A handful of moons orbiting space rocks on the fringes of the solar system and discovered over the past year or so have astronomers puzzling over their presence and size.
The Kuiper Belt region of the solar system, which stretches from just past Neptune to beyond the farthest reaches of Pluto's orbit, was confirmed to exist in 1992 and some 500 KBOs have since been found. Last year, the first satellite orbiting a KBO was discovered. The total such moons in the belt now stands at seven.
"Surprisingly, observations by both ground-based telescopes and the Hubble Space Telescope have indicated that, in many cases, the KBO satellites are as large or nearly as large as the KBOs around which they orbit," says Alan Stern of the Southwest Research Institute (SwRI). "That so many binary or quasi-binary KBOs exist came as a real surprise to the research community."
Celestial objects and satellites are often thought to form by collision. In a paper to be published in the October issue of The Astronomical Journal, Stern questions the standard model for how such large KBO-satellite pairs could form.
The collision model for large satellite formation is based on collisions between an interloping body and the parent object around which the satellite orbits. This model has successfully explained binary systems around asteroids, as well as Pluto and its moon Charon. It also is the leading theory explaining the formation of the Earth-moon system.
But collisions of the magnitude required to form KBO satellites, Stern found, appear to be energetically improbable, given the number and masses of potential impactors in both the ancient (more massive) and modern day (eroded) Kuiper Belts.
This likely implies one of two alternatives:
- either KBO satellites were not formed by collisions, as has been commonly assumed;
- or the surface reflectivities (which help determine size) of KBOs with satellites, or the reflectivity of the satellites themselves, have been significantly underestimated.
"If the surfaces of KBOs with satellites, or the satellites themselves, are more reflective than previously thought," says Stern, "these objects would be smaller and less massive, and would therefore require smaller, less energetic impacts to create the satellite systems we see."
NASA's new Space Infrared Telescope Facility (SIRTF), set for launch early next year, will help resolve these two alternatives, Stern says, by directly measuring the reflectivities and sizes of numerous KBOs, including those with satellites.
In addition to this work, Stern serves as principal investigator of the NASA New Horizons mission to Pluto and the Kuiper Belt. Tentatively slated to launch in January 2006 but officially on hold for now, this spacecraft would make the first ever flyby reconnaissance of the Pluto-Charon system and then go on to explore KBOs as it leaves the solar system. New Horizons is the only NASA mission planned to study Kuiper Belt Objects at close range.
The NASA Origins of Solar Systems program provided funding for Stern's research.
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