If the asteroid Apophis were to collide with Earth 30 years from now, it would strike with the force of 57,000 Hiroshima bombs. The aircraft-carrier-size rock could wipe out a city the size of Los Angeles and cause a 30-foot tsunami to smack into southern California. There's only a 1 in 5,500 chance of impact, but it's a chance that NASA is not willing to take.

So in late October, the space agency outlined a three-phase plan to observe, track, and deflect the rogue rock over the next two decades.