Russian scientists have recently raised the spectre of global disaster once again in the form of an impact of the asteroid Apophis, which is about 270 m across, with the Earth. The asteroid was christened Apophis, after an ancient Egyptian spirit of evil and destruction, when it was thought that it might impact the Earth on Friday the 13th April 2029. Refinement of Apophis’s trajectory now suggests that it will pass the Earth harmlessly at a distance of around 5 Earth radii on that date. Perhaps the recent concern surrounds the idea that the object may pass through a so-called ‘gravitational keyhole’, and then return to impact the Earth many years later. So what’s this all about? I don’t know how good you are at thinking in 3D, but give this a go. Imagine a plane passing through the Earth’s centre, which is perpendicular to Apophis’s trajectory at its point of closest approach in 2029. You might refer to this as the ‘impact plane’, as it allows the miss-distance to be easily measured. Now, there are small elliptical areas on this plane of the order of a few 10s of metres in size referred to as keyholes. If Apophis passes through one of these, it’s perturbed trajectory will take it into an orbit around the Sun which is resonant with the Earth’s orbit. This means that after an exact number of orbits the asteroid can return to impact the Earth in the future. The main concern with the Russian research is the keyhole that will cause an Earth impact exactly 7 years later on Friday 13th April 2036. I thought this had already been investigated and the risk found to be negligible, so I’m not sure why the prospect of impact with Apophis has been resurrected? Anyway, one good thing is that the Russian scientists were kind enough to give our research at Southampton a plug – our work looked at the cost of such an impact in terms of human life and infrastructure damage – so thank you for that !
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