RADAR PUSHES LIMITS OF ASTEROID IMPACT PREDICTION
(JPL Press Release)

Applying unprecedented refinements to the analysis of celestial hazards, NASA astronomers have identified a potential close encounter with Earth more than eight centuries in the future by an asteroid two-thirds of a mile (one kilometer) wide.

What will most likely be a miss, even without preventive measures, will come on March 16, 2880. Odds for a collision are at most one in 300, and probably even more remote, based on what is known about the asteroid so far. Still, that makes this object, named 1950 DA, a greater hazard than any other known asteroid.

ASTEROID 1950 DA (NASA NEO Program's web page)
"Asteroid 1950 DA's Encounter with Earth: Physical Limits of Collision Probability Prediction"
(paper published in Science magazine)

[This paper is Copyright 2002 American Association for the Advancement of Science. Its reproduction in any form requires written permission of author and publisher.]

Radar images of asteroid 1950 DA obtained at Arecibo on Mar. 4, 2001, from a distance of 0.052 AU (22 lunar distances, eight million kilometers, five million miles):

The resolution is roughly 15 meters. The asteroid, a kilometer-wide, slightly asymmetrical spheroid, is illuminated by the radar from the top.