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[1] [2] The planet orbits the Sun in 687 days [3] and travels 9.55 AU in doing so, [4] making the average orbital speed 24 km/s. The eccentricity is greater than that of every other planet except Mercury, and this causes a large difference between the aphelion and perihelion distances—they are respectively 1.666 and 1.381 AU. [5]
The size of solid bodies does not include an object's atmosphere. For example, Titan looks bigger than Ganymede, but its solid body is smaller. For the giant planets , the "radius" is defined as the distance from the center at which the atmosphere reaches 1 bar of atmospheric pressure.
Apparent sizes of the moons of Mars, Deimos and Phobos, and the Moon as viewed from the surface of their respective planets (Mars' moons imaged by the Curiosity rover, 1 August 2013) Size comparison between Phobos, Deimos and the Moon (right)
A 2023 study shows evidence, based on the orbital inclination of Deimos (a small moon of Mars), that Mars may once have had a ring system 3.5 billion years to 4 billion years ago. [32] This ring system may have been formed from a moon, 20 times more massive than Phobos , orbiting Mars billions of years ago; and Phobos would be a remnant of that ...
Unlike Deimos, Phobos is heavily cratered, [30] with one of the craters near the equator having a central peak despite the moon's small size. [31] The most prominent of these is the crater Stickney, a large impact crater some 9 km (5.6 mi) in diameter, which takes up a substantial proportion of the moon's surface area.
Size comparison between Phobos, Deimos and the Moon (right) Deimos is a gray-colored body. Like most bodies of its size, Deimos is highly non-spherical with triaxial dimensions of 16.1 km × 11.8 km × 10.2 km (10.0 mi × 7.3 mi × 6.3 mi), corresponding to a mean diameter of 12.5 km (7.8 mi) which makes it about 57% the size of Phobos. [ 7 ]
On the other hand, an observer on Mars would see the Moon rotate, with the same period as its orbital period, and would see far side features that can never be seen from Earth. Since Earth is an inferior planet, observers on Mars can occasionally view transits of Earth across the Sun. The next one will take place in 2084.
Areosynchronous orbit (ASO): A synchronous orbit around the planet Mars with an orbital period equal in length to Mars' sidereal day, 24.6229 hours. Areostationary orbit (AEO): A circular areosynchronous orbit on the equatorial plane and about 17,000 km (10,557 miles) above the surface of Mars. To an observer on Mars this satellite would appear ...