A recent study has updated the impact hazard assessment for 99942 Apophis, a 325-meter diameter near-Earth asteroid that has been the focus of considerable attention after it was found in December 2004 to have a significant probability of Earth impact in April 2029.

While the 2029 potential impact was ruled out within days through the measurement of archival telescope images, the possibility of a potential impact in the years after 2029 continues to prove difficult to rule out.

Based on extensive optical and radar position measurements from 2004-2012, Apophis will pass the Earth in 2029 at an altitude of 31900 +/- 750 km (about 5 +/- 0.1 Earth-radii above the surface of the Earth).

That altitude is close enough that the Earth's gravity could deflect the asteroid onto a trajectory that brings it back to an Earth impact. Such impact trajectories require Apophis to pass the Earth at a precise altitude, known as a keyhole, in 2029 en route to a subsequent impact.

Recent observations from Pan-STARRS PS1 telescope at Haleakala, Hawaii have reduced the current orbital uncertainty by a factor of 5, and radar observations in early 2013 from Goldstone and Arecibo will further improve the knowledge of Apophis' current position.

However, the current knowledge is now precise enough that the uncertainty in predicting the position in 2029 is completely dominated by the so-called Yarkovsky effect, a subtle nongravitational perturbation due to thermal re-radiation of solar energy absorbed by the asteroid.

The Yarkovsky effect depends on the asteroid's size, mass, thermal properties, and critically on the orientation of the asteroid's spin axis, which is currently unknown.

This means that predictions for the 2029 Earth encounter will not improve significantly until these physical and spin characteristics are better determined.

The new report, which does not make use of the 2013 radar measurements, identifies over a dozen keyholes that fall within the range of possible 2029 encounter distances.

Notably, the potential impact in 2036 that had previously held the highest probability has been effectively ruled out since its probability has fallen to well below one chance in one million.

Indeed only one of the potential impacts has a probability of impact greater than 1-in-a-million; there is a 2-meter wide keyhole that leads to an impact in 2068, with impact odds of about 2.3 in a million.

The full report is available here.