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.

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 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.

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.

‘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.”

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 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,” Dr. 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. Siding Spring may well produce atmospheric dynamics that the Maven team expected to study in a far more static state.

“If particles from the comet hit the atmosphere, we’ll absolutely be able to measure what happens,” said Bruce Jakosky, principal investigator for the satellite mission. Initially worried that the comet could damage Maven just as its mission began, he now sees the flyby as exploration science at its best.

“We’ll follow how different chemical processes play out and will be looking to see if the arrival of those fast-moving dust particles, with all their energy, heats up the atmosphere,” he said. “We know there were lots of comet and asteroid impacts and near misses on early Mars, and now we’re in a position to learn about some possible consequences.”

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.

Dr. McEwen said the team also planned to photograph jets of water vapor and dust that often shoot out of the nucleus.

Most comets that form in the Oort cloud stay in place, orbiting the sun once every million to 30 million years. Sometimes, however, gravitational forces from nearby stars, or giant planets that many scientists believe wander in space, push a comet out of orbit and send it toward the sun. For Comet Siding Spring, that voyage has taken a million years.

Astronomers will also be using Earth and space observatories to identify the comet’s chemical makeup. Of particular interest is what carbon-based organic compounds might be detected. These compounds, the building blocks of life, are known to reside in comets and asteroids. NASA’s Stardust mission to the comet Wild 2 collected samples in 2006. In labs, scientists found not only organics in the stardust, but small yet detectable amounts of evolved amino acids.

“We don’t know how life begins, but we do know that organics are necessary,” Dr. Glavin said. “And how do organics appear? Maybe they’re formed on the surface of a planet like Earth, or maybe they get delivered by comets like Siding Spring.”

One of the main goals of the Curiosity mission is to search for organic compounds that might help show whether Mars was once habitable.

Siding Spring is not expected to get close enough to send organic compounds to the surface, but Dr. Green, NASA’s Mars program director, does not want to take any chances. Although Curiosity will be in a defensive position for the flyby, he has plans for the small scoop that the rover uses to deliver crushed rock samples to the instruments inside.

“What I told the Curiosity team is that the chances are very slight that organics or comet dust would fall on the rover,” he said. “But we should have the scoop out to catch some just in case.”