The tumbling and irregularly-shaped moon Hyperion rotates away from Cassini in this movie taken during a distant encounter in Dec. 2005. A shadow closes over the large crater at bottom as the movie progresses.
Hyperion (270 kilometers, 168 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 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 movie was made from 40 images taken over about two hours as Cassini sped past the icy moon. The still image is taken from the movie sequence.
The images were taken in visible light with the Cassini spacecraft narrow-angle camera on Dec. 23, 2005 at distances ranging from 228,000 kilometers (142,000 miles) to 238,000 kilometers (148,000 miles) from Hyperion and at a Sun-Hyperion-spacecraft, or phase, angle ranging from 77 to 86 degrees. Resolution in the original images was about 1.4 kilometers (0.9 mile) per pixel. The images have been magnified by a factor of two and contrast-enhanced to aid visibility.
[Caption updated on July 4, 2007.]
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 Cassini-Huygens mission for NASA's Science Mission Directorate, Washington, D.C. The imaging team consists of scientists from the US, England, France, and Germany. The imaging operations center and team lead (Dr. C. Porco) are based at the Space Science Institute in Boulder, Colo.