1899

Phoebe

Edward Charles Pickering (1846–1919), William Henry Pickering (1858–1938), DeLisle Stewart (1870–1941)

By the end of the nineteenth century, most astronomers realized that simply building progressively larger telescopes was not the only way to study progressively fainter objects. Just as important as increasing light-gathering power was increasing the sensitivity of the “detector” used to record that light. Thus, more observatories were converting from the human eye to the photographic plate as their detector of choice.

The Harvard College Observatory (HCO) was no exception; indeed, Edward Charles Pickering, the late-nineteenth-century HCO director, was a pioneer in the use of astrophotography to collect and record high-resolution spectra of stars. Pickering’s brother, William Henry Pickering, was also an HCO astronomer. In 1899 he analyzed photographic plates of the sky near Saturn taken a year earlier by HCO staff member DeLisle Stewart and discovered a faint new moon orbiting the ringed planet. But this was quite an oddball moon: it orbited backward compared to Saturn’s other moons, in a highly eccentric and tilted orbit four times farther from Saturn than the next closest moon (Iapetus). W. H. Pickering named the new moon Phoebe, following the theme of naming Saturn’s satellites after Titans from Greek mythology. Phoebe was the first moon in our solar system to be discovered using photography rather than the naked eye.

It wasn’t until the Voyager 2 Saturn flyby in 1981 and then especially the Cassini Saturn Orbiter mission beginning in 2004 that astronomers got more detailed information about Phoebe. It is a relatively large and somewhat spherical moon about 134 miles (220 kilometers) in diameter, with a low reflectivity (approximately 6 percent) and moderately low density (1.6 grams per cubic centimeter). Bright icy patches appear beneath the dark surface layers along steep slopes, and spectroscopy measurements show some to be carbon dioxide ice. Phoebe’s composition and strange orbit suggest that it might be a captured Centaur object—an interloper that was somehow diverted inward from the Kuiper belt. Impacts into Phoebe have created an enormous, tilted, dark, and diffuse distant ring of icy, rocky material around Saturn, some of which appears to be responsible for darkening the leading hemisphere of two-toned Iapetus.

NASA Cassini Saturn orbiter photo of part of the heavily cratered, dark surface of Saturn’s outer moon, Phoebe. Bright crater walls reveal icy deposits beneath a layer of darker materials.