The global threat level from asteroid impact can be lowered from Lime Green to Forest Green. A potential killer rock, lost for 66 years, has finally been sighted again, and astronomers are relieved to announce that the kilometer-wide asteroid known as Hermes will not get uncomfortably close to Earth within the next century.
"Hermes is a dangerous object," says Timothy Spahr of the International Astronomical Union's Minor Planet Center (MPC) in Cambridge, Massachusetts. The asteroid, named after the swift messenger god of the ancient Greeks, was discovered by German astronomer Karl Reinmuth in 1937, when it zipped past Earth at about 800,000 kilometers, just twice the distance to the moon. Unfortunately, it was lost from sight before astronomers could precisely determine its orbit. "Every once in a while someone said: 'Hey, where's Hermes?' It was kind of nerve-racking," says Spahr. "We're happy to have it back."
On 15 October Hermes turned up as a faint speck of light in photos made by the Lowell Observatory Near-Earth Object Search program in Flagstaff, Arizona. Brian Skiff, who noted the object, immediately sent the images to the MPC, where Spahr used old-fashioned astronomical detective work to track down a couple of other recent observations of the asteroid and concluded it almost certainly had to be Hermes. Final confirmation came when the new orbit was firmly linked to the scant observations of 1937.
The sighting comes as no surprise to Lutz Schmadel and Joachim Schubart of the Astronomisches Rechen-Institut in Heidelberg, Germany. Two years ago they predicted that Hermes would approach Earth in October 2003, based on some 1937 photographic plates that had never been analyzed before. "Unfortunately, the American astronomers apparently were not aware of our prediction," says Schmadel. "So the recovery is truly accidental."
This time around, Hermes will make its closest approach to Earth on 4 November, at a safe distance of 7 million kilometers. In the next hundred years or so, it won't get any closer than about 3 million kilometers, but since Hermes is in a very eccentric, chaotic orbit, astronomers say it's hard to predict what will happen in the distant future.
But thanks to the new observations, at least Hermes won't get lost again. And it will now finally receive a proper number, like more than 60,000 other asteroids for which a precise orbit has been determined. According to Spahr, it's quite likely that it will get a nice, round number, to set it apart from the rest. Says Schmadel: "This exceptional asteroid deserves an exceptional number, like 66666 or 70000."