Mountain Sized Comet to Pass By Mars

A comet on a once-in-a-million-years journey will graze the atmosphere of Mars, astronomically speaking, Sunday, October 19th, according to NASA. This historic game of cosmic chicken will be documented by more than a dozen spacecraft, including Mars orbiters in the neighborhood that must then "duck and cover" behind the Red Planet to avoid damage from the comet's 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.

"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 the Curiosity and 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.