Here are the back-of-the-envelope numbers for July 21, 2013.
Today, Comet ISON is approximately 2.823 AU from the sun. There are 130 days until Comet ISON reaches perihelion.
Over the past 5 days, between July 16th and July 21th, Comet ISON traveled a distance of approximately 0.071 AU.
That’s an average speed of roughly 1.32 million miles per day, or 2.12 million kilometers per day.
On July 7th, the time projected (tp) value calculated by NASA for perihelion (closest to the sun) was 2013-Nov-28.77833817.
That’s the same as November 28, 2013 at 18:40:48 UTC.
The TP value was updated yesterday on July 20th at 08:13:10 UTC.
The current time projected by NASA for perihelion is 2013-Nov-28.77831993
That’s the same as November 28, 2013 at 18:40:46 UTC. That’s a small difference of 2 seconds from the prior projection.
Here are some news snippets from the past week.
“About 10,000 years ago, Comet ISON left our solar system’s distant shell, a region known as the Oort cloud, and began streaking toward the sun. This November, the icy wanderer will reach the climax of its journey, potentially providing a stunning skywatching show here on Earth.”
— Discovery News 07/18/2013
Comet ISON began its trip from the Oort cloud region of our solar system and is now traveling toward the sun. The comet will reach its closest approach to the sun on Thanksgiving Day — Nov. 28, 2013 — skimming just 730,000 miles above the sun’s surface. If it comes around the sun without breaking up, the comet will be visible in the Northern Hemisphere with the naked eye, and from what we see now, ISON is predicted to be a particularly bright and beautiful comet.
— NASA 07/16/2013