Near-Earth object atmosphere, passing Earth's surface only away (about one Earth radius or one-sixtieth of the distance to the Moon). Because it was very small (6 meters/20 feet), FU was detected only hours before its closest approach. If it had collided with Earth, it probably would have disintegrated harmlessly in the atmosphere. On February 4, 2011, an asteroid designated , estimated at in diameter, passed within of the Earth, setting a new record for closest approach without impact, which still stands . On November 8, 2011, asteroid , relatively large at about in diameter, passed within (0.85 lunar distances) of Earth. On
Orbit determination the returned signal strength from radar decreases rapidly, as the inverse fourth power of the range to the object. This limits radar observations to objects relatively near the Earth, such as artificial satellites and Near-Earth objects. Orbit determination must take into account that the apparent celestial motion of the body is influenced by the observer's own motion. For instance, an observer on Earth tracking an asteroid must take into account both the motion of the Earth around the Sun, the rotation of the Earth, and the observer's local latitude and longitude, as these affect the apparent position of the body.