Apophis is an asteroid with an slightly offset orbit to that of Earth's. Discovered in June 2004, astronomers have determined that it will make a very close flyby on April 13th, 2029, where it will pass to within 5 Earth diameters of us. The exact path the asteroid follows on its flyby in 2029 will determine whether it smashes into the Earth seven years later.

In 2004, it was first thought that the asteroid would hit us in the first flyby in 2029. The initial calculation for the orbit was made using only two sets of observations, those made in June and a subsequent set made in December.

From those observations, astronomers calculated a 1-in-200 chance that the asteroid would hit the Earth. If this was true, then this asteroid had become the most dangerous asteroid ever found. After taking more observations , the chances climbed even higher and by the end of December 2004, the chances of the Earth being struck by this asteroid climbed as high as 1-in-37.

Normally, when more observations are taken the chances of this kind of collision decrease. Not so here, it seemed like this thing was really going to hit us.

Luckily, some other observations from other sources were located and they allowed astronomers to calculate a more precise orbit. From those images they were able to conclude that there was no way that Apophis was going to hit the Earth. Whew!

For a while there though, you can bet there were some astronomers with some pretty tight sphincters.

We're not out of the woods yet. As you may know, when one celestial object passes close to another one, their orbits are altered a bit by the gravitational forces interacting between the two bodies. Since Apophis is much smaller than the Earth (it is 320 meters - or 1050 feet- across), its orbit will be the one affected. As it flies by our humble little planet, its orbit will change.

So, after astronomers had determined that the April 2029 encounter wasn't going to impact the Earth, they ran some simulations and found that the orbit of the asteroid will bend about 28 degrees, altering its course.

The flyby will make the orbit a bit bigger and Apophis will travel a bit slower. How much the orbit changes depends on how close it gets to us. If it flies through a specific 610-meter wide region of space as it goes past us in 2029, then Apophis' and the Earth will be in the exact same spot 7 years later on Friday April 13th, 2036.

So, what are the chances that Apophis will pass through that tiny region of space? Rest assured that astronomers are observing like crazy to get the best possible estimate.

Right now, it stands at 1-in-48,000.

One more time: Whew.

Just for reference, there is a 1-in-354,319 chance that you'll be killed in an airplane accident.

More definitive measurements still need to be made. Keep in mind that this thing is pretty small as astronomical bodies go. Even though its effects could be huge on the Earth if it hit us, relatively speaking and because of the vastness of space, this asteroid is tiny and very hard to see. It is currently hovering just outside of out ability to see it in telescopes.

That will change in the early 2010's. At that time, the asteroid will be close enough to us that optical and radar observations can be made accurately enough to make a better estimate.

I think I'll be paying attention to those results.

We are very fortunate that there are people out there whose job is to look for these things and warn us about them.

This sort of impact has happened before. It is widely believed that such an impact by an asteroid is responsible for changing the Earth in such a way as to adversely affect the dinosaurs, wiping them out. Also, The Tunguska Event is believed to be an explosion of a meteorite in the air above Siberia.

There are all sorts of objects like Apophis whirling over our heads. There is a real chance that the Earth can be struck by an asteroid we haven't seen yet. Because they are so small, they are dim and are easily missed by our telescopes.

Luckily, Apophis was observed early and we have many years to watch it and prepare for what it might do. This raises all sorts of interesting questions such as, what is our plan in the event of such a disaster? How can we prepare ourselves? How many resources should be made available for such an event?

We should be able to learn sometime in the 2010 decade whether this thing will actually hit us or not. If it is found that it will, then we'll have roughly 20 years to prepare. By that time, there is no doubt that many options will be explored (and fast).

This event, however, raises the spectre of those asteroids we do not know about and that may not give us as much warning. What about them? Perhaps we should be spending some money on more telescopes whose sole purpose is to look for such objects.

As with all things, we need to weight the chance of getting hit by an asteroid against the cost of getting prepared. There are many things that would probably be prudent to develop in case such an event actually does transpire.


Finally, let's address the issue of what would happen if the Earth was hit by this asteroid. A lot depends on where it hits. The ocean would be a great place for this thing to hit, for example. While it would still make a mess (tidal waves, etc), a lot of the energy would be absorbed by the water.

NASA estimates the energy from this particular asteroid to be roughly the same as if 65,000 nuclear bombs were dropped on us. What happens next depends on where it hits. It would certainly trash the immediate area, and it looks like estimates (from Wikipedia) tell us that an impact winter (a period of extended cold weather brought on by dust and particles shrouding the Earth and blocking the Sun) is unlikely.

Talk about astronomy you can use...

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Further Reading:
Friday the 13th, 2029
Caveat Impactor
Wikipedia entry on Apophis