This movie sequence provides the record of Cassini’s first close brush with Hyperion, Saturn’s chaotically tumbling moon. As the spacecraft whizzes past, Hyperion’s unusual shape is most apparent. The jagged outlines are indicators of large impacts chipping away at Hyperion’s shape as a sculptor does to marble.
Hyperion’s unusual dimensions are 328 by 260 by 214 kilometers (204 by 162 by 132 miles).
These Cassini images are the best views yet of one of the large, low density objects that orbit Saturn. Hyperion is close to the size limit where, like a child compacting a snowball, internal pressure due to the moon’s gravity will begin to crush weak materials like ice, closing pore spaces and eventually creating a more spherical shape.
However, this moon has a very irregular shape and preliminary estimates of its density show that it is only about 60 percent as dense as solid water ice. This suggests that much of its interior (40 percent or more) must be empty space.
The low density further suggests that Hyperion is mostly made of water ice, with a low rock and metal content. If the moon had significant higher density components, its implied porosity would be significantly higher than 50 percent. The dark material on the surface is therefore likely a minor component, possibly originating from impacts of dark material, as seen on Iapetus.
Hyperion’s elliptical orbit and irregular shape influence its chaotic tumbling. Further, because it is in a resonance orbit with the giant moon Titan, impact debris ejected with sufficient energy does not come to rest again on Hyperion. Instead, debris is tugged gravitationally into Titan’s orbit, where it impacts the large smoggy moon.
This series of 25 images was taken over a period of nearly two and a half days, between June 9 and June 11, 2005, as Cassini’s orbit took it close to Hyperion.
Credit: NASA/JPL/Space Science Institute