Phobos (moon)

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Phobos colour 2008.jpg
Enhanced-color view of Phobos obtained by Mars Reconnaissance Orbiter on March 23 2008.
Discovered byAsaph Hall
Discovery dateAugust 18, 1877
Reference date J2000
Shortest distance from what it orbits9235.6 km
Longest distance from what it orbits9518.8 km
Longest distance from the center of its orbital path
("semi-major axis")
9377.2 km[1]
How long it takes to complete an orbit0.31891023 d
(7 h 39.2 min)
Average speed2.138 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 orbitsMars
Size and other qualities
Measurements26.8 × 22.4 × 18.4 km[2]
Average radius11.1 km[3]
(0.0021 Earths)
Surface area~6100 km²
(11.9 µEarths)
Volume5680 km³[4]
(5.0 nEarths)
Mass1.072×1016 kg[5]
(1.8 nEarths)
Average density1.887 g/cm³[4]
Surface gravity0.0084–0.0019 m/s²
(8.4-1.9 mm/s²)
(860-190 µg)
Escape velocity11.3 m/s (40 km/h)[5]
Rotation periodsynchronous
Turning speed11.0 km/h (at longest axis' tips)
Angle at which it turns
(in relation to its orbit)
How much light it reflects0.071[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 in tidal drag, with its orbit lowering roughly 1.8 meters per century. In about 50 million years, Phobos will reach the Roche limit, where it is likely to be torn apart. Some fragments will fall on Mars and some will form a planetary ring or rings 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 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.