Risk level for asteroid 99942 Apophis drops to nearly zero, Earth safe for the time being

Thursday, October 08, 2009

Drat. Looks like new orbital measurements have shown that the asteroid 99942 Apophis will not have a 1 in 45,000 chance of hitting the Earth in 2036 but rather 1 in 250,000. It's a pity because the asteroid is 270 metres in diameter, which is just enough that we should be able to dislodge it from its orbit fairly easily given the amount of time, and having a practice run deflecting an asteroid would bring about a sudden realization that we aren't just a planet floating out here on its own but rather a planet that gets hit by things every once in a while, pretty large things that we need to be able to predict and defend ourselves against.

Asteroids often sneak up on us though, so this is certainly not the only asteroid that could hit us. Many smaller ones stay unseen until just a few days before they fly by, and we have no way to defend ourselves from those.

At least Apophis will still have a record-setting close approach to Earth in 2029, when it will pass a mere 29,450 km above the surface. Compare that to asteroid 2009 DD45, which flew by at a distance 20% that of the Moon from us (76800 km or so) and had a diameter of just 35 metres, and even that achieved a magnitude of 11 and made headlines for a few days. Even a few videos were uploaded to YouTube after it flew by.



Apophis has almost ten times the diameter and will fly by at a distance less than half that, so you can be certain it'll be a big event when it finally happens.

One interesting proposed method for changing the orbit of a small asteroid is a so-called 'gravity tractor', which would have a mass of about 10 tonnes. It would reach an asteroid some 15 years before the expected impact and would then approach it from a very close distance, exerting a very tiny gravitational tug that would eventually send it into a different orbit from the previous one, just enough to avoid hitting the Earth.

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