NASA should extend its search for near-Earth objects to smaller asteroids, a team of experts has warned. The panel compared the added cost of tracking smaller asteroids with the benefits of knowing what risk they pose, and concludes that it 鈥渏ustifies substantial investment鈥�.
Since 1998, NASA has been tracking asteroids larger than a kilometre across that come within 45 million kilometres of Earth鈥檚 orbit. These big rocks are the easiest to find, and the programme is on track to meet its 2008 goal of finding 90 per cent of the 1000 or so such objects thought to exist. Yet while such an asteroid could cause global devastation, an impact is extremely unlikely.
In contrast, half a million or so objects larger than 50 to 100 metres across come within seven million kilometres of the planet. A strike is likely roughly once every millennium, causing regional devastation. The panel compared the risk from different categories of asteroid by dividing the likely death toll by the typical number of years between impacts. Their analysis confirms warnings that smaller impacts are the most dangerous, posing 70 per cent of the total asteroid threat 鈭�53 per cent due to tsunamis caused by ocean strikes, and 17 per cent due to land impacts.
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The search should now focus on objects over 140 metres across, which account for 90 per cent of the potential damage from small objects, says the panel. It suggests a small telescope orbiting the sun at about the distance of Venus would have the best view and cover more of the sky than ground-based telescopes. It could do the job in seven years, for around $400 million 鈥� many times the few million dollars a year NASA spends hunting big asteroids.
Looking for fainter objects makes sense, says Brian Marsden of the Harvard-Smithsonian Center for Astrophysics. But it could lead to even more false alarms. 鈥淭he fainter the objects are, the more difficult it is to get adequate follow-up observations鈥� to calculate an accurate orbit, he warns.