The tumbling and irregularly shaped moon Hyperion hangs before Cassini in this image taken during a distant encounter in December 2005. This still image is part of a movie sequence of 40 images taken over about two hours as Cassini sped past the icy moon (see the related movie).
Hyperion (280 kilometers, or 174 miles across) is covered with closely packed and deeply etched pits. Scientists originally thought the warming action of the sun on water ice lying beneath a darkened layer of surface material apparently had deepened and exaggerated the depressions already created by impacts.
Cassini scientists now think that Hyperion's unusual appearance can be attributed to the fact that it has an unusually low density for such a large object, giving it weak surface gravity and high porosity. These characteristics help preserve the original shapes of Hyperion's craters by limiting the amount of impact ejecta coating the moon's surface. Impactors tend to make craters by compressing the surface material, rather than blasting it out. Further, Hyperion's weak gravity, and correspondingly low escape velocity, means that what little ejecta is produced has a good chance of escaping the moon altogether.
The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Dec. 23, 2005, at a distance of 228,000 kilometers (142,000 miles) from Hyperion and at a sun-Hyperion-spacecraft, or phase, angle of 77 degrees. Resolution in the original image was about 1.4 kilometers (0.9 mile) per pixel. The image was magnified by a factor of two and contrast-enhanced to aid visibility.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo.
Credit: NASA/JPL/Space Science Institute