Newspaper article International New York Times

A Celestial Traveler Closes on Mars ; Comet Will Skirt Planet, Offering a First-Time View of Such an Encounter

Newspaper article International New York Times

A Celestial Traveler Closes on Mars ; Comet Will Skirt Planet, Offering a First-Time View of Such an Encounter

Article excerpt

On its first visit to the inner solar system, Comet Siding Spring will pass within 82,000 miles of the planet, offering a first-time view of such an encounter.

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

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 Mr. McNaught sighted it, but he and other so-called comet modelers were nonetheless able to predict its 125,000-mile-an-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 traveling at 125,000 m.p.h. (35 miles a 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.

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 program.

'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."

A 'Dirty Snowball' Comet Siding Spring is especially interesting because of its formation in the Oort cloud during the early days of the solar system, making it a "long period" comet with an orbit of millions of years. What's more, it is believed to be what comet specialists call a virgin -- one that has never reached the inner solar system.

As a result, its icy nucleus -- the "dirty snowball" at its core - - 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," Mr. …

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