Early this morning, while most of the U.S. slept, a once-menacing asteroid drew close to Earth on its usual rounds through the inner solar system. The 300-meter asteroid, known as Apophis, kept a comfortable distance, flying well beyond the orbit of the moon. But Apophis has not drawn so near to Earth since 2004, when it was first discovered—and when it was briefly feared to be on a possible collision course with Earth. Further observations of the asteroid have cooled those worries, but Apophis will come much closer in 2029, passing within about 35,000 kilometers of Earth, and still holds a minuscule chance—about 1 in 230,000—of an impact when it comes back around in 2036.
Such is often the case with near-Earth asteroids (NEAs) and the troublemakers among them, potentially hazardous asteroids (PHAs). Uncertainties in a PHA’s orbit allow the possibility that the object will strike Earth until better observations reduce those uncertainties and all but eliminate the risk.
That is just what happened last month with a PHA known as 2011 AG5, which carried a small risk of impact in 2040 [right]. Discovered two years ago, the 140-meter rock had been one of only two known asteroids to score a 1 on the Torino Impact Hazard Scale, which ranks comet and asteroid hazards from 0 to 10. A 1 on the Torino scale means a collision is “extremely unlikely with no cause for public attention or public concern,” but 2011 AG5 was still tied for the most threatening known PHA. (Put another way, all the known asteroids are pretty tame—the only remaining PHA to rate above a zero on the Torino scale is 2007 VK184, which holds a 1-in-1,820 chance of impact in 2048.)
Now the 2011 AG5 threat has been retired, thanks to new data released in December. Astronomer David Tholen of the University of Hawaii and his colleagues observed the asteroid several times throughout the month of October and pinned down its orbit with sufficient precision to significantly limit the possible paths it might follow in the future. The new data [left] show that 2011 AG5 will cruise past Earth at a comfortable distance of roughly 900,000 kilometers. “The bottom line is that there is no impact risk for 2040,” Tholen says.
For researchers who study asteroid properties and orbits, close approaches of Earth are not cause for hand-wringing—rather these flybys are rare opportunities to get a good look at objects that usually hide from view. European researchers, for instance, used the Herschel space telescope to observe Apophis as it drew near over the weekend. Preliminary data from Herschel indicate that the asteroid may be somewhat larger, and significantly more massive, than had been assumed.
Next month astronomers will have a much closer target to examine. A near-Earth asteroid called 2012 DA14, just 45 meters or so in diameter, will approach Earth on February 15, passing inside the realm of some orbiting satellites. No reason to worry, though—the asteroid rates as a solid zero on the Torino scale, with a negligible impact risk until 2026 and only a 1-in-3,000 shot after that. Nevertheless, during its February flyby 2012 DA14 is expected to miss Earth by just 30,000 kilometers or so—the closest approach of a known asteroid in the foreseeable future.