Radar images of the asteroid, known as 1999 JM8, reveal an oddly shaped object several miles wide with an unusually slow and possibly complex spin, said Lance Benner of Nasa's Jet Propulsion Laboratory in Pasadena, California.
The images were taken by a radio telescope at Arecibo in Puerto Rico in conjunction with Nasa's Goldstone Solar System Radar in California. The images show that the asteroid is dotted with impact craters, some as small as 100 metres across and others one kilometre wide.
Michael Nolan, a scientist at the Arecibo observatory, said: "The density of craters suggest that the surface is geologically old, and is not simply a `chip' off a parent asteroid."
Arecibo has recently been fitted with a powerful upgrade to its radar transmitter, enabling it to take detailed images of the asteroid, which was first identified in 1990 but went missing until earlier this year.
The radar waves took a full minute to bounce off the asteroid and return to Earth. Scientists hope to develop the technology to forewarn of any potentially dangerous asteroids that could collide with Earth.
"This is the first good opportunity for radar imaging of an asteroid in a very long time," said Jean-Luc Margot, an Arecibo astronomer. "You don't get these kinds of objects passing near the Earth every day."