Phobos, which is one of the two moons of Mars, was discovered by American astronomer Asaph Hall on August 18, 1877. The name Phobos is derived from the Greek word for "fear," reflecting its association with the symbol of fear in Greek mythology, and it is named after one of the sons of Ares, the Greek god of war, who is equivalent to the Roman god Mars. Phobos is the larger of Mars' two moons and is notable for its irregular shape and surface characteristics.
With a size of approximately 22 kilometers (14 miles) in diameter, Phobos is significantly smaller than Earth's moon, and it orbits Mars at an exceptionally close distance, about 6,000 kilometers (3,700 miles) above the Martian surface. This proximity is unique among the moons of the solar system because Phobos orbits Mars more rapidly than the planet rotates on its axis, leading to some fascinating phenomena. For instance, observers on the Martian surface would see Phobos rising in the west and setting in the east, contrary to the usual celestial motion.
Phobos exhibits a heavily cratered surface, with some of its craters measuring over 8 kilometers (5 miles) in diameter. One of the most prominent features is Stickney Crater, a massive impact crater that is nearly half the size of Phobos itself. This suggests that Phobos has undergone numerous collisions throughout its history, contributing to its uneven topology. The surface of Phobos is coated with a fine layer of dust consisting of regolith, created by the impact of meteoroids.
Interestingly, Phobos is slowly spiraling inward toward Mars and is predicted to either crash into the planet or break apart to form a ring system in about 50 million years. The moon's surface is also marked by grooves and channels, which are believed to be the result of tidal forces exerted by Mars as well as impacts from space debris.
The exploration of Phobos has been a subject of interest for scientists and space agencies. Proposed missions, such as the joint Russian and Japanese Fobos-Grunt mission and the NASA Phobos Sample Return mission, aim to learn more about this intriguing moon and its potential history, composition, and the implications for Mars exploration.