Neptune has been visited by only one spacecraft, Voyager 2 on Aug 25 1989. It was the last stop in 1989 for the Voyager 2 spacecraft on its grand tour of the solar system.
Recent knowledge has been gained by ground-based telescopes and the Hubble Space Telepscope.
Due to Pluto's eccentric orbit, Neptune at times becomes the most distant planet from the Sun for a few years.
Planet Neptune Facts:
Mass: 17.15 Earth-masses
Number of known satellites: 8
Length of Year: 164.8 Earth-years
Mean Distance from the Sun: 4,500 million kilometers
Mean Orbital Velocity: 5.4 kilometers per second
Length of Day: 16.11 hours, 0.67 Earth-day
Equatorial diameter: 49,500 kilometers
Atmospheric components: 74% hydrogen, 25% helium, 1% methane
Moons of Planet Neptune
Neptune has 8 known moons: Triton, Thalassa, Naiad, Despina, Galatea, Larissa, Proteus and Nereid
Triton has the coldest temperatures in the solar system. Voyager observed geysers.
The Great Dark Spot
Voyager 2 observed the Great Dark Spot. Recent observations made with the Hubble Space Telescope indicate that the Great Dark Spot no longer exists.
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