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Asaph Hall discovered Phobos on August 12, 1877, at the US Naval Observatory in Washington. It was named after one of the mythological sons of Ares (Mars) and Aphrodite (Venus). Phobos is Greek for "fear". Phobos is closer to its primary than any other moon in the Solar System: it in fact orbits below the synchronous orbit radius. It rises in the west and sets in the east as seen from the Martian surface, crossing the sky in 4.5 hours. To a Martian stargazer Phobos would appear as bright as the planet Venus as seen from Earth. Phobos is so close to the surface that it cannot be seen at latitudes higher than 69º. Tidal forces will likely destroy Phobos within 100 million years. Phobos is heavily cratered: its largest impact feature, Stickney, was named after the wife of Asaph Hall. Stickney crater is 10 km in diameter. Phobos probably came close to breaking up from the impact that produced Stickney. Radiating away from Stickney are sets of parallel grooves or striations.



Hall discovered Deimos on August 10, 1877. In Greek mythology, Deimos is one of the sons of Ares and Aphrodite, "deimos" is Greek for "panic", usually rendered as "terror". Deimos would be visible at latitudes lower than 82º and would appear as bright as the star Sirius, the most luminous star as seen from Earth - or Mars.


Both moons are likely captured C-type asteroids, composed of carbon-rich rock and ice. The Soviet spacecraft Phobos 2 detected a faint but steady outgassing from Phobos, possibly water.

Property Phobos Deimos
Mean distance from center of Mars (km) 9270 23400
Mean Siderial period (days) 0.3189 1.2624
Mean Synodic period 7 hr 39 min 26.6 sec 1 d 6 hr 21 min 15.7 sec
Orbital eccentricity 0.0210 0.0028
Orbital inclination (degrees) 1.1 1.8
Diameter (km) 20 x 23 x 28 10 x 12 x 6
Mass (kg) 9.6 x 1015 2.0 X 1015
Mean density (g/cm3) 1.9 2.1
Escape velocity (m/sec) 15 10
Magnitude at mean opposition 11.6 12.8
Magnitude from surface of Mars -3.9 -0.1
Siderial period
The time taken for a planet or satellite to complete one revolution about its primary.
Synodic period
The interval of time between any planetary configuration of a celestial body with respect to the sun and the next successive configuration of that body.
Orbital eccentricity
e=sqrt(1-b2/a2) where a is the semi major axis, b is the semi minor axis. e for a perfectly circular orbit is 0
Orbital inclination
The angle between the plane of an orbit and a reference plane. The equator is the reference for geocentric orbits and the ecliptic for heliocentric orbits.

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