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Mars has two relatively small (compared to Earth's) natural moons, Phobos (about 22 kilometres (14 mi) in diameter) and Deimos (about 12 kilometres (7.5 mi) in diameter), which orbit close to the planet. The origin of both moons is unclear, although a popular theory states that they were asteroids captured into Martian orbit. [199]
Among the other dwarf planets, Ceres has no known moons. It is 90 percent certain that Ceres has no moons larger than 1 km in size, assuming that they would have the same albedo as Ceres itself. [6] Eris has one large known moon, Dysnomia. Accurately determining its size is difficult: one indicative estimate of its radius is 350 ± 57.5 km. [7]
Compared to the Earth's Moon, the moons Phobos and Deimos are small. Phobos has a diameter of 22.2 km (13.8 mi) and a mass of 1.08 × 10 16 kg, while Deimos measures 12.6 km (7.8 mi) across, with a mass of 1.5 × 10 15 kg.
The sizes and masses of many of the moons of Jupiter and Saturn are fairly well known due to numerous observations and interactions of the Galileo and Cassini orbiters; however, many of the moons with a radius less than ~100 km, such as Jupiter's Himalia, have far less certain masses. [5]
Pluto (bottom left) compared in size to the Earth and the Moon. Pluto's diameter is 2 376.6 ± 3.2 km [5] and its mass is (1.303 ± 0.003) × 10 22 kg, 17.7% that of the Moon (0.22% that of Earth). [125] Its surface area is 1.774 443 × 10 7 km 2, or just slightly bigger than Russia or Antarctica (particularly including the Antarctic sea ice ...
The Moon is Earth's only natural satellite.It orbits at an average distance of 384,400 km (238,900 mi), about 30 times the diameter of Earth. Tidal forces between Earth and the Moon have synchronized the Moon's orbital period (lunar month) with its rotation period at 29.5 Earth days, causing the same side of the Moon to always face Earth.
At least 146 moons [33] orbit the planet, of which 63 are officially named; these do not include the hundreds of moonlets in the rings. Titan, Saturn's largest moon and the second largest in the Solar System, is larger (and less massive) than the planet Mercury and is the only moon in the Solar System that has a substantial atmosphere. [34]
The outer planets Jupiter, Saturn, Uranus and Neptune, compared to the inner planets Earth, Venus, Mars, and Mercury at the bottom right. The four outer planets, called giant planets or Jovian planets, collectively make up 99% of the mass orbiting the Sun. [h] All four giant planets have multiple moons and a ring system, although only Saturn's ...