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 Friday, 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
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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 in 2026. Whew!
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 Sunday, 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
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