
The smaller of the two moons of Mars
This photo of Deimos was featured recently on NASA’s Astronomy Picture of the Day website, which says: “Deimos is one of the smallest known moons in the Solar System measuring only about 15 kilometers across. The diminutive Martian moon was discovered in 1877″.
AbsoluteAstronomy.com explains: “In Greek mythology, Deimos was the personification of dread. He was the son of Ares and Aphrodite. He, his brother Phobos and the goddess Enyo accompanied Ares into battle, as well as his father’s attendants, Trembling, Fear, Dread and Panic. His Roman equivalent was Formido or Metus. Asaph Hall (a 19th century American astronomer), who discovered the moons of Mars, named one Deimos, and the other Phobos.”
A recent entry in the Planetary Society website has more:
Deimos is relatively poorly studied because all modern Mars spacecraft orbit at altitudes much lower than Deimos’ 20,000 kilometers. Since Deimos, like nearly every moon in the solar system (including our own), is tidally locked to its planet, that means that all orbiters see only one face of the moon, the “sub-Mars hemisphere,” and that at a great distance. Phobos orbits much closer to Mars (at 9,400 kilometers), which is still above the altitude of the circular, polar orbiters like Mars Global Surveyor, Odyssey, and Mars Reconnaissance Orbiter, but it’s below the apoapsis of Mars Express, and is now being well mapped from all sides.