The Detection of Asteroids in the Final Days Before Earth Impact
Session 48 -- Solar and Solar-System Physics
Display presentation, Wednesday, 9:20-6:30, Pauley Room

## [48.10] The Detection of Asteroids in the Final Days Before Earth Impact

P. J. T. Leonard, J. G. Hills (LANL)

The problem of the detection of small asteroids in the final days before Earth impact is considered. Low-velocity (i.e., relative velocity of approach ${}^<_\sim~12$ km~s$^{-1}$) asteroids are scattered many times via close encounters with the Earth before they finally hit the Earth. Therefore, their velocity distribution should be nearly isotropic with respect to the moving Earth, so they approach the Earth from all directions. There is a lack of higher velocity asteroids approaching the Earth from the direction opposite to that in which the Earth is moving, since these asteroids have been scattered out of the solar system. This zone of avoidance in the sky increases in area for higher velocities of approach. Let us consider a hypothetical stone impactor with a diameter of 100 meters, and a visual Bond albedo of 0.2. Such impactors strike the Earth every $\simeq200$ years, and are large enough to cause considerable damage at ground level even though they fragment and dissipate their energy in the atmosphere (Hills \& Goda 1993, AJ, 105, 1114). Let us consider such impactors approaching with a relative velocity of 10 km~s$^{-1}$. At 10 days before impact, their median apparent visual magnitude is $V\simeq18$. Those approaching from the anti-solar direction (opposition) can be brighter than $V\simeq16$. Those approaching from within $30^\circ$ of the Sun are fainter than $V\simeq21$. Since these impending impactors are heading straight at Earth, they move very slowly on the sky. Those approaching from within $10^\circ$ of the solar or anti-solar directions, or from within $\pm10^\circ$ of $90^\circ$ from the Sun move less than $\simeq1$ arc minute per day on the sky. The maximum motion is $\simeq5$ arc minutes per day for intermediate directions of approach. Results for other velocities of approach and times before impact will be presented, as well as strategies for detecting these impending impactors.