A survey of near-Earth asteroids shows they come in a far wider variety of shapes, sizes and surfaces than previously thought, U.S. researchers say.

The findings are based on infrared scanning of about 100 asteroids by NASA's Spitzer Space Telescope, SPACE.com reported Thursday.

The effort is part of a larger Spitzer telescope project to look at 700 near-Earth objects to identify their individual characteristics.

"These rocks are teaching us about the places they come from," lead study author David Trilling of Northern Arizona University, Flagstaff, said. "It's like studying pebbles in a stream bed to learn about the mountains they tumbled down.

"Very little is known about the physical characteristics of the near-Earth population," Trilling said. "Our data will tell us more about the population, and how it changes from one object to the next. This information could be used to help plan possible future space missions to study a near-Earth object."

The asteroids observed so far have a greater degree of diversity than expected, scientists say, indicating that they might have different origins.

Some might come from the main asteroid belt between Mars and Jupiter, and others could come from farther out in the solar system, they say.

The research is being published in the September issue of the Astronomical Journal.