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Mercury, the smallest and innermost planet, has no moons, or at least none that can be detected to a diameter of 1.6 km (1.0 mi). [2] For a very short time in 1974, Mercury was thought to have a moon. Venus also has no moons, [3] though reports of a moon around Venus have circulated since the 17th century.
For the small outer irregular moons of Uranus, such as Sycorax, which were not discovered by the Voyager 2 flyby, even different NASA web pages, such as the National Space Science Data Center [6] and JPL Solar System Dynamics, [5] give somewhat contradictory size and albedo estimates depending on which research paper is being cited.
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]
NASA said 2024 PT5 does not qualify as a "mini-moon" because it was never captured by Earth's gravity and remained more of a "distant companion of our planet" rather than a satellite.
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.
The faint object has a visual magnitude of 24.64, comparable to the visual magnitudes of Pluto's smaller moons. [9] [19] It was initially estimated to be 300 km (190 mi) in diameter under the assumption of an albedo of 0.15, [5] though observations of a single-chord stellar occultation at Penticton, Canada on 20 October 2018 suggested a smaller ...
NASA's Galileo ' s measurements suggest that large moons can have magnetic fields; it found Ganymede has its own magnetosphere, even though its mass is only 2.5% of Earth's. [18] Alternatively, the moon's atmosphere may be constantly replenished by gases from subsurface sources, as thought by some scientists to be the case with Titan.
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.