[meteorite-list] Apophis Risk Assessment Updated

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 21 Feb 2013 17:08:48 -0800 (PST)
Message-ID: <201302220108.r1M18nTn009124_at_zagami.jpl.nasa.gov>

http://neo.jpl.nasa.gov/news/news178.html

Apophis Risk Assessment Updated
Steve Chesley, Davide Farnocchia
NASA/JPL Near-Earth Object Program Office
February 21, 2013

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 at http://arxiv.org/abs/1301.1607
Received on Thu 21 Feb 2013 08:08:48 PM PST