Asaph Hall discovers Deimos, the outer moon of Mars

Background. The United States Naval Observatory, established in 1830, is a leading facility for astronomical data in Washington, D.C. In 1875, Asaph Hall was given responsibility for the observatory's 26-inch telescope, the largest refracting telescope in the world at that time. This instrument was crucial for his subsequent astronomical discoveries.

Discovery Context. During Mars's closest approach in 1877, Hall was encouraged by his wife, Angeline Stickney, to search for the Martian moons. He was specifically searching for these satellites when he discovered Deimos on August 12, 1877, at about 07:48 UTC. The discovery was made using the 26-inch telescope at the USNO.

Naming and Characteristics. Deimos is the smaller and outer of Mars's two natural satellites, with a mean radius of 6.2 km. It is named after the Ancient Greek god of dread and terror, a name suggested by academic Henry Madan based on Book XV of the Iliad. The moon has a highly non-spherical shape with triaxial dimensions.