From the archive, originally posted by: [ spectre ]

http://planetary.org/programs/projects/apophis_competition/

“Apophis is a near-Earth object (NEO), which will come closer to
Earth in 2029 than the orbit of our geostationary satellites. On that
pass, the asteroid will be gravitationally perturbed to an unknown
orbit, one that that may have the slim possibility of hitting Earth in
2036. Very precise tracking may be needed to determine the probability
of such a collision. Such precise tracking may require “tagging” the
asteroid, perhaps with a beacon — a transponder or reflector — or
some other method. Exactly how an asteroid could best be tagged is not
yet known, nor is it obvious, which is why The Planetary Society is
holding a competition.”