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.
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