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WE ARE not alone. The most accurate survey yet of asteroids that are closely aligned with Earth鈥檚 orbit and which could come into contact with our planet reveals that there are twice as many as previously thought.
One reason that the new figure is so high, says Amy Mainzer of the Jet Propulsion Laboratory in Pasadena, California, is that the data for this NEOWISE survey came from NASA鈥檚 Wide-field Infrared Survey Explorer space telescope. The telescope searches in infrared, so is equally sensitive to large dim asteroids and small bright ones. Previous surveys had trouble judging the size, and therefore threat level, of dimmer asteroids.
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Mainzer and colleagues looked at a representative sample of 107 potentially hazardous asteroids and extrapolated to predict the total number. They found that about 4700 of them are 100 metres or more in diameter. Only 20 to 30 per cent of these have actually been observed.
Of the 4700, twice as many as previously thought are on the 鈥渓ow-inclination orbits鈥� that could lead to a collision with Earth.
But there鈥檚 a silver lining: the easy-access asteroids could be a boon for scientific exploration or space mining.