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I read about the near-earth asteroid 99942 Apophis. It is in the first place of Potentially Hazardous Asteroids (visiting earth at an altitude of about 36,000km on 2028). But, scientists have calculated that the asteroid would impact earth on 2036. What is the fact behind this? I mean, Why is this difference in period of 8 years for a close-encounter and then impact? Is Apophis, a part of the asteroid belt between Mars and Jupiter or is it a part of the solar system? Would we actually get an "Armageddon"? Even if it doesn't impact, it could cause some damages either to our atmosphere or even our environment, Nah?

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The orbital period of Apophis is about 324 days, but the orbits of asteroids tend to be chaotic. Because their orbits are eccentric they may make close approaches to other bodies like the Earth or Venus, and this will change the orbit in ways that are difficult to calculate. Well, difficult to calculate far into the future anyway. Technically the orbits of all Solar System bodies, including the Earth, are chaotic but since the Earth doesn't make any close approaches to other heavy objects it's orbit changes only slowly and within narrow bands.

Anyhow, the variable orbit of Apophis is why it's hard to be precise about whether it will hit the Earth. The close approach in 2029 will change it's orbit in such a way that 7 years (i.e. 8 orbits) later it could collide wth Earth. But then we don't know exactly how close it will come in 2029 so we can't predict exactly how close it will come in 2036.

I'm not sure that anyone knows where Apophis came from. A quick Google shed no light on the subject. I would guess it originated in the Asteroid belt because if it came from much farther out it's orbital period would be longer.

As to the impact, there are so many variables it's difficult to say what damage there would be. Apophis is a lot smaller than the asteroid that killed off the dinosaurs, so we're not looking at the end of all life on Earth. However if it hit a big city there would be little if anything left of the inhabitants!

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JPL has a very nice page detailing impact energies and probabilities. In the case of Apophis, the impact probability in 2036 is quite small ($4.3\cdot10^{-6}$) and the impact energy is large but not overwhelming (~500 megatons). –  mmc Aug 14 '12 at 16:55
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No, Apophis takes 324 days to orbit the Sun, but when it completes one orbit the Earth will have moved. So even though it will come close in 2029 it won't come close in 2030. It will take another eight orbits before it comes close to the Earth again, in 2036, and that's when we get the close call. –  John Rennie Aug 14 '12 at 17:14
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