Exploring the Planets

Planetary Satellites

In 1900, the planets were known to have 22 satellites or moons. The number of known moons has now more than quadrupled and is still increasing.

Discovery of the Satellites of Mars

The scientists of Laputa "discovered two lesser stars, or satellites, which revolve about Mars; whereof the innermost is distant from the primary planet exactly three of his diameters, and the overmost five; the former revolved in a space of ten hours, the latter in twenty-one and a half."

— Johnathan Swift, Gulliver's Travels, 1726

Phobos and Deimos

Mars' moons were actually discovered a century and a half later by Asaph Hall (1829-1907) at the Naval Observatory in Washington, DC. Hall named the satellites for the sons of Ares (Mars) in Greek mythology, Phobos, and Deimos (meaning fear and terror, respectively).

Mars' Moon Phobos

Mars' Moon Phobos

Two Views of Mars' Moon Deimos

Two Views of Mars' Moon Deimos

Discovery of Uranus' Rings

Uranus Rings Discovery Telegram

Telegram from James L. Elliott
Image ©2002 Smithsonian Institution #W1998EP0003

The rings of planets, such as those of Saturn, are a system of countless small satellites. In 1977, it was discovered that Uranus was also encircled by a system of rings. Uranus' rings were discovered because they occulted (blocked from view) a star as Uranus passed between the Earth and the star. The discovery of Uranus' rings occurred through observations made from the Kuiper Airborne Observatory. The discovery telegram (shown at left) sent by Kuiper observers is on display in the Exploring The Planets exhibit in the National Air and Space Museum.