Reflecting on the Kuiper belt. (Astronomy).

Over the past 2 years, scientists have discovered that 7 of the roughly 500 known objects in the Kuiper belt--the reservoir of comets and other frozen objects just beyond Neptune's orbit--have moons (SN: 5/4/02, p. 285). Several of the moons appear nearly as large as the bodies they circle.

According to a common theory, each moon was created when an interloping body smashed into a large Kuiper belt object. However, the estimated number of potential impactors is too few to explain the number of large moons, says S. Alan Stern of the Southwest Research Institute in Boulder, Colo.

In the October Astronomical Journal, he suggests a solution. Stern proposes that the moons and the Kuiper belt objects they orbit reflect nearly four times more sunlight than typically estimated. Since astronomers calculate the mass and size of Kuiper belt objects from surface reflectivity, this correction would yield moons one-fourth as large and one-sixty-fourth as massive as the present estimates. Smaller moons are easier to explain by collisions, Stern notes.

The reflectivity value typically cited, 4 percent, is based on spacecraft data collected from comets Halley and Borrelly. But both of these dirty snowballs have visited the inner solar system many times, and a significant fraction of their ice may have evaporated, leaving behind a dirtier, less reflective surface than Kuiper belt objects have, Stern says. The chilly denizens of the distant Kuiper belt are likely to have retained more of their ice.

The Space Infrared Telescope Facility, scheduled for launch next year, will test Stern's ideas by measuring the size and reflectivity of Kuiper belt objects.--R.C.