[159]

See also


Book: Solar System
Comet vintages
List of comets
The Big Splash (book)
Solar System portal
Space portal
Astronomy portal

References

Notes

^ “What is the difference between asteroids and comets”. Rosetta’s Frequently Asked Questions. European Space Agency. Retrieved 30 July 2013.
^ “What Are Asteroids And Comets”. Near Earth Object Program FAQ. NASA. Retrieved 30 July 2013.
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Tempel 1: Using a spherical diameter of 6.25 km; volume of a sphere * a density of 0.62 g/cm3 yields a mass of 7.9E+13 kg.
19P/Borrelly: Using the volume of an ellipsoid of 8x4x4km * a density of 0.3 g/cm3 yields a mass of 2.0E+13 kg.
81P/Wild: Using the volume of an ellipsoid of 5.5x4.0x3.3km * a density of 0.6 g/cm3 yields a mass of 2.28E+13 kg.
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Sources
Sagan, Carl; Druyan, Ann (1997). Comet. ISBN 9780747276647.
Further reading
Schechner, S. J. (1997). Comets, Popular Culture, and the Birth of Modern Cosmology. Princeton University Press. .
Brandt, J.C. and Chapman, R.D.: Introduction to comets, Cambridge University Press 2004
External links
Find more about Comet at Wikipedia’s sister projects
 Definitions and translations from Wiktionary
 Media from Commons
 Quotations from Wikiquote
 Textbooks from Wikibooks
 Learning resources from Wikiversity
Comets on the Open Directory Project
Comets Page at NASA’s Solar System Exploration
International Comet Quarterly
How to Make a Model of a Comet audio slideshow – National High Magnetic Field Laboratory
Catalogue of the Solar System Small Bodies Orbital Evolution
Information about comets and asteroids
V
T
E
COMETS
FEATURES
Nucleus
Coma
Tails
Antitail
Dust trail
Dust


TYPES
Names
Antimatter
Encke-types
Exocomets
Extinct
Great
Halley-types
Interstellar
Lost
Main-belt
Sungrazing (Kreutz Sungrazers)
RELATED
Asteroid
Centaur
Comet discoverers
LINEAR
Extraterrestrial atmospheres
Oort cloud
Small Solar System body
See also
Comet vintages
Comets in fiction
Fictional comets
SPACE
MISSIONS
Comet Hopper
CONTOUR
CRAF
Deep Impact/EPOXI
Deep Space 1
Giotto
Hayabusa Mk2
ICE
Marco Polo
MarcoPolo-R
Rosetta
Philae
Sakigake
Stardust/NeXT
Suisei
Ulysses
Vega program
Vega 1
Vega 2
Vesta
LATEST
C/2013 R1 (Lovejoy)
C/2013 A1 (Siding Spring)
P/2013 P5 (PANSTARRS)
C/2012 S1 (ISON)
C/2012 K1 (PANSTARRS)
C/2012 F6 (Lemmon)
C/2012 E2 (SWAN)
276P/Vorobjov
2P/Encke

LISTS (MORE)
PERIODIC
COMETS
NUMBER
1P/Halley
2P/Encke
3D/Biela
4P/Faye
5D/Brorsen
6P/d’Arrest
7P/Pons–Winnecke
8P/Tuttle
9P/Tempel
10P/Tempel
11P/Tempel–Swift–LINEAR
12P/Pons–Brooks
13P/Olbers
14P/Wolf
15P/Finlay
16P/Brooks
17P/Holmes
18D/Perrine–Mrkos
19P/Borrelly
20D/Westphal
21P/Giacobini–Zinner
22P/Kopff
23P/Brorsen–Metcalf
24P/Schaumasse
25D/Neujmin
26P/Grigg–Skjellerup
27P/Crommelin
28P/Neujmin
29P/Schwassmann–Wachmann
30P/Reinmuth
31P/Schwassmann–Wachmann
32P/Comas Solá
33P/Daniel
34D/Gale
35P/Herschel–Rigollet
36P/Whipple
37P/Forbes
38P/Stephan–Oterma
39P/Oterma
40P/Väisälä
41P/Tuttle–Giacobini–Kresák
42P/Neujmin
43P/Wolf–Harrington
44P/Reinmuth
45P/Honda–Mrkos–Pajdušáková
46P/Wirtanen
47P/Ashbrook–Jackson
48P/Johnson
49P/Arend–Rigaux
50P/Arend
51P/Harrington
52P/Harrington–Abell
53P/Van Biesbroeck
54P/de Vico–Swift–NEAT
55P/Tempel–Tuttle
56P/Slaughter–Burnham
57P/du Toit–Neujmin–Delporte
60P/Tsuchinshan
62P/Tsuchinshan
65P/Gunn
67P/Churyumov–Gerasimenko
68P/Klemola
69P/Taylor
71P/Clark
73P/Schwassmann–Wachmann
74P/Smirnova–Chernykh
75D/Kohoutek
77P/Longmore
78P/Gehrels
81P/Wild
82P/Gehrels
84P/Giclas
85P/Boethin
87P/Bus
88P/Howell
91P/Russell
94P/Russell
95P/Chiron
96P/Machholz
98P/Takamizawa
99P/Kowal
100P/Hartley
101P/Chernykh
102P/Shoemaker
103P/Hartley
105P/Singer Brewster
107P/Wilson–Harrington
109P/Swift–Tuttle
111P/Helin–Roman–Crockett
112P/Urata–Niijima
113P/Spitaler
114P/Wiseman–Skiff
115P/Maury
116P/Wild
117P/Helin–Roman–Alu
118P/Shoemaker–Levy
119P/Parker–Hartley
120P/Mueller
121P/Shoemaker–Holt
122P/de Vico
125P/Spacewatch
128P/Shoemaker–Holt
129P/Shoemaker–Levy
130P/McNaught–Hughes
132P/Helin–Roman–Alu
133P/Elst–Pizarro
136P/Mueller
138P/Shoemaker–Levy
139P/Väisälä–Oterma
143P/Kowal–Mrkos
144P/Kushida
147P/Kushida–Muramatsu
152P/Helin–Lawrence
153P/Ikeya–Zhang
157P/Tritton
158P/Kowal–LINEAR
159P/LONEOS
160P/LINEAR
161P/Hartley–IRAS
163P/NEAT
164P/Christensen
165P/LINEAR
166P/NEAT
167P/CINEOS
168P/Hergenrother
169P/NEAT
170P/Christensen
171P/Spahr
172P/Yeung
173P/Mueller
174P/Echeclus
176P/LINEAR
177P/Barnard
178P/Hug–Bell
206P/Barnard–Boattini
208P/McMillan
209P/LINEAR
238P/Read
246P/NEAT
255P/Levy
271P/van Houten–Lemmon
276P/Vorobjov
289P/Blanpain
NONE
D/1770 L1
D/1993 F2
P/1997 B1
P/2007 R5
P/2010 A2
P/2010 B2
P/2010 V1
P/2011 NO1
P/2013 P5
NON
PERIODIC
COMETS
BEFORE
1910
C/-43 K1 (Comet Caesar)
X/1106 C1
C/1577 V1 (Great Comet of 1577)
C/1652 Y1
C/1680 V1 (Kirsch’s Comet)
C/1702 H1
C/1729 P1 (Comet of 1729)
C/1743 X1
C/1760 A1
C/1769 P1
C/1811 F1 (Great Comet of 1811)
C/1823 Y1 (Great Comet of 1823)
C/1843 D1 (Great March Comet of 1843)
C/1847 T1 (Miss Mitchell’s Comet)
C/1858 L1 (Comet Donati)
C/1861 G1 (Comet Thatcher)
C/1861 J1 (Great Comet of 1861)
X/1872 X1
C/1882 R1 (Great September Comet of 1882)
C/1887 B1 (Great Southern Comet of 1887)
C/1910 A1 (Great January Comet of 1910)
AFTER
1910
C/1911 O1 (Brooks)
C/1911 S3 (Beljawsky)
C/1927 X1 (Skjellerup–Maristany)
C/1948 V1 (Eclipse)
C/1956 R1 (Arend–Roland)
C/1961 R1 (Humason)
C/1965 S1 (Ikeya-Seki)
C/1969 Y1 (Bennett)
C/1970 K1 (White–Ortiz–Bolelli)
C/1973 E1 (Kohoutek)
C/1975 V1 (West)
C/1980 E1 (Bowell)
C/1989 X1 (Austin)
C/1989 Y1 (Skorichenko–George)
C/1993 Y1 (McNaught–Russell)
C/1995 O1 (Hale–Bopp)
C/1996 B2 (Hyakutake)
C/1997 L1 (Zhu–Balam)
C/1998 H1 (Stonehouse)
C/1999 F1 (Catalina)
C/1999 S4 (LINEAR)
C/2000 U5 (LINEAR)
C/2000 W1 (Utsunomiya-Jones)
C/2001 Q4 (NEAT)
C/2002 T7 (LINEAR)
C/2004 Q2 (Machholz)
C/2006 A1 (Pojmański)
C/2006 M4 (SWAN)
C/2006 P1 (McNaught)
C/2007 F1 (LONEOS)
C/2007 N3 (Lulin)
C/2007 Q3 (Siding Spring)
C/2007 W1 (Boattini)
C/2009 F6 (Yi–SWAN)
C/2009 R1 (McNaught)
C/2010 X1 (Elenin)
C/2011 L4 (PANSTARRS)
C/2011 W3 (Lovejoy)
C/2012 E2 (SWAN)
C/2012 F6 (Lemmon)
C/2012 K1 (PANSTARRS)
C/2012 S1 (ISON)
C/2013 A1 (Siding Spring)
P/2013 P5 (PANSTARRS)
C/2013 R1 (Lovejoy)
AFTER
1910
(ABC’S)
Arend–Roland
Austin
Beljawsky
Bennett
Boattini
Bowell
Brooks
Catalina
Eclipse
Elenin
Hale-Bopp
Humason
Hyakutake
Ikeya-Seki
ISON
Kohoutek
Lemmon
LINEAR
C/1999 S4
C/2000 U5
C/2002 T7
LONEOS
Lovejoy
C/2011 W3
C/2013 R1
Lulin
Machholz
McNaught
C/2006 P1
C/2009 R1
McNaught–Russell
NEAT
Pan-STARRS
C/2011 L4
C/2012 K1
P/2013 P5
Pojmański
Siding Spring
C/2007 Q3
C/2013 A1
Skjellerup–Maristany
Skorichenko–George
Stonehouse
SWAN
C/2006 M4
C/2012 E2
Utsunomiya–Jones
West
White–Ortiz–Bolelli
Yi–SWAN
Zhu–Balam
MISC
ASTEROID
COMETS
2060 Chiron (95P/Chiron)
7968 Elst–Pizarro (133P/Elst–Pizarro)
4015 Wilson–Harrington (107P/Wilson-Harrington)
60558 Echeclus (174P/Echeclus)
118401 LINEAR (176P/LINEAR, prev 1999 RE70)
LOST
COMETS
11P/Tempel–Swift–LINEAR
15P/Finlay
17P/Holmes
27P/Crommelin
54P/de Vico–Swift–NEAT
69P/Taylor
73P/Schwassmann–Wachmann
113P/Spitaler
107P/Wilson–Harrington
177P/Barnard
206P/Barnard–Boattini
D/1770 L1 (Lexell)
3D/Biela
5D/Brorsen
18D/Perrine–Mrkos
20D/Westphal
25D/Neujmin
34D/Gale
UNKNOWN
ORBITS
X/1106 C1
X/1872 X1
V
T
E
THE SOLAR SYSTEM

The Sun
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Earth
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Jupiter
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List of minor planets
Solar System → Local Interstellar Cloud → Local Bubble → Gould Belt → Orion Arm → Milky Way → Milky Way subgroup → Local Group → Virgo Supercluster → Pisces–Cetus Supercluster Complex → Observable universe → Universe

Each arrow (→) may be read as “within” or “part of”.
PORTALS: Astronomy
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Solar System
V
T
E
SMALL SOLAR SYSTEM BODIES
MINOR PLANETS
Designation
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Meanings of names
Pronunciation of names
ASTEROIDS
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Spectral types
DISTANT MINOR PLANETS
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LISTS / CATEGORIES
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V
T
E
MISSIONS TO COMETS
NEAR FLYBY
Vega program
Vega 1
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Giotto
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Deep Space 1
CONTOUR (failed)
Deep Impact/EPOXI
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 Spaceflight portal
FAR FLYBY
International Cometary Explorer
Ulysses
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SAMPLE RETURN
Stardust/NExT
LANDER
Deep Impact/EPOXI
Rosetta/Philae
PROPOSED
MarcoPolo-R
CANCELLED
Comet Hopper
CRAF
Hayabusa Mk2
Marco Polo
Vesta





Ephemeris – November

Date TT R. A. (2000) Decl. Delta r Elong. Phase m1
2013 11 01 11 12 23.8 +06 21 42 1.2300 0.9969 52.0 51.7 7.9
2013 11 02 11 17 32.6 +05 41 31 1.2024 0.9725 51.5 53.0 7.8
2013 11 03 11 22 55.3 +04 59 16 1.1751 0.9479 51.0 54.5 7.7
2013 11 04 11 28 32.7 +04 14 49 1.1484 0.9229 50.4 56.0 7.5
2013 11 05 11 34 26.0 +03 28 02 1.1222 0.8975 49.8 57.5 7.4
2013 11 06 11 40 36.3 +02 38 47 1.0965 0.8719 49.0 59.2 7.2
2013 11 07 11 47 04.8 +01 46 58 1.0716 0.8458 48.2 60.9 7.1
2013 11 08 11 53 52.7 +00 52 26 1.0474 0.8193 47.3 62.7 6.9
2013 11 09 12 01 01.5 -00 04 55 1.0240 0.7924 46.3 64.6 6.7
2013 11 10 12 08 32.6 -01 05 09 1.0016 0.7651 45.2 66.6 6.6
2013 11 11 12 16 27.3 -02 08 21 0.9802 0.7372 43.9 68.7 6.4
2013 11 12 12 24 47.2 -03 14 32 0.9600 0.7089 42.6 70.9 6.2
2013 11 13 12 33 33.9 -04 23 42 0.9411 0.6799 41.1 73.3 6.0
2013 11 14 12 42 48.8 -05 35 46 0.9236 0.6504 39.6 75.7 5.8
2013 11 15 12 52 33.4 -06 50 35 0.9077 0.6202 37.9 78.2 5.6
2013 11 16 13 02 49.2 -08 07 56 0.8936 0.5892 36.0 80.8 5.4
2013 11 17 13 13 37.4 -09 27 28 0.8813 0.5574 34.1 83.6 5.2
2013 11 18 13 24 59.3 -10 48 46 0.8713 0.5247 32.0 86.4 5.0
2013 11 19 13 36 55.6 -12 11 17 0.8635 0.4910 29.8 89.3 4.7
2013 11 20 13 49 27.4 -13 34 19 0.8584 0.4561 27.5 92.3 4.4
2013 11 21 14 02 35.1 -14 57 05 0.8561 0.4199 25.0 95.3 4.1
2013 11 22 14 16 19.5 -16 18 39 0.8569 0.3821 22.5 98.4 3.8
2013 11 23 14 30 41.4 -17 37 59 0.8612 0.3423 19.9 101.4 3.5
2013 11 24 14 45 42.7 -18 53 56 0.8693 0.3002 17.1 104.3 3.0
2013 11 25 15 01 27.3 -20 05 10 0.8819 0.2551 14.3 107.0 2.5
2013 11 26 15 18 04.6 -21 09 58 0.8998 0.2058 11.4 109.3 1.8
2013 11 27 15 35 58.3 -22 05 30 0.9244 0.1502 8.2 110.4 0.7
2013 11 28 15 56 28.2 -22 43 29 0.9594 0.0826 4.6 106.9 -1.3
2013 11 29 16 23 17.5 -19 52 57 0.9762 0.0322 1.8 107.7 -4.5
2013 11 30 16 21 22.4 -16 20 32 0.9125 0.1145 5.3 127.4 -0.2

Ephemeris – October

Date TT R. A. (2000) Decl. Delta r Elong. Phase m1
2013 10 01 09 34 34.6 +17 37 30 2.1508 1.6510 47.6 26.6 10.9
2013 10 02 09 36 43.1 +17 25 01 2.1213 1.6321 48.1 27.1 10.8
2013 10 03 09 38 53.9 +17 12 12 2.0918 1.6131 48.5 27.7 10.8
2013 10 04 09 41 07.1 +16 59 03 2.0622 1.5939 48.9 28.2 10.7
2013 10 05 09 43 22.7 +16 45 30 2.0325 1.5747 49.3 28.8 10.6
2013 10 06 09 45 40.8 +16 31 35 2.0028 1.5553 49.7 29.3 10.5
2013 10 07 09 48 01.7 +16 17 14 1.9729 1.5358 50.0 29.9 10.5
2013 10 08 09 50 25.4 +16 02 28 1.9430 1.5162 50.4 30.5 10.4
2013 10 09 09 52 52.1 +15 47 14 1.9130 1.4965 50.8 31.1 10.3
2013 10 10 09 55 21.9 +15 31 32 1.8830 1.4766 51.1 31.8 10.2
2013 10 11 09 57 55.0 +15 15 19 1.8529 1.4566 51.4 32.4 10.1
2013 10 12 10 00 31.7 +14 58 34 1.8228 1.4364 51.7 33.0 10.1
2013 10 13 10 03 12.0 +14 41 15 1.7926 1.4161 52.0 33.7 10.0
2013 10 14 10 05 56.3 +14 23 20 1.7624 1.3957 52.2 34.4 9.9
2013 10 15 10 08 44.7 +14 04 47 1.7322 1.3751 52.5 35.1 9.8
2013 10 16 10 11 37.4 +13 45 34 1.7020 1.3544 52.7 35.8 9.7
2013 10 17 10 14 34.8 +13 25 38 1.6718 1.3335 52.9 36.6 9.6
2013 10 18 10 17 37.1 +13 04 56 1.6416 1.3124 53.1 37.4 9.5
2013 10 19 10 20 44.6 +12 43 26 1.6115 1.2912 53.2 38.2 9.4
2013 10 20 10 23 57.6 +12 21 05 1.5814 1.2698 53.4 39.0 9.3
2013 10 21 10 27 16.6 +11 57 49 1.5513 1.2482 53.4 39.8 9.2
2013 10 22 10 30 41.8 +11 33 35 1.5213 1.2265 53.5 40.7 9.1
2013 10 23 10 34 13.6 +11 08 20 1.4915 1.2045 53.5 41.6 9.0
2013 10 24 10 37 52.6 +10 41 58 1.4617 1.1824 53.5 42.6 8.9
2013 10 25 10 41 39.1 +10 14 26 1.4320 1.1600 53.5 43.6 8.8
2013 10 26 10 45 33.6 +09 45 39 1.4025 1.1374 53.4 44.6 8.7
2013 10 27 10 49 36.8 +09 15 33 1.3731 1.1146 53.3 45.6 8.6
2013 10 28 10 53 49.1 +08 44 01 1.3440 1.0916 53.1 46.7 8.4
2013 10 29 10 58 11.3 +08 10 58 1.3151 1.0683 52.9 47.9 8.3
2013 10 30 11 02 44.0 +07 36 18 1.2864 1.0448 52.7 49.1 8.2
2013 10 31 11 07 27.9 +06 59 55 1.2581 1.0210 52.3 50.3 8.1