On the lookout for asteroids

5 per cent, and for smaller rocks that could still inflict big damage.

Altogether, Nasa has catalogued 18,310 objects of all sizes. Slightly more than 800 are 140m or bigger.

There’s no quick solution if a space rock is suddenly days, weeks or even months from striking, according to Johnson. But such short notice would give the world time, at least, to evacuate the area it might hit, he said.

Ground telescopes are good at picking up asteroids zooming into the inner solar system and approaching from the night side of Earth. What’s difficult to detect are rocks that have already zipped past the Sun and are heading out of the solar system, approaching from the day side.

That’s apparently what happened in 2013 when an asteroid about 20m in size suddenly appeared and exploded over Chelyabinsk, Russia, damaging thousands of buildings and causing widespread injuries.

An asteroid double or even triple in size exploded over Tunguska, Russia in 1908, levelling 2000 square km of forest. According to the new report, casualties could be in the millions if a similar event struck New York City.

Johnson stressed that it would take years to attempt to turn away a potential killer asteroid – several years to build a spacecraft, then another few years to get it to the target. Ideally, he’d like at least 10 years’ advance notice.

A mission to defend Earth could involve hitting the asteroid or comet with robotic spacecraft to try to change its path; or in the worst-case scenario, launching a nuclear device – not to blow up the asteroid but rather to superheat its surface and blow off enough material to divert it.

The bottom line, officials said, was the US government wanted to be prepared to decide which action was best if needed.

Scientists hope to learn a lot more about asteroids from a pair of missions currently under way. Nasa’s Osiris-Rex spacecraft will reach the asteroid Bennu later this year and return samples in 2023, and Japan’s Hyabusa 2 is closing in on the asteroid Ryugu, with samples to be returned in 2020. –AP

AP A meteor streaks through the sky over Chelyabinsk, Russia in February 2013. The US National Science and Technology Council is calling for improved asteroid detection, tracking and deflection.