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Phobos orbits so fast (with a period of just under one third of a sol) that it rises in the west and sets in the east, and does so twice per sol; Deimos on the other hand rises in the east and sets in the west, but orbits only a few hours slower than a Martian sol, so it spends about two and a half sols above the horizon at a time.
After years of analysis, Kepler discovered that Mars's orbit was likely to be an ellipse, with the Sun at one of the ellipse's focal points. This, in turn, led to Kepler's discovery that all planets orbit the Sun in elliptical orbits, with the Sun at one of the two focal points. This became the first of Kepler's three laws of planetary motion.
(The two Mars Exploration Rovers happen to be approximately 12 hours and one minute apart.) Since the late 1990s and arrival of Mars Global Surveyor at Mars, the most widely used system for specifying locations on Mars has been planetocentric coordinates, which measure longitude 0°–360° East and latitude angles from the center of Mars. [11]
In gravitationally bound systems, the orbital speed of an astronomical body or object (e.g. planet, moon, artificial satellite, spacecraft, or star) is the speed at which it orbits around either the barycenter (the combined center of mass) or, if one body is much more massive than the other bodies of the system combined, its speed relative to the center of mass of the most massive body.
Phobos orbits closer to Mars, with a semi-major axis of 9,377 km (5,827 mi) and an orbital period of 7.66 hours; while Deimos orbits farther with a semi-major axis of 23,460 km (14,580 mi) and an orbital period of 30.35 hours. Two major hypotheses have emerged as to the origin of the moons: The first suggests that they originated from Mars ...
Several factors make placing a spacecraft into an areostationary orbit more difficult than a geostationary orbit. Since the areostationary orbit lies between Mars's two natural satellites, Phobos (semi-major axis: 9,376 km) and Deimos (semi-major axis: 23,463 km), any satellites in the orbit will suffer increased orbital station keeping costs due to unwanted orbital resonance effects.
A satellite in areosynchronous orbit does not necessarily maintain a fixed position in the sky as seen by an observer on the surface of Mars; however, such a satellite will return to the same apparent position every Martian day. The orbital altitude required to maintain an areosynchronous orbit is approximately 17,000 kilometres (11,000 mi).
Deimos (/ ˈ d aɪ m ə s /; systematic designation: Mars II) [11] is the smaller and outer of the two natural satellites of Mars, the other being Phobos. Deimos has a mean radius of 6.2 km (3.9 mi) and takes 30.3 hours to orbit Mars. [5] Deimos is 23,460 km (14,580 mi) from Mars, much farther than Mars's other moon, Phobos. [12]