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Sky show. In October, Comet Siding Spring will pass a mere 131,000 kilometers from Mars, giving scientists unprecedented opportunities to observe the icy visitor and its possible effects on the Red Planet.

Sky show. In October, Comet Siding Spring will pass a mere 131,000 kilometers from Mars, giving scientists unprecedented opportunities to observe the icy visitor and its possible effects on the Red Planet.

© 2014 Jerusha Lederman, for York University/CRESS

Comet fireworks on Mars?

In mid-October, a comet sweeping through our inner solar system for the first time will pass near Mars—so close, in fact, that if it were buzzing Earth at the same distance it would fly by well inside our moon’s orbit. And while material spewing from the icy visitor probably won’t trigger the colossal meteor showers on the Red Planet that some scientists predicted, dust and water vapor may still slam into Mars, briefly heating up its atmosphere and threatening orbiting spacecraft. However it affects the planet, the comet should give scientists their closest view yet of a near-pristine visitor from the outer edges of our solar system.

Astronomers first spotted comet C/2013 A1—dubbed Comet Siding Spring, after the Australian observatory where it was discovered—early in 2013. Researchers quickly realized the object would pass near Mars. At first, when observations of the comet were sparse and its orbit wasn’t well defined, they suggested that the cosmic iceball even had a small chance of striking Mars. Now, researchers estimate the comet will pass about 131,000 kilometers from the Red Planet on 19 October, says John Moores, a planetary scientist at York University in Toronto, Canada. (In comparison, the average distance between Earth and moon is a little more than 384,000 km.)

Initially, scientists thought this comet’s close pass might be a little too close, and that the comet’s coma—the hazy cloud of dust and water vapor spewed from the iceball’s surface as it warmed—would slam into Mars with spectacular effect. In March, one team predicted a “meteor hurricane” on Mars, with billions of bits of dust streaking through the Red Planet’s atmosphere each hour for about 5 hours. “Now, we realize the comet is much smaller than expected,” says Jeremie Vaubaillon, an astronomer at the Institute of Celestial Mechanics and Calculation of Ephemerides in Paris, who led that team. Although early data hinted that Comet Siding Spring might be as much as 50 kilometers in diameter, he notes, estimates now range between 500 meters and 2 km. As a result, Vaubaillon says, C/2013 A1 “is not likely to be the comet of the century.”

“Comets can be unpredictable,” says Mark Lemmon, a planetary scientist at Texas A&M University, College Station, who wasn’t involved with either team’s research. “They can range from really spectacular to kind of a dud.” Comet Siding Spring “is running a little brighter” than comets normally do, Lemmon adds, but that’s a far cry from the supercomet that some astronomers had hoped for.

The comet’s dust tail might or might not wash across Mars, but some of the coma’s water vapor is sure to strike the planet, says Roger Yelle, a planetary scientist at the University of Arizona in Tucson. Considering the relative velocities of Mars and the comet, that material will slam into the Red Planet’s atmosphere at more than 57 km/sec—a process that will heat the air and cause it to expand, fluffing upward to increase atmospheric drag on craft orbiting the planet (thereby slowing down the orbiters slightly but not substantially threatening them). Such physical changes to the atmosphere might last only hours or days, he notes, but any subtle chemical changes—including those resulting from the extra hydrogen added to the air when ultraviolet light breaks down the water vapor—would persist much longer.

Any dust from Comet Siding Spring that does strike the planet would be a small addition to the overall amount of dust in Mars’s atmosphere, but in some regions—especially outside the martian tropics—it could have noticeable effects, Moores and his colleagues report in Geophysical Research Letters. The most visible effects might include seeding clouds at very high altitudes, a process that could boost the frequency and thickness of such clouds for some as-yet-unknown interval, Moores says.

Studying the extent and duration of any atmospheric changes caused by the comet will help scientists better understand how the martian atmosphere works, Yelle says.

And although Mars’s atmosphere will protect landers and rovers from speeding dust particles, probes orbiting the Red Planet might be at risk. The greatest danger will occur when Mars passes through the debris trail following the comet, says Richard Zurek, chief scientist in the Mars Program Office at Jet Propulsion Laboratory in Pasadena, California. The window of danger will start about 90 minutes after the comet’s closest approach to Mars and will last between 20 and 30 minutes. Long before that time, Zurek says, NASA and other space agencies can adjust their satellites’ orbits such that the craft swing behind Mars for protection during the brief interval of highest threat.  Planning for such adjustments is already under way, he notes.

When the orbiters aren’t safely tucked away behind the planet, Zurek says, orbiters will have an unprecedented look at the comet. For example, the HiRISE camera on board the Mars Reconnaissance Orbiter should be able to see features on the comet about 140 meters across, he notes: “This will be the first time we’ve ever imaged the nucleus of a long-period comet with that resolution.” Even down on the Red Planet’s surface, the Curiosity rover might be able to get in on the act: Because Mars’s atmosphere has no ozone to block ultraviolet light, sensors on the rover will be able to detect those wavelengths and thereby monitor certain trace gases spewing from the comet—unless a dust storm blocks the view to space, Lemmon says.

Besides the craft now orbiting Mars, in October there will be two more: a probe launched by India early last November, and NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) mission, launched 2 weeks later. Yelle, who works with the MAVEN team, is excited. Because there typically isn’t enough time to design and launch a scientific mission to intercept a first-pass-through-the-solar-system comet before it swoops back into the depths of space, “[i]t’s almost impossible to send a spacecraft to one of these comets,” he notes. “In this case, we have a comet coming to us.” He adds: “This is a remarkable event, and we’re going to make the most of it.”