Spaceflight Now: Breaking News

Hubble finds possible crater of fresh ice on space rock
Posted: September 14, 2000

The unexpectedly varied surface of a wayward piece of space debris has given Hubble telescope astronomers new insights into the characteristics and behavior of a ghostly population of faintly observed comet-like bodies that lie just beyond Pluto's orbit.

Artist's concepts of the crater. Photos: Bryan Preston (STScI/AVL)
While observing an object called 8405 Asbolus, a 48-mile-wide (80-kilometer) chunk of ice and dust that lies between Saturn and Uranus, astronomers using NASA's Hubble Space Telescope were surprised to find that one side of the object looks like it has a fresh crater less than 10 million years old, exposing underlying ice that is apparently unlike any yet seen. This shows that these mysterious objects do not have a simple homogenous surface, say researchers.

Hubble didn't directly see the crater -- the object is too small and far away -- but a measure of its surface composition shows a complex chemistry.

"To wildly speculate, there may have been an impact that heated this surface and did some different chemistry of the hydrocarbons present. This may be a solar system ice that hasn't yet been seen in other objects or generated in the lab," said Donald W. McCarthy of the University of Arizona (UA) in Tucson, Arizona.

"The ice does have some strong similarities to water ice, but in places it really doesn't match," adds lead investigator Susan D. Kern of UA. "This could be a new mixture of things we've seen before, but not in this combination."

By latest count, scientists have discovered a total of 21 Centaurs, which are dim, small bodies, which are icy like comet nuclei. These objects are considered escapees from a vast reservoir of comets, the Kuiper Belt beyond Pluto. The Centaurs' orbits were perturbed into the region between the orbits of Neptune and Jupiter.

"Perhaps the event that caused the impact crater on 8405 Asbolus also knocked it out of the Kuiper belt," McCarthy speculates.

As part of a survey of 10 Centaur objects, the Arizona team measured the surface composition of 8405 Asbolus on June 11, 1998. This was done by taking near-infrared spectra that measure the colors of sunlight reflected off the surface.

Fortuitously, as it turned out, that day the Space Telescope instruments were briefly shut down to protect them from radiation interference as the spacecraft passed through one of the Earth's strong radiation belts of charged particles. Instead of observing the Centaur for an uninterrupted 40 minutes, the astronomers studied it in two separate sessions spanning almost two hours over which both hemispheres of the asteroid were visible.

These results will be published in the Astrophysical Journal Letters.

The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract with NASA's Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency.