CSM 7/26/05:

"An asteroid, headed our way"

Excerpts:

"Humans live in a vast solar system where 2,000 feet seems a razor-thin
distance. Yet it's just wide enough to trigger concerns that an asteroid
due to buzz Earth on April 13, 2029 may shift its orbit enough to return and
strike the planet seven years later. The concern: Within the object's range
of possible fly-by distances lie a handful of gravitational 'sweet spots,'
areas some 2,000 feet across that are also known as keyholes.

The physics may sound complex, but the potential ramifications are plain
enough. If the asteroid passes through the most probable keyhole, its new
orbit would send it slamming into Earth in 2036. It's unclear to some
experts whether ground-based observatories alone will be able to provide
enough accurate information in time to mount a mission to divert the
asteroid, if that becomes necessary. So NASA researchers have begun
considering whether the US needs to tag the asteroid, known as 99942
Apophis, with a radio beacon before 2013."

[snip]

"Based on available data, astronomers give Apophis - a 1,000-foot wide chunk
of space debris - a 1-in-15,000 chance of a 2036 strike. Yet if the asteroid
hits, they add, damage to infrastructure alone could exceed $400 billion.
When the possibility of the asteroid passing through two other keyholes is
taken into account, the combined chance of the asteroid hitting the planet
shifts to 1 in 10,000, notes Clark Chapman, a senior scientist with the
Southwest Research Institute in Boulder, Colo."

Text:

http://www.csmonitor.com/2005/0726/p01s04-stss.html