The asteroid 99942 Apophis is likely to fly past Earth in 2029 and 2036 without any impact. However, experts believe there is a chance the asteroid (of 270m diameter) could hit our planet on a third fly-by, in 2068.
The good news is the odds of Apophis hitting the planet are only 2.3 in a million, due to need of a very specific gravitational keyhole. A gravitational keyhole is tiny space within which the Earth's gravity can alter a passing asteroid's trajectory, literally turning it on to itself.
The Times of India indicates there was an earlier probability of impact in 2029 and archival data now suggests there are over a dozen keyholes that fall within range of the 2029 impact. However, the probability of Apophis finding one of those keyholes (and the trajectory thereby altering to a 2036 impact) was calculated to be less than one in one million; this almost certainly rules out an impact.
The bad news, though, is that a two-meter keyhole has been isolated and there is 2.3 in a million chance Apophis will find that space. And if it does, scientists expect an impact in 2068.
How Bad Will it Be?
The Sentry Risk Table estimates the asteroid's atmospheric entry would measure 750 megatons of kinetic energy. In comparison, the biggest hydrogen bomb ever exploded measured 57 megatons. The Tunguska event, which was caused by an asteroid in Russia, was estimated to be between three and 10 megatons.
Apophis came under considerable scrutiny in June 2009 because it was believed the 2029 fly-by would, in fact, trigger the gravitational keyhole, leading to an impact with the planet on its next fly-by.
Check out this video that explains the impact scenarios: