Visiting comets don’t come around very often, particular ones on their first trek through the inner solar system. The chance to study pristine materials believed to date back from the formation of the solar system is one reason why scientists and armchair astronomers alike are eagerly awaiting Comet ISON. The celestial guest, which likely originated in the Oort cloud located about 50,000 times farther away from the sun than Earth, is due to make a close flyby of the sun on Thanksgiving Day, Nov. 28.
Read on to learn about the top 5 things you need to know about this famous comet.
Observations of Comet ISON as imaged by the HiRISE camera aboard NASA's Mars Reconnaissance Orbiter.
So far, Comet ISON has been a bit of a puzzle. Discovered in September 2012 by two Russian astronomers, ISON (an acronym for the International Scientific Optical Network near Kislovodsk, Russia, which made the find) was extremely bright, given its distance at the time far beyond the orbit of Jupiter. That led some scientists to predict that Comet ISON might be so bright by the time it got to Earth that it could be visible even in daylight.
Over the past few months, however, Comet ISON proved once again just how variable comets can be. It hasn’t brightened as much as original predicted, perhaps because it has fewer volatiles to be vaporized by the sun, or perhaps because it is smaller than expected, or made of different materials.
This week, Comet ISON passed just 6.5 million miles from Mars. It was too dim to be seen by NASA’s Mars rover Curiosity, but the agency’s sharp-eyed Mars Reconnaissance Orbiter managed to take a shot, MRO project scientist Richard Zurek told Discovery News.
Comet Lovejoy (C/2011 W3) was a sungrazing comet that dove deep into the solar corona... and survived the searing heat.
The next eight weeks are a critical time for the comet. On Nov. 28 -- Thanksgiving Day, as it turns out -- Comet ISON will pass about 680,000 miles above the surface of the sun.
The encounter may prove deadly. Solar radiation will boil off the comet’s water and other volatiles. Gravitational stresses will be extreme. Many comets with flight paths that bring them close to the sun do not survive. Comet Lovejoy (pictured here), which made its closest approach to the sun on Dec. 15, 2011, was a rare exception.
If Comet ISON makes it through, its present course will bring it within about 40 million miles of Earth on Dec. 26.
Infrared observations by NASA's Spitzer Space Telescope captured the glow of Comet ISON's nucleus and coma in June 2013.
Even if Comet ISON survives its brush with the sun, its voyage in the solar system may be over. "ISON’s orbit suggests that it may gain enough momentum to escape the solar system entirely, and never return," NASA noted in a status report.
Scientists estimate Comet ISON began its journey from the Oort cloud, a collection of icy objects orbiting far beyond Neptune, about 10,000 years ago.
Hubble observations of Comet ISON's coma and developing tail.
Back in May, scientists were surprised when Hubble Space Telescope observations showed the nucleus of Comet ISON appeared to be no more than three to four miles in diameter.
"Since the comet was so bright and so active, scientists had assumed the nucleus was larger. Hubble found the dusty coma, or head of the comet, to be around 3,100 miles across and the tail to be more than 57,000 miles long," NASA wrote on a timeline of the comet’s journey.
Additional Hubble observations are expected this month to check on the size of the comet’s nucleus again. By Oct. 10, Comet ISON should be close enough to the sun that a wide-angle instrument on NASA’s Solar Terrestrial Relations Observatories, STEREO-A, should be able to get its first glimpse of the comet as well. At that point, Comet ISON will be about 94.5 million miles from the sun. In November, the comet should be within range of NASA’s Mercury-orbiting MESSENGER spacecraft.
The location of Comet ISON in the night sky through to the end of October 2013.
Though early predictions of a comet brighter than the full moon seem unlikely, Comet ISON should be quite prominent in the night-time sky -- if it survives its close approach to the sun.
In early December, the comet is expected to be visible in the Northern Hemisphere in the morning, low on the horizon to the east-southeast -- no telescope or binoculars needed.
A month later, the comet will be visible all night long.