Emily LakdawallaJan 02, 2008

Mars impact chances at 1 in 28

The Near Earth Object program has posted an update on the status of asteroid 2007 WD5, which now has a reported 1 in 28 chance of hitting Mars (or, as several other bloggers have put it, a 27 in 28 chance of doing nothing exciting).

I've been getting two main questions on this asteroid. First of all, people ask, when the trajectory for WD5 is being calculated, is the gravity of Mars being taken into account? Mars is pretty big, so if something swings by sufficiently closely, the planet's gravity will pull that object even closer, making an impact more likely. The answer is, yes, they are taking Mars' gravity into account.

The more difficult question to answer is, what will happen to the asteroid after it flies by (assuming it doesn't hit)? Again, Mars' gravity plays a big role; the asteroid's orbit will not be the same after this flyby as it was before the Mars encounter. And since this asteroid also crosses Earth's orbit, changes in the orbit are quite relevant to those of us who would rather not be hit by an asteroid. The problem is that the uncertainty in where exactly the asteroid will fly by Mars hugely magnifies the uncertainty in predictions in shape of its future orbit; we won't know exactly where WD5 will go next until astronomers observe its travels following its close brush with Mars (again, assuming it doesn't hit). However, I think that if there was any chance that the Mars flyby could cause it to come back and hit Earth any time soon, we'd have heard about it already. That's exactly the worry we have about the near-Earth object called Apophis -- that its 2029 flyby past Earth (when it has no chance of hitting us) could, if it happens to pass through a sweet spot, change the orbit exactly so it comes back to hit us in 2036. Still, it's worth tracking WD5 following its Mars encounter to see if it'll come any closer to Earth in the future than it already has.

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