Asteroid Eros, seen here by NASA's NEAR spacecraft, is 33 kilometres wide, making it the second largest near-Earth asteroid
Existing sky surveys miss many asteroids smaller than 1 kilometre across, leaving the door open to damaging impacts on Earth with little or no warning, a panel of scientists reports. Doing better will require devoting more powerful telescopes to asteroid hunting, but no one has committed the funds needed to do so, it says.

Near-Earth asteroids larger than 1 kilometre across could blast huge amounts of sunlight-blocking dust into Earth's atmosphere in an impact, causing devastating climate change. The US Congress asked NASA in 1998 to find 90 per cent of those in this size range within 10 years, a goal that has now nearly been reached.

Astronomers have now found 784 of them, mostly using telescopes funded by NASA. That works out to 83 per cent of the 940 estimated to be out there by astronomer Alan Harris of the Space Science Institute in Boulder, Colorado.

But asteroids below 1 kilometre in size can cause serious harm, too, and they hit Earth more frequently because they are more numerous. To address the small-asteroid threat, Congress told NASA in 2005 to find 90 per cent of the near-Earth asteroids larger than 140 metres across by 2020.

NASA asked the US National Research Council in 2008 to figure out the best way to survey small asteroids and meet the 2020 goal. Now, the NRC panel has issued an interim report, saying that without new money for more powerful surveys, NASA will not be able to meet the goal.

Surprise hit

"To achieve this goal, or to even come close to achieving it, new facilities capable of detecting fainter asteroids and having wider fields of view to cover larger portions of the sky each night are required," the report says.

Panel leader Irwin Shapiro of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, says there is wide agreement on this point. "Pretty well everyone agrees now that [just] continuing with what we have, there's no way we could reach the 2020 goal," he told New Scientist.

The report also points out that existing surveys are designed to gradually build up a catalogue of near-Earth objects over time, not to watch out for incoming asteroids that are just days or weeks from colliding with our planet.

Small asteroids could easily slip past existing surveys unnoticed until the moment of collision because telescopes currently devoted to the task are only capable of imaging a small part of the sky each night. And even then, clouds can prevent them from spotting asteroids, says Timothy Spahr of the Minor Planet Center in Cambridge, Massachusetts, a central clearinghouse for asteroid and comet data.

'No free lunch'

Asteroids approaching from the direction of the sun would also be missed, at least by ground-based telescopes, says Alan Harris. Sending a telescope to another vantage point in space could overcome this problem. "[But] it could be seriously expensive," says Harris. "So one must ask, 'What's it worth?'"

"There is no free lunch," Shapiro agrees. But he adds, "We're talking about investing in an insurance policy."

A comet or asteroid as small as 30 metres across is thought to have exploded in the atmosphereMovie Camera over Siberia in 1908, unleashing hundreds of times the energy of the nuclear bomb dropped on Hiroshima, Japan, in 1945, and flattening trees in a zone dozens of kilometres across.

A small asteroid impact in the ocean could also flood coastal cities by triggering huge waves, though scientists are still debating how far such waves could travel before petering out.