In just 4 days four Mars probes will focus cameras on Comet ISON as it makes its closest approach to the planet on Oct. 1. NASA’s Mars Reconnaissance Orbiter will study and photograph the comet from orbit on three days: Sept. 29, Oct. 1 and Oct. 2. Europe’s Mars Express orbiter, which began its ISON observing campaign on Sept. 21, will study the coma, the tenuous atmosphere that surrounds the comet’s icy nucleus. The probe will examine and photograph ISON through about October 5. As soon as photos are released, you’ll see them here.
Both the Curiosity and Opportunity rovers will also be pressed into service to spy ISON from the ground during the same time. Seen from Mars, the comet should glow about magnitude 2.5-3, plenty bright to see with the naked eye if you could somehow wish yourself there.
Amateur and professional astronomers have been watching Comet ISON evolve since its discovery on Sept. 21, 2012. It began as a dim 15th magnitude patch of haze and brightened very slowly. Amateurs reported a nice uptick in activity beginning late this summer into early fall. ISON now shines at about magnitude 11.5-12 (still faint and requiring a telescope to see) and sports a short tail pointing to the northwest. I was able to see the tail and brighter coma in my 15-inch (37 cm) scope without difficulty about a week ago in a dark sky just before dawn.
Time exposure photos show a classic beauty of a comet with a bright, compact head and elegant tail. According to NASA’s Comet ISON Observing Campaign site, astronomers consider ISON “somewhat more active” that a typical comet. It’s kicking out a lot of dust right now as the sun’s heat vaporizes ice on the comet’s surface. That’s why we see a substantial tail in recent photos. Any guesses as to exactly how bright ISON will become when it zips nearest the sun on Thanksgiving Day are just that … educated guesses.
Movie made from images taken by NASA’s Deep Impact probe in mid-Jan. 2013
Astronomers use formulas based on comet size, distance and dust production rate to come up with brightness predictions. ISON could rival Venus for a short time on Nov. 28 or be substantially fainter or brighter. If it survives its searingly close passage of 745,000 miles (1.2 million km) from the sun’s surface, it will likely become a beautiful sight for northern hemisphere skywatchers during the first half of December. Southern hemisphere observers will have their best views before closest approach.
Much depends on the the comet’s size, currently estimated at 1.8 miles (3 km), pretty typical for a comet. The bigger the iceball, the better the chance of surviving the sun’s battering. Let’s hope ISON keeps it together.