From Science:
Astronomers say they can now compute with great confidence which asteroids pose a threat to our planet. The problem, they told the annual meeting of the American Association for the Advancement of Science (which publishes ScienceNOW), is that preventing a collision would require mounting a not-yet-planned space mission by a not-yet-identified governmental body.
The next known close encounter with an asteroid will occur, somewhat ominously, on Friday the 13th of April 2029. Then, Near-Earth Object 99942–also known as Apophis (Greek for “The Demon of Darkness”)–is expected to miss the planet by a mere 30,000 kilometers. The real sweating begins soon after, when astronomers must determine whether Earth’s gravity has steered Apophis onto a course for impact seven years later. Current calculations place the chance of that happening at about one in 30,000.
At 250 meters wide, Apophis is five times larger than the object that hit Earth 50,000 years ago and blew out the 1200-meter-wide Meteor Crater in Arizona. It’s also six times larger than the Tunguska object, which grazed Earth’s atmosphere before exploding over Siberia in 1908, flattening 2100 square kilometers of forest.
More here.