A celestial traveller closes on Mars

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-mile-per-hour 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.

Shock waves may rock the atmosphere. The dust particles may be tiny, but when travelling at 125,000 mph 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 programme 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.

Up-close perspective

But for the most part, the initial worries have given way to excitement about the scientific opportunities presented by the very close encounter. The satellites and rovers — along with ground and space observatories such as the Spitzer and Hubble Space Telescopes — will offer a front-row seat to the event, which may provide important images and science for days.

“This is an entirely unprecedented situation,” said James Green, director of Nasa’s planetary science division and of its Mars programme. ‘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.”

The implications for Mars science are substantial. The Curiosity rover has confirmed and substantially expanded earlier findings that Mars was warmer and much wetter a long time ago. But the question of how and when the planet lost those potentially life-supporting conditions remains largely unresolved.

Because all the cameras orbiting above Mars are designed to focus on the planet, they are not expected to produce the best images of the flyby. That role is likely to be played by the Hubble and by observatories on Earth. Some believe that Curiosity might be lucky and snap an image of the comet passing above.

One especially powerful orbiting camera, however, has a chance of capturing what is considered the most important and interesting part of the comet — its nucleus, the “dirty snowball.” Little changed for billions of years, the ball of dust and ice warms as it enters the inner solar system and emits a vast surrounding cloud of material called the coma, followed by the long tail.

The camera, named HiRise for High Resolution Imaging Science Experiment, produces finely detailed images that have revolutionized our understanding of the Martian surface; now its operators will try to do the same for the comet’s primordial nucleus.