The Kuiper Belt region of the
solar system, which stretches from just past Neptune to beyond the farthest
reaches of Pluto’s orbit, was only discovered in 1992, but continues to
reveal new knowledge into the formation processes of the planets. Now, in a
paper to be published in the October issue of The Astronomical Journal, a
Southwest Research Institute (SwRI) scientist reveals a new mystery about
Kuiper Belt Objects (KBOs).

The study examined the formation of KBO satellites, which have been observed
only since 2001 and continue to be discovered around an unexpectedly large
number of the more than 500 known KBOs.

“In just over a year since the first satellite of a KBO was found,
scientists have discovered a total of seven KBO satellites. 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 Dr. S. Alan
Stern, director of the SwRI Space Studies Department. “That so many binary
or quasi-binary KBOs exist came as a real surprise to the research

The focus of Stern’s work was not observational in nature, but rather it
sought to understand how such large KBO-satellite pairs could form. The
standard 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 and
the Pluto-Charon system, and also has direct relevance to the formation of
the Earth-moon system.

Stern’s findings call into question the formation of KBO satellites by
standard collisional processes. Collisions of the magnitude required, 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

“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. Expected to launch in
January 2006, this spacecraft will 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 this

EDITORS: An image to accompany this story is available from, or download a preprint of “Implications
Regarding the Energetics of the Collisional Formation of Kuiper Belt
Satellites” from