Nasa has revealed its plan to protect Earth from asteroids that could wipe out entire continents. The world needs better ways of spotting asteroids and then swatting them out of our way, according to a major new report issued by many of the most important bodies in the US. It would take years to build the space craft experts hope would be able to move an asteroid out of our way, but knowing when one is coming would at least give people time to evacuate the area, the report warns.

There is no immediate threat from a killer rock. But some scientists fear that we might not know about one until a very late stage — and so the White House has requested better plans to ensure we are safe from them. The report was issued by the National Science and Technology Council, which includes participation from Nasa along with federal emergency, military, White House and other officials.

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Nasa’s planetary defence officer Lindley Johnson said scientists have found 95 per cent of near-Earth objects measuring a kilometre (0.62 miles) or bigger but the hunt is on for the remaining five per cent and smaller rocks that could still inflict big damage.

Nasa has catalogued 18,310 objects of all sizes —just over 800 are 140 metres or bigger. There is 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 to evacuate the area it might hit.

Ground telescopes are good at picking up asteroids zooming into the inner solar system and approaching from the night side of Earth, Johnson said. What is 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 is apparently what happened in 2013 when an asteroid about 20 metres in size suddenly appeared and exploded over Chelyabinsk, Russia, damaging thousands of buildings and causing widespread injuries.

An asteroid double or even triple that size exploded over Tunguska, Russia, in 1908, levelling 770 square miles of forest. According to the report released last week, casualties could be in the millions if a similar event struck New York City. A giant space rock wiped out the dinosaurs when it smacked into Mexico’s Yucatan peninsula 65 million years ago.

Johnson stressed 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 would like at least 10 years’ notice.

A mission to defend Earth could involve hitting the asteroid or comet with big, fast moving robotic spacecraft in the hope of changing its path, or in the worst case, launching a nuclear device not to blow up the asteroid but to superheat its surface and blow off enough material to divert it.

All that involves current technology, Johnson said, “Part of what this action plan is about is to investigate other technologies, techniques for both deflection and disruption of the asteroid.”

Scientists hope to learn 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.

Forget about sending astronauts, Hollywood style, as Johnson said, “It makes a good movie, but we did not see in our study any technique that would require the involvement of astronauts.” Missions like this lasting months or years make it difficult if not impossible for humans, given current technology.

The independent