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 on January 9, 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 our planet. 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 it still holds a vanishingly small chance of an impact when it comes back around in 2036.
Such is often the case with near-Earth asteroids and the troublemakers among them, potentially hazardous asteroids. Uncertainties in an asteroid's orbit allow the possibility that the object will strike Earth until better observations reduce ambiguity and all but eliminate the risk.
That is just what happened with 2011 AG5, which had carried a small risk of impact in 2040. Discovered two years ago, the 140-meter rock had been one of only two known asteroids to score above 0 on the Torino Impact Hazard Scale, which ranks comet and asteroid hazards from 0 to 10. (Apophis scores 0.) Asteroid 2011 AG5 was only 1, meaning a collision is “extremely unlikely with no cause for public attention or public concern,” but it was still tied for the most threatening known asteroid, along with 2007 VK184, which holds a one-in-1,820 chance of impact in 2048.
Now the 2011 AG5 threat has been retired, thanks to new data released last December. Throughout the month of October, astronomer David Tholen of the University of Hawaii and his colleagues had observed the asteroid several times and pinned down its orbit with sufficient precision to significantly limit the possible paths it might follow in the future. The recent data show that 2011 AG5 will cruise past Earth at a comfortable distance of roughly 900,000 kilometers. Says Tholen, “The bottom line is that there is no impact risk for 2040.”