The announcement from NASA yesterday that two small asteroids would today miss our planet by less than the distance between the Earth and the Moon brings into focus the short notice that we are likely to have of any collision from smaller space rocks.

One asteroid, 2010 RX30, estimated by NASA to be between 10 and 20 metres across, passed within 248,000 kilometres of Earth at 10.51 am BST today. The second, 2010 RF12, estimated to be only 6 to 14 metres across, will pass within 79,000 kilometres at 10:12 pm tonight. Both were only discovered on Sunday, just three days before their time of closest approach. That is due to their small size; smaller objects are harder for telescopes to pick up.

It’s unknown exactly how much damage such objects would do if they did hit Earth. In 2008, Mark Boslough and David Crawford of Sandia National Laboratories published simulations showing that a 30 to 50 metre asteroid could cause substantial damage to Earth’s surface by depositing energy several kilometers up in the atmosphere, and causing a jet of expanding gas to plunge to the ground.

U.S. policy on hazardous asteroids is largely determined by the 2005 mandate to NASA, in the George E. Brown Jr. Near-Earth Object Survey Act, to detect and track 90% of Near-Earth Objects over 140 metres in size by 2020. As I report in a story this week on the NASA Ad-Hoc Task Force on Planetary Defense, NASA is unlikely to meet the goal, but is detecting many smaller objects because of it, some of which are likely to have initial probabilities of hitting Earth. During their deliberations, panel members noted that the agency ought to keep track of objects smaller than 140 metres when they are detected – although not if this comes at the expense of tracking the larger ones.

Image: Devastation at Tunguska in Russia in 1908 is now thought to have been caused by a 30-50 metre asteroid / Leonid Kulik Expedition