Number of asteroids that pose risk to Earth is doubled

The Earth is at risk from more asteroids than previously thought, according to a new survey. New data from the asteroid-tracking NEOWISE mission reveals that twice as many asteroids as previously thought are on low-inclination orbits that could bring them into contact with our home planet.

"We were very surprised to find that," says Amy Mainzer of the Jet Propulsion Laboratory in Pasadena, California. "We were not expecting to find [that result] at all."

NEOWISE, which took data with NASA's infrared WISE space telescope from 2010 to 2011, has completed the most accurate census yet of potentially hazardous asteroids: those that come within 8 million kilometres of Earth, and are large enough to survive the trip through the atmosphere.

Because WISE searched in infrared wavelengths, it was equally as sensitive to large dark asteroids as to small bright ones. Previous surveys that looked only at visible light couldn't tell those two populations apart, Mainzer says.

Mainzer and colleagues sampled 107 potentially hazardous asteroids (a representative sample of the entire population) and extrapolated to predict the total number. They found that about 4700 of them ? give or take 1500 ? are 100 metres or larger in diameter. Only 20 to 30 per cent of these have actually been observed.

Twice as many of those as previously thought are on so-called low-inclination orbits, which means they are closely aligned with Earth's orbit and therefore have a higher chance of hitting us.

"It's easier for them to make close approaches to the Earth more often," Mainzer says. "It's a hazard and a risk."

Our best recourse is to make sure we know where they are, where they're going and what they're made of. "If you find things 20 to 30 years before a potential impact would take place, then you have time to deflect them," she says. "If we find them days before impact, then it's much worse."

But there's a silver lining: the new population of easy-access asteroids is also an opportunity for scientific exploration or the proposed asteroid mining industry. "If the asteroid is naturally carried near the Earth, it's easier to reach with a spacecraft, either robotic or with astronauts," Mainzer says.

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