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Far beyond the orbit of Neptune, at the gravitational fringes of our solar system, sits a vast field of icy debris known as the Oort cloud.

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Extending nearly one light-year from the Sun, the Oort cloud is thought to be the reservoir of comets

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like C/2013 A1, better known as Comet Siding Spring.

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This chunk of frozen water and gas, with the estimated mass of a small mountain,

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left the Oort cloud's southern hemisphere millions of years ago.

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It was discovered by astronomers at Australia's Siding Spring Observatory in 2013,

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and is now approaching the plane of the solar system, set to make a remarkably close encounter with Mars.

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On October 19, Comet Siding Spring will come within 88,000 miles of the Red Planet, roughly one third of the distance from the Earth to the Moon.

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Anyone standing on Mars might be treated to a spectacular view, with the comet's long tail of gas and dust dominating the northern skies.

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At Gale Crater, NASA's Curiosity rover will turn its gaze skyward for several days surrounding the flyby.

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Curiosity's Mastcam will study the comet's general features, while the ChemCam instrument will look at its chemical makeup.

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Although Curiosity and its fellow rover Opportunity will be protected from comet dust by the Martian atmosphere, the same cannot be said for spacecraft orbiting above Mars.

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Scientists believe that the odds of hitting dust are low, but with particles moving at 33 miles per second,

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NASA isn't taking chances, and will position its orbiters behind Mars during the period of greatest danger.

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For orbiters like MAVEN, Comet Siding Spring is not just a risk, but also an opportunity to study a first-time visitor from the Oort cloud.

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MAVEN will join NASA's Mars Reconnaissance Orbiter and Mars Odyssey in observing the comet,

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looking at its composition and the effects of its coma on the Mars upper atmosphere.

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One planet over, the Hubble Space Telescope and other space and ground-based observatories will also be keeping an eye on Comet Siding Spring,

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as will legions of amateur astronomers armed with backyard telescopes,

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all excited to witness this historic close encounter between Mars and its icy visitor.

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