Phobos (mythology) of Cypselus on the shield of Agamemnon. American astronomer Asaph Hall named a satellite of the planet Mars, "Phobos", which he discovered along with the second Mars satellite, "Deimos", in 1877. Phobos is the larger of the two satellites. The word "phobia" derives from "phobos", (Φόβος), meaning fear. The two brothers Deimos and Phobos were particularly worshipped in the city state of "Sparta" as they were the sons of Ares, the god of war. Phobos in Greek literally means "fear" (). Spartan soldiers would idolize Phobos, because he symbolized discipline, and consistency of the armed forces. Because Phobos and Deimos
Phobos (moon) presence of voids and indicate that it is not a solid chunk of rock but a porous body. The porosity of Phobos was calculated to be 30% ± 5%, or a quarter to a third being empty. Phobos has been photographed in close-up by several spacecraft whose primary mission has been to photograph Mars. The first was "Mariner 7" in 1969, followed by "Viking 1" in 1977, "Mars Global Surveyor" in 1998 and 2003, "Mars Express" in 2004, 2008, and 2010, and "Mars Reconnaissance Orbiter" in 2007 and 2008. On August 25, 2005, the "Spirit" rover, with an excess of