Apophis is a 250-meter-wide rock with a special designation: it's a near-Earth asteroid, meaning it passes close to our planet. In fact, in April of 2029 it will pass so close to the Earth -- just under 30,000 kilometers (18,000 miles) -- that it will actually get between us and some of our geosynchronous satellites!
Because it will come so close, the Earth's gravity will change its orbit. There is a region of space called a keyhole (it's actually kidney-bean shaped) and if Apophis passes through it like an arrow through a bulls-eye, the Earth's gravity will change the asteroid's orbit enough that in seven more years, in 2036, Apophis will hit the Earth! The odds of it passing through the keyhole are low; up until recently they've been quoted as 1 in 45,000. Not a huge concern, but worth keeping an eye on.
However, new observations have lowered these odds even more, to only 1 in 250,000. Pretty much at this point I'm not worried about this particular rock any more. The odds before weren't great, but they're so bad now it's no big deal.
How does this work? The orbit of an asteroid is calculated using measurements of its position in the sky over time. There is a tiny uncertainty in those positions for many reasons: atmospheric distortion blurring the asteroid image being one of if not the biggest. The way to minimize that is to get lots of images so that the errors average out, but even then the orbit calculated has uncertainties. And the longer into the future you project the orbit, the worse it gets. In the case of Apophis, astronomer Dave Tholen used hundreds of new images of Apophis to refine the orbit and get the better statistics for its impact risk.
This happens quite frequently: a potentially dangerous asteroid gets better observations made of it, and the risk drops. In this case, that's definitely a good thing. At 250 meters wide, an Apophis impact could do considerable damage to civilization. It wouldn't be an extinction level event, but it would put serious hurt on humans. Still, it bears watching. I doubt the odds will go back up, but the more we pay attention to these things, the better!
Image credit: UH/IA.