NASA’s Cassini spacecraft made its final close approach to Saturn’s large, odd, tumbling, irregularly shaped moon Hyperion on May 31, 2015. The first three images on this page are from Sunday’s close pass.
The Saturn-orbiting spacecraft passed Hyperion at a distance of about at approximately 9:36 a.m. EDT (1336 UTC) on Sunday. Mission controllers expected images from the encounter to arrive on Earth 24 to 48 hours later, and so they have.
As they analyze and process these images, scientists have hopes of seeing different terrain on Hyperion than the mission has previously explored in detail during the encounter, but this is not guaranteed.
As you can see, Hyperion is very heavily cratered, with little, if any in the way of more recent geological activity present. It is likely to be a huge frozen ball of ice with a little rock. Many of its craters are deep fluted, and that is because Hyperion’s density is so low. The density is only 0.55 g/cm3, or about half the density of water. As a result, incoming objects – colliding asteroids or comets – can penetrate Hyperion’s surface very deeply, compressing the ice. Also the darker carbon-rich material sometimes found on these bodies can warm up slightly and can ‘burn’ into Hyperion’s surface.
Hyperion is one of the largest known irregularly shaped objects in the solar system with mean dimensions of 360 x 280 x 225 kilometers (223 x 174 x 137 miles).
The moon orbits Saturn at a mean distance of 1,481,100 kilometers (920,300 miles) once every 21 days.
Cassini’s next notable flyby after May 31 is slated for June 16, when the spacecraft will pass 321 miles (516 kilometers) above icy Dione. That flyby will represent the mission’s penultimate close approach to that moon. In October, Cassini will make two close flybys of the active moon Enceladus, with its jets of icy spray, coming as close as 30 miles (48 kilometers) in the final pass. In late 2015, the spacecraft will again depart Saturn’s equatorial plane — where moon flybys occur most frequently — to begin a year-long setup of the mission’s daring final year.
For its grand finale, Cassini will repeatedly dive through the space between Saturn and its rings.
Bottom line: Cassini’s final close approach to Saturn’s large, irregularly shaped moon Hyperion was Sunday, May 31, 2015 at approximately 6:36 a.m. PDT (9:36 a.m. EDT, 1336 UTC). First images from that close pass here.
Andrew R. Brown, an avid follower of the space program, writes frequently about space topics for EarthSky. Over several years, he has also suggested observations that were carried out by imaging teams of some space missions. He has lives in Ashford, Kent, United Kingdom and works for local government, Kent County Council.