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Pluto's origin and identity had long puzzled astronomers. One early hypothesis was that Pluto was an escaped moon of Neptune [161] knocked out of orbit by Neptune's largest moon, Triton. This idea was eventually rejected after dynamical studies showed it to be impossible because Pluto never approaches Neptune in its orbit. [162]
Proposed to orbit Pluto for three years, investigating the possibility of a subsurface ocean. [144] 4 New Horizons 2: New Horizons 2: N/A N/A NASA: Flyby Cancelled A proposed probe that would've flew by trans-Neptunian objects using a gravity assist from Uranus. [139] 5 Pluto Hop, Skip, and Jump: Pluto Hop, Skip, and Jump: N/A N/A NASA: Lander ...
A total of five planets are going retrograde between May and September: Mercury, Saturn, Uranus, Neptune and Pluto. "Retrograde" is a term used to describe when a planet's orbit appears to slow.
The nine missions include two, Ulysses and New Horizons, whose primary objectives were not outer planets, but which flew past Jupiter to gain gravity assists en route to a polar orbit around the Sun (Ulysses), and to Pluto (New Horizons). Pluto was considered a planet at the time that New Horizons launched, but was reclassified as a dwarf planet.
When Pluto was discovered, it was considered a planet, and Neptune thus became the second-farthest known planet, except for a 20-year period between 1979 and 1999 when Pluto's elliptical orbit brought it closer than Neptune to the Sun, making Neptune the ninth planet from the Sun during this period.
On August 25, 2014, New Horizons crossed the orbit of Neptune, exactly 25 years after the planet was visited by the Voyager 2 probe. [123] This was the last major planet orbit crossing before the Pluto flyby.
Timelapse of Voyager 2 approaching Jupiter. The plains of Pluto, as seen by New Horizons after its nearly 10-year voyage. Remotely guided space probes have flown by all of the observed planets of the Solar System from Mercury to Neptune, with the New Horizons probe having flown by the dwarf planet Pluto and the Dawn spacecraft currently orbiting the dwarf planet Ceres.
Neptune 25 August 1989 4389 days (12 yr, 6 days) Voyager 2 flew by Neptune and was the first spacecraft to visit it. Voyager 1: Jupiter 5 September 1977 5 March 1979 547 days (1 yr, 6 mo, 1 d) Voyager 1 flew by Jupiter and returned the first detailed images. [94] Saturn 12 November 1980 1165 days (3 yr, 2 mo, 8 d) Voyager 1 flew by Saturn. ICE