Mars-based satellites and rovers will offer a front-row seat to viewing Comet Siding Spring.

One day early last year, the Australian comet hunter Robert H. McNaught spotted something unusual from his post at the Siding Spring Observatory in the foothills of the Warrumbungle Mountains.

As a member of a team sponsored by NASA that searches the skies for potentially dangerous asteroids and comets, he generally focuses on objects that orbit the sun on the same plane as the planets. But coming up from below that plane was a comet that had apparently originated in the Oort cloud, a vast, primordial region that surrounds the solar system.

The comet was well beyond Jupiter when McNaught sighted it, but he and other so-called comet modelers were nonetheless able to predict its 125,000-mph path into the inner solar system. To their surprise and consternation, it appeared to be heading straight for Mars, and some of their most precious equipment.

Comet trajectories are notoriously changeable, and more recent projections suggest the comet, named Siding Spring, is highly unlikely to strike the planet or to do much damage to the two NASA rovers on its surface or the five research satellites orbiting it.

Still, on Oct. 19, the comet is expected to pass within 82,000 miles of Mars, a stone’s throw in astronomical terms — one-third the distance between Earth and the moon, and much closer to Mars than any comet has come to Earth in recorded history.

The dust, water vapor and other gases spewed by a comet can spread for tens of thousands of miles, so the upper reaches of the Martian atmosphere are expected to be showered by Siding Spring — perhaps briefly, perhaps more extensively.

The dust particles may be tiny, but when traveling at 125,000 mph (35 miles per second) they would pierce the skin of any satellite orbiting the planet. “Essentially, they would be like bullets out there,” said Richard Zurek, the chief scientist of the Mars program at NASA’s Jet Propulsion Laboratory.

He added that although the danger to satellites and rovers appeared to be limited, there was a small possibility that the comet could break up as it approaches Mars — a fate similar to that of Comet ISON as it neared the sun last year. As a precaution, the five satellites’ orbits have been tweaked so they will be on the far side of the planet when the greatest threat from dust arrives.

“We have an opportunity to see what happens when a comet comes so close to a planet,” he continued. “We can follow the planet as it responds to the dust and water and shock, and hope to learn more about how it processes it all. Comets have played a huge role in transforming planets, and now we’ll see the process as it’s happening.”

As a result, its icy nucleus (the “dirty snowball” at the core of a comet) has never been thawed and reshaped, like those of comets that pass by more regularly.

“We’ve studied the nuclei of comets before but never a long-period comet from the Oort cloud,” Zurek said. “The comet may well be bringing us primordial material unchanged since the creation of the solar system.”

As luck would have it, Siding Spring will pass Mars just a month after the arrival of NASA’s newest orbiter, Maven, short for Mars Atmosphere and Volatile Evolution. That satellite has instruments designed to study the Martian atmosphere, and in particular how water vapor and other material escape into space.

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