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 Mr. 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.