Ground telescopes are good at picking up asteroids zooming into the inner solar system and approaching from the night side of Earth, Johnson says. 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 on Wednesday, 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 says, adding: ”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 says, “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.