A few days ago there was a news item about a proposal by the Russian Space Agency that they were considering a mission to this asteroid to deflect its course, because there is a risk that it might hit the earth in 2036.
Most asteroids orbit the Sun between Mars and Jupiter, but not all of them. Those with Earth-like orbits are monitored, because they might collide with the earth, with devastating consequences.
On 19-6-2004 an asteroid was discovered, which might collide with the earth in 2029. It was named Apophis, after the demon of darkness and chaos in Egyptian Mythology.
More accurate observations followed, and showed that this asteroid will NOT hit the Earth in 2029, although it will pass the Earth on Friday, April 13 (yes!) 2029 at a distance of only 30.000 km.
Mind you, that is closer than the altitude of the TV-satellites, circling the Earth!
At such a close distance the course of Apophis will be disturbed by the Earth. Calculations show that there is a small probability that at their next encounter, on Sunday, April 13 2036, Apophis could hit the earth. The estimated size of this asteroid is about 300 m and an impact could release an energy of ~ 900 megaton TNT (the Hiroshima atomic bomb was ~ 15 kiloton TNT, 60.000 times smaller!)
Here is the estimated risk path for such a collision
No wonder that it is the Russian Space Agency coming with this proposal.
When you want to know more about how to deflect such a massive piece of rock, there is a really fascinating educational website about asteroids, Apophis, and how to deflect it. Choose the Expert option.