A space-based survey of 101 objects that pass relatively close to Earth has found they have a wide range of reflectivity, indicating that their composition is varied as well.

Thousands of such near-Earth objects, or NEOs, most of them asteroids, have been catalogued in the past several years, but most of those have been found by optical telescopes that cannot determine an NEO's diameter—only how bright an object appears, which depends both on its diameter and its reflectance. NEO-trackers, then, have to assume some plausible reflectance, or albedo, for all such objects in order to estimate their size.

Only by obtaining complementary radar or infrared observations of NEOs can astronomers accurately determine their size. The ExploreNEOs project is aimed at doing just that, surveying a sample of some 700 known NEOs with the infrared Spitzer Space Telescope to pin down their albedos and diameters.

The group has now published preliminary results from that survey, based on observations of 101 near-Earth objects, in the September issue of The Astronomical Journal. Among the findings: The albedo of NEOs is not at all constant—it varies greatly from asteroid to asteroid. In other words, "the NEO population appears to be compositionally diverse," lead study author David Trilling of Northern Arizona University wrote in an e-mail. Some may be simply rocky, some may be laden with organic compounds, and some may be so-called dead comets—rocky objects from the outer solar system that once had volatile-rich tails.

The preliminary data from ExploreNEOs also show that larger objects, those wider than a kilometer, tend to be less reflective than smaller ones. Might that latter discovery indicate that asteroid-watchers have underestimated the size of the largest asteroids? (After all, a bright asteroid that turns out to have a very dull surface would have to be very large to reflect so much light.)

That's a possibility, Trilling says, but a significant observational bias makes it hard to tell. Small, dark NEOs reflect very little light and hence tend to escape detection from optical telescopes. "Since our Spitzer project uses known NEOs (discovered by ground-based optical surveys) as our input target list, we expected from the outset for our results to show that the smallest objects are bright (high albedo), and that there would be no small, dark objects in our sample," he says.

Another possible source for the albedo discrepancy beyond the composition variable is space weathering, which darkens asteroids over time. Smaller objects, which may be fragments produced in asteroid collisions, would be younger and less weathered than larger objects that have largely avoided collisions. The picture may become clearer as the ExploreNEOs project gathers more data; Trilling says that the group has now observed more than 300 objects with Spitzer and that the results of the first half of the survey will be submitted for publication in the fall.

Photo of near-Earth asteroid Eros, whose properties are well known thanks in part to a visit from the NEAR mission in 2000: NASA/JPL/JHUAPL