A comet on a once-in-a-million-years journey will graze the atmosphere of Mars, astronomically speaking, this Sunday. This historic game of cosmic chicken on October 19 will be documented by more than a dozen spacecraft, includingin the neighborhood that must then "duck and cover" behind the Red Planet to avoid damage from the comet's coma of debris.
Comet Siding Spring was named for the Australian observatory that first detected it in early 2013 and is currently expected to pass within 87,000 miles (139,500 kilometers) of Mars on Sunday -- that's about one-third the distance between Earth and our moon. No comet has ever come anywhere near that close to Earth in recorded history, which is a very, very short span of time on a geological scale, but nonetheless it's fortunate that we've got a number of voyeuristic robots on and above Mars today.
"Think about a comet that started its travel probably at the dawn of man and it's just coming in close now," said Carey Lisse, a senior astrophysicist at the Johns Hopkins University Applied Physics Laboratory, in a NASA news briefing about Comet Siding Spring last week. "And the reason we can actually observe it is because we have built satellites and rovers. We've now got outposts around Mars."
In all, a dozen spacecraft and theand Opportunity rovers on Mars will have a chance to observe and document Siding Spring's visit. NASA's Mars Odyssey orbiter, Mars Reconnaissance Orbiter (MRO) and Mars Atmosphere and Volatile EvolutioN (MAVEN) are taking measures to hide behind the far side of the Red Planet as Siding Spring passes, enveloping Mars in a cloud of potentially dangerous dust and particles traveling at high enough speeds to act like a bullet should they impact part of an orbiter.
The European Space Agency's Mars Express orbiter will also take precautions to shield itself from debris, along with India's, which just arrived in Martian orbit in September.
"It only takes a half a millimeter-sized particle traveling at 56 kilometers per second to injure one of these spacecraft," Don Yeomans from NASA's Near-Earth Object Program says in the video embedded below.
Because Siding Spring is on its first journey around our sun from the distant Oort cloud at the farthest reaches of our solar system, it's considered to be unpredictable compared with other comets that visit regularly on the celestial equivalent of a time share arrangement. Don't worry, though; it has become clear in recent months as it has approached that there is almost no chance it will collide with Mars. I guess the Red Planet wins this game of chicken.
We've had plenty of opportunities to get even closer and observe other comets up close, but we've never had an encounter this close to a new comet fresh in from its first trip from the Oort cloud, which is basically a spherical layer of icy, rocky particles left over from the formation of our solar system. Because Siding Spring is a newcomer, it likely hasn't had many of its icy layers burned off on previous trips, so it also acts a bit like a time machine or a stack of decades-old comics in your basement, providing a glimpse into an earlier period of galactic history.
The nucleus of Siding Spring should be closest to Mars at 11:28 a.m. PT on Sunday. It might be possible to view it with binoculars or telescopes from the Southern Hemisphere, particularly South Africa or Australia. Likely the best bet to check out this historic event will be the Slooh network of telescopes, which will be broadcasting live on Sunday starting at 11:15 a.m.
Meanwhile, check out NASA's animated simulation of Siding Spring's fly-by below.