Spacecrafts from many countries will get a rare opportunity to study a comet as it hurtles past Mars on Sunday, October 19. For any comet to come so close to a planet is unusual, but the comet's history should make this encounter particularly interesting. Amateur astronomers may also be able to join in as the comet is visible with moderate-sized telescopes.
Comet Siding Spring was reported by Rob McNaught last year. Having found 82 comets, more than any other person in history, and having the brightest comet of recent years named after him, McNaught credited this discovery to the observatory where the observations were made.
When McNaught discovered Comet Siding Spring, or C/2013 A1 as it was originally known, its orbit was calculated as taking it so close to Mars that it was considered possible it might collide. It took more than a year before this was ruled out, with an estimated clearance of 139,500 kilometers (87,000 miles)—less than half the distance between the Earth and our moon.
The paths of the missions orbiting Mars have been adjusted, where necessary, to ensure they are far enough away to be safe, not only from the comet itself but from the trail of debris it will leave behind. Meanwhile, NASA’s Mars orbiters will keep a close eye on the comet and the influence its tail has on the Martian atmosphere.
Comets are divided into two categories, short and long-period comets. Long-period comets have spent most of their existence in the Oort Cloud, at distances where the sun appears as little more than a bright star. Consequently, they are time capsules of the early solar system. Short-period comets, on the other hand, have become trapped in the inner or middle solar system and pass close enough to the sun to lose their outer layers regularly, gradually shrinking away to nothing but grains of dust.
Comet Siding Spring doesn't appear to be a particularly large comet, but as an intruder from the Oort Cloud on its first pass close to the sun, it provides a contrast to the short-period comets to which we have sent spacecraft.
“This is a cosmic science gift that could potentially keep on giving, and the agency’s diverse science missions will be in full receive mode,” says NASA's John Grunsfeld. “This particular comet has never before entered the inner solar system, so it will provide a fresh source of clues to our solar system's earliest days.”
This will be the second time we have had a good view of the interaction of a comet and a planet after Shoemaker-Levy 9's spectacular collision with Jupiter in 1994.
The comet may be among McNaught's last, with the Australian government refusing to match NASA funding to keep his program going.