1848

Hyperion

William Bond (1789–1859), William Lassell (1799–1880), George Bond (1825–1865)

No new moons around Jupiter, Saturn, or Uranus were discovered for more than a half century after the 1789 discovery of Mimas by William Herschel, but the eighth known satellite of Saturn was independently discovered by two groups within days of each other in late 1848. The American father-and-son astronomer team of William Bond and George Bond apparently observed the new moon first, but the English astronomer William Lassell was the first to announce his observations. All three astronomers are credited with the discovery. Just one year earlier, John Herschel had published a compendium of new observations in which he had proposed a Greek Titans naming scheme for the moons of Saturn; Lassell had endorsed the scheme and suggested that the newest moon be named Hyperion, after the Greek Titan and brother of Cronus, the Greek equivalent of Saturn.

Little was known about Hyperion, except that it traveled in a relatively eccentric orbit at an average distance of about 25 times the radius of Saturn (well outside the planet’s ring system), until the Voyager flybys in the 1980s and then the Cassini Saturn Orbiter mission’s encounters with the moon in the early 2000s. Hyperion turns out to have been the first, and largest, irregular (nonspherical) moon discovered by telescope. Space mission images show it to be an elongated body with dimensions around 204 × 162 × 133 miles (328 × 260 × 214 kilometers), and more recent spectroscopy from telescopes shows it to have a surface composed of dark, reddish, “dirty” water ice, perhaps similar to that on the dark side of Iapetus. Hyperion has an extremely weird, spongy surface, characterized by tons of deep impact craters and sharp-edged pits and cliffs. One whole side of the moon is scarred by a giant impact crater 75 miles (120 kilometers) wide and 6 miles (10 kilometers) deep, suggesting that Hyperion is only a fragment of a larger satellite broken apart by a giant impact. Hyperion’s density is only about 0.56 grams per cubic centimeter, suggesting that it is made mostly of very porous water ice. Indeed, astronomers think that a remarkable 40–50 percent of Hyperion’s interior is empty space between icy fragments.

NASA Cassini Saturn orbiter enhanced color photo of the irregular, almost spongy-looking moon Hyperion, taken in September 2005. The long axis of the satellite is about 205 miles (330 kilometers) across; the impact crater Helios (75 miles [120 kilometers] wide) dominates most of this hemisphere of the satellite.