So far, images of the irregularly-shaped 168 mile (270 kilometers) wide Hyperion have only been captured of the approximate same side, showing the moon’s signature “spongy” appearance. During the final flyby, expected on Sunday (May 31) at 6:36 a.m. PDT (9:36 a.m. EDT), mission scientists hope that Cassini will have the opportunity to photograph the other side of the moon so we can gain a better picture of its composition.
But predicting which side of Hyperion will be facing Cassini at flyby is a nigh-on impossible task; the moon is tumbling chaotically as it orbits Saturn, denying orbital dynamics experts the chance to accurately calculate the object’s spin.
Hyperion has an unusually low density (and therefore gravity) for an object of its size, a factor that scientists believe can be to blame for the moon’s porous appearance. Through Hyperion’s evolution, rocky impactors have struck the satellite, compressing the surface rather than excavating it, so any material blown off the surface is lost to space, leaving this weird pock-marked surface.
Unfortunately, Sunday’s flyby won’t be as close as the closest Cassini has come to Hyperion. In 2005, Cassini cruised past Hyperion at a distance of 314 miles (505 km) — the final flyby will be 21,000 miles (34,000 kilometers) from the moon.
This flyby will be the first of several flybys of Saturn’s moons. On June 16, Cassini will buzz Dione at only 321 miles (516 kilometers) — the first of two planned flybys. Then Enceladus will be the next, daring target; the second of two close passes skating across the moon’s icy surface at a distance of only 30 miles (48 km)!
All of these maneuvers past the Saturnian moons represents the beginning of Cassini’s final stage, where the spacecraft will move out of Saturn’s equatorial plain and, through 2016, begin a series of deep dives through Saturn’s rings.
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