On the morning of 15 February 2013 a research team in central Europe was preparing its final recommendations to a United Nations subcommittee on establishing an international response mechanism to prepare for an asteroid impact on Earth. Driving the point home would be a presentation by US space agency NASA showing asteroid ‘2012 DA14’ would pass within 27,700 kilometres of Earth that very afternoon – closer than the orbit of meteorology and communications satellites. But the point would be well made before NASA’s presentation: later that morning a different asteroid exploded 30 kilometres above Chelyabinsk in Russia. The shockwave knocked down walls, blew out windows and injured over 1,600 people. No one knew the asteroid was coming.
The atmosphere protects Earth from asteroids 30 metres across or smaller – most of the time. The one that exploded over Chelyabinsk was 14 metres to 17 metres across. The asteroid’s shallow entry angle and high speed of 19 kilometres per second relative to Earth allowed it to skip through.
This asteroid’s surprise appearance is far from unique. Asteroid 2015 HD1 was discovered by NASA’s Mt Lemmon Survey Telescope on 18 April 2015, three days before it made its closest approach to Earth at 73,385 kilometres. If it had been on track to hit Earth, there would have been no time for deflection or disruption. Those in the impact zone could only have prepared to manage the consequences.
