Phobos (moon)

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Enhanced-color view of Phobos obtained by Mars Reconnaissance Orbiter on March 23 2008.
Discovered by Asaph Hall
Discovery time August 18, 1877
Reference date J2000
Shortest distance from what it orbits around 9235.6 km
Longest distance from what it orbits around 9518.8 km
Longest distance from the center of its orbital path
("semi-major axis")
9377.2 km[1]
How egg-shaped its orbit is
How long it takes to complete an orbit 0.31891023 d
(7 h 39.2 min)
Average speed 2.138 km/s
Angle above the reference plane
1.093° (to Mars's equator)
0.046° (to local Laplace plane)
26.04° (to the ecliptic)
What it orbits Mars
Size and Other Qualities
Measures 26.8 × 22.4 × 18.4 km[2]
Average distance from its center to its surface 11.1 km[3]
(0.0021 Earths)
Area of its surface ~6100 km²
(11.9 µEarths)
Volume inside it 5680 km³[4]
(5.0 nEarths)
Mass 1.072×1016 kg[5]
(1.8 nEarths)
Average density 1.887 g/cm³[4]
Gravity at its surface 0.0084–0.0019 m/s²
(8.4-1.9 mm/s²)
(860-190 µg)
Slowest speed able to escape into space
("escape velocity")
11.3 m/s (40 km/h)[5]
How long it takes to turn around one time synchronous
Turning speed 11.0 km/h (at longest axis' tips)
Angle at which it turns
(in relation to its orbit)
How much light it reflects 0.071[3]
Avg. surface temp. ~233 K
Seeming brightness
("apparent magnitude")
Orbits of Phobos and Deimos (to scale), as seen from above Mars

Phobos (or Mars I) is one of Mars' moons. The other is Deimos.

Phobos is the larger of the two moons, and is only 27 kilometers in diameter. This is about as far as a car can travel on the highway in 15 minutes. It is covered with craters, as Earth's moon is.[7]

It is named after the god Phobos in Greek mythology. Its name means "fear".

Phobos is trapped into tidal drag, and its lowering orbit roughly 1.8 meters per century. In about 50 million years, Phobos is likely to collide into Martian atmosphere, or get torn apart and form some ring systems around Mars.

The other moon, Deimos, is the smaller of the two.

Spacecraft[change | change source]

The Russians have sent at least two space craft to this moon, Phobos 1 and Phobos 2. Both failed or lost contact with Earth, but Phobos 2 managed to take some pictures of the moon before dying.

Features[change | change source]

There is one large crater on Phobos called Stickney. It is 1/3 the size of the moon itself.

References[change | change source]

  1. NASA Celestia
  2. "Mars: Moons: Phobos". NASA Solar System Exploration. 2003-09-30. Retrieved 2008-08-18.
  3. 3.0 3.1 "Planetary Satellite Physical Parameters". JPL (Solar System Dynamics). 2006-07-13. Retrieved 2008-01-29.
  4. 4.0 4.1 "Mars Express closes in on the origin of Mars' larger moon". DLR. 2008-10-16. Retrieved 2008-10-16.
  5. 5.0 5.1 use a spherical radius of 11.1 km; volume of a sphere * density of 1.877 g/cm³ yields a mass (m=d*v) of 1.07×1016 kg and an escape velocity (sqrt((2*g*m)/r)) of 11.3 m/s (40 km/h)
  6. "Classic Satellites of the Solar System". Observatorio ARVAL. Retrieved 2007-09-28.
  7. Gater, Will (2009). Space 3D. Bristol Magazines. p. 67.