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Emily LakdawallaOctober 6, 2008

A three-meter-diameter piece of the sky is falling

Many, many congratulations are due to Richard Kowalski and the rest of the team at Mt. Lemmon Observatory in Arizona, who, last night, made the first discovery of an object that proved to be on a collision course with Earth. The fact that I'm offering congratulations and not screams of panic should tell you that this thing presents no possibility of harm to humans or indeed any life on Earth; it's so small (between 2 and 5 meters diameter) that it will burn up in the atmosphere before it hits the ground. But it should make an absolutely amazing fireball in the process, visible mostly to people in northeastern Africa. Atmospheric entry will occur at about 02:46 UTC Tuesday morning (i.e. in a little under six hours), over northern Sudan. The object's name is 2008 TC3.

Objects of this size hit Earth a few times a year, but no one has ever spotted one on the way in before, so, kudos to Kowalski et al. The fact that it's been predicted allows astronomers, both professional and amateur, to study the color of the asteroid as it approaches. Then, if there are any appropriately instrumented observers in the right place (I don't know how many well-equipped astronomers there are in Sudan), they could capture spectra of the fireball, which would tell them volumes about what the asteroid was made of. If, by some chance, any pieces survive to hit the ground and get recovered (a very, very unlikely scenario, but it could happen), we could actually study the body's composition in a third way, with real hand samples. That would be an unprecedented data set on asteroids, helping to tie what we've learned from meteorites found on Earth to what we've learned from studying their light from space.

The location of the fireball is a bit unfortunate for science -- it's not exactly the best-instrumented region of the globe. On the other hand, at least it's coming in over land and not over ocean; and, when it's not in the middle of a civil war, Sudan is actually a great place to look for meteorite fragments, being a desert. Anyway, the fact that this discovery has been made once seems to increase the odds that it could happen again. Even if we don't get that sky-to-ground set of science data on this one, we can look forward to it happening some time soon.

It's amusing to visit the Close Approaches page at JPL's Near Earth Object program and look up the entry for 2008 TC3. The table lists objects by name, with their close approach date and their miss distances in astronomical units and lunar distances. For 2008 TC3, the miss distance in astronomical units is "0.0000." Ha! The miss distance in lunar distances is nonzero, 0.02. Does that mean it'll miss? No, because, I assume, the miss distances are calculated by assuming that both Earth and the incoming object are point masses -- so the distance is between the center of the object and the center of Earth. Ordinarily this is a fair enough approximation. But when you're dealing with an object that is coming within 0.02 times 384,000 kilometers, or under 8,000 kilometers, you're talking about something that might hit Earth.

Anyway, if you happen to be in northeastern Africa, make sure you get out of bed early to look up for this fireball -- it should be dramatic. Checking a map, I don't see a lot of big population centers near there. The closest large cities I see to the entry point are Jiddah and Mecca. Wouldn't it be something if the fireball were visible from Mecca? It'll probably be too far away though.

I'm getting conflicting information on which direction the object is supposed to be traveling. A JPL press release states it'll be traveling from northeast to southwest, so it would be coming in over the Middle East. But other information indicates it'll be coming in to its atmospheric entry point from an azimuth of about 280, or pretty much due west. That puts its entry path over Chad and Niger. I'm no astronomer myself, so I'm not really sure how to read the predictions.

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Emily Lakdawalla

Senior Editor and Planetary Evangelist for The Planetary Society
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