Author: Dr. Tony Phillips
Production editor: Dr. Tony Phillips
August 23, 2013: Around the world, astronomers are buzzing with anticipation over the approach of Comet ISON. On Thanksgiving Day 2013, the icy visitor from the outer solar system will skim the sun's outer atmosphere and, if it survives, could emerge as one of the brightest comets in years.
First, though, it has to fly by Mars.
"Comet ISON is paying a visit to the Red Planet," says astronomer Carey Lisse of the Johns Hopkins University Applied Physics Lab. "On Oct 1st, the comet will pass within 0.07 AU from Mars, about six times closer than it will ever come to Earth."
Mars rovers and satellites will get a close-up view. It's too early to say whether Curiosity will be able to see the comet from the surface of Mars-that depends on how much ISON brightens between now and then. Lisse says the best bet is NASA's Mars Reconnaissance Orbiter. The MRO satellite is equipped with a powerful half-meter telescope named HiRISE that is more than capable of detecting the comet's atmosphere and tail. Observations are planned on four dates: August 20th, Sept 29th, and Oct 1st and 2nd.
HiRISE wasn't sent to Mars to do astronomy, notes the telescope's principal investigator Alfred McEwen of the University of Arizona. "The camera is designed for rapid imaging of Mars. Our maximum exposure time is limited compared to detectors on other space telescopes. This is a major limitation for imaging comets. Nevertheless, I think we will detect Comet ISON."
The Mars flyby comes at a key time in Comet ISON's journey. It will have just crossed the "frost line," a place just outside the orbit of Mars where solar heating is enough to start vaporizing frozen water.
"The volatiles in a comet are 80% to 90% water ice," notes Lisse. "Right now in August almost all the water is still frozen, and the outgassing we see in ISON is driven by carbon dioxide and other lesser constituents. Probably only isolated patches of the comet's nucleus are active."
But when ISON crosses the frost line, "the whole comet could erupt in geysers of gas," says Lisse. "Mars orbiters will have a ringside seat."
The amount of outgassing at Mars will give researchers clues to the size of ISON's nucleus, which is hidden from view deep within the comet's dusty atmosphere.
"If ISON's nucleus is much bigger than 0.5 km, it will probably survive its Thanksgiving Day brush with the sun," says Lisse. "It could turn into one of the most spectacular comets in many years."
McEwen sees this as a tune-up for another comet encounter next year. "The science value of observing Comet ISON is hard to predict. We've never tried such a thing before. However, this is good practice for Comet Siding Spring, which will pass much closer to Mars in 2014."
For now all eyes are on Comet ISON. An unprecedented number of NASA spacecraft - 16 - will be observing the comet. Astronauts on board the International Space Station will be watching, too.
Meanwhile back on Earth, Lisse is working with NASA to organize a worldwide observing campaign for Comet ISON. "Our goal is to have every telescope on Earth pointed at the comet when it emerges from the sun," says Lisse. "The Mars flyby will give us a sneak preview, providing data we need to predict what we might see."
Last Updated: 23 September 2013