Photograph taken by NASA's Voyager 2 in 1989
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William Lassell, the discoverer of Triton
Galileo Galilei
The orbit of Triton (red) is opposite in direction and tilted −23° compared to a typical moon's orbit (green) in the plane of Neptune's equator.
Urbain Le Verrier
Animation of Triton
A size comparison of Neptune and Earth
The Kuiper belt (green), in the Solar System's outskirts, is where Triton is thought to have originated.
Combined colour and near-infrared image of Neptune, showing bands of methane in its atmosphere, and four of its moons, Proteus, Larissa, Galatea, and Despina
Artist's impression of Triton, showing its tenuous atmosphere just over the limb.
Bands of high-altitude clouds cast shadows on Neptune's lower cloud deck.
Clouds observed above Triton's limb by Voyager 2.
The Great Dark Spot (top), Scooter (middle white cloud), and the Small Dark Spot (bottom), with contrast exaggerated.
Interpretative geomorphological map of Triton
Four images taken a few hours apart with the NASA/ESA Hubble Space Telescope Wide Field Camera 3
Triton's bright south polar cap above a region of cantaloupe terrain
Neptune (red arc) completes one orbit around the Sun (centre) for every 164.79 orbits of Earth. The light blue object represents Uranus.
Cantaloupe terrain viewed from 130,000 km by Voyager 2, with crosscutting Europa-like double ridges. Slidr Sulci (vertical) and Tano Sulci form the prominent "X".
A diagram showing the major orbital resonances in the Kuiper belt caused by Neptune: the highlighted regions are the 2:3 resonance (plutinos), the nonresonant "classical belt" (cubewanos), and the 1:2 resonance (twotinos).
Tuonela Planitia (left) and Ruach Planitia (center) are two of Triton's cryovolcanic "walled plains". The paucity of craters is evidence of extensive, relatively recent, geologic activity.
A simulation showing the outer planets and Kuiper belt: a) before Jupiter and Saturn reached a 2:1 resonance; b) after inward scattering of Kuiper belt objects following the orbital shift of Neptune; c) after ejection of scattered Kuiper belt bodies by Jupiter
NASA illustration detailing the studies of the proposed Trident mission
Natural-colour view of Neptune with Proteus (top), Larissa (lower right), and Despina (left), from the Hubble Space Telescope
Neptune (top) and Triton (bottom) three days after flyby of Voyager 2
Neptune's moon Proteus
thumb|Close up of the volcanic province of Leviathan Patera, the caldera in the center of the image. Several pit chains extend radially from the caldera to the right of the image, while the smaller of the two cryolava lakes is seen to the upper left. Just off-screen to the lower left is a fault zone aligned radially with the caldera, indicating a close connection between the tectonics and volcanology of this geologic unit.
A composite Hubble image showing Hippocamp with other previously discovered inner moons in Neptune's ring system
thumb|Dark streaks across Triton's south polar cap surface, thought to be dust deposits left by eruptions of nitrogen geysers
Neptune's rings
thumb|Two large cryolava lakes on Triton, seen west of Leviathan Patera. Combined, they are nearly the size of Kraken Mare on Titan. These features are unusually crater free, indicating they are young and were recently molten.
In 2018, the European Southern Observatory developed unique laser-based methods to get clear and high-resolution images of Neptune from the surface of Earth.
A Voyager 2 mosaic of Triton
The appearance of a Northern Great Dark Spot in 2018 is evidence of a huge storm brewing.<ref>{{cite web |title=A storm is coming |url=https://www.spacetelescope.org/images/potw1907a/ |website=spacetelescope.org |access-date=19 February 2019 |language=en |archive-url=https://web.archive.org/web/20190220062857/https://www.spacetelescope.org/images/potw1907a/ |archive-date=20 February 2019 |url-status=live }}</ref>
The Northern Great Dark Spot and a smaller companion storm imaged by Hubble in 2020<ref>{{cite web|url=https://hubblesite.org/contents/news-releases/2020/news-2020-59.html|title=Dark Storm on Neptune Reverses Direction, Possibly Shedding Fragment|author1=Michael H. Wong|author2=Amy Simon|publisher=Hubblesite|date=15 December 2020|access-date=25 December 2020|archive-date=25 December 2020|archive-url=https://web.archive.org/web/20201225153808/https://hubblesite.org/contents/news-releases/2020/news-2020-59.html|url-status=live}}</ref>
The Great Dark Spot, as imaged by Voyager 2
Neptune's shrinking vortex<ref>{{cite web|title=Neptune's shrinking vortex|url=http://www.spacetelescope.org/images/potw1808a/|website=spacetelescope.org|access-date=19 February 2018|archive-url=https://web.archive.org/web/20180219125043/http://www.spacetelescope.org/images/potw1808a/|archive-date=19 February 2018|url-status=live}}</ref>

Triton is the largest natural satellite of the planet Neptune, and was the first Neptunian moon to be discovered, on October 10, 1846, by English astronomer William Lassell.

- Triton (moon)

Its largest moon, Triton, was discovered shortly thereafter, though none of the planet's remaining 13 known moons were located telescopically until the 20th century.

- Neptune
Photograph taken by NASA's Voyager 2 in 1989

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Neptune (top) and Triton (bottom), three days after the Voyager 2 flyby in 1989

Moons of Neptune

Neptune (top) and Triton (bottom), three days after the Voyager 2 flyby in 1989
The number of moons known for each of the four outer planets up to October 2019. Neptune currently has 14 known satellites.
Orbit diagram of Neptune's inner moons including Triton, with their names and orbit directions indicated
Size comparison of Neptune's seven inner moons
The orbit of Triton (red) is different from most moons' orbit (green) in the orbit's direction, and the orbit is tilted −23°.
The diagram illustrates the orbits of Neptune's irregular moons excluding Triton. The eccentricity is represented by the yellow segments extending from the pericenter to apocenter with the inclination represented on Y axis. The moons above the X axis are prograde, those beneath are retrograde. The X axis is labeled in Gm and the fraction of the Hill sphere's radius.
The relative masses of the Neptunian moons

The planet Neptune has 14 known moons, which are named for minor water deities in Greek mythology.

By far the largest of them is Triton, discovered by William Lassell on October 10, 1846, 17 days after the discovery of Neptune itself; over a century passed before the discovery of the second natural satellite, Nereid.

Voyager 2

Space probe launched by NASA on August 20, 1977, to study the outer planets and interstellar space beyond the Sun's heliosphere.

Space probe launched by NASA on August 20, 1977, to study the outer planets and interstellar space beyond the Sun's heliosphere.

see diagram
Animation of Voyager 2s trajectory around Jupiter ·····
The trajectory of Voyager 2 through the Jovian system
Voyager 2 left the heliosphere on November 5, 2018.
Voyager 1 and 2 speed and distance from Sun
On Voyager 2, both PWS and PRS have remained active, whereas on Voyager 1 the PRS has been off since 2007
NASA map showing trajectories of the Pioneer 10, Pioneer 11, Voyager 1, and Voyager 2 spacecraft.
The current position of Voyager 2 as of December 2018. Note the vast distances condensed into an exponential scale: Earth is one astronomical unit (AU) from the Sun; Saturn is at 10 AU, and the heliopause is at around 120 AU. Neptune is 30.1 AU from the Sun; thus the edge of interstellar space is around four times as far from the Sun as the last planet.
Voyager Golden Record
RTG inner heat source
RTG assembly
RTG unit
Voyager 2 launch on August 20, 1977, with a Titan IIIE/Centaur.
thumb|Animation of Voyager 2{{'s}} trajectory from August 20, 1977, to December 30, 2000
Trajectory of Voyager 2 primary mission.
Plot of Voyager 2{{'s}} heliocentric velocity against its distance from the Sun, illustrating the use of gravity assists to accelerate the spacecraft by Jupiter, Saturn and Uranus. To observe Triton, Voyager 2 passed over Neptune's north pole, resulting in an acceleration out of the plane of the ecliptic, and, as a result, a reduced velocity relative to the Sun.<ref>{{cite web |url=https://solarsystem.nasa.gov/basics/bsf4-1.php |title=Basics of space flight: Interplanetary Trajectories}}</ref>

A part of the Voyager program, it was launched 16 days before its twin, Voyager 1, on a trajectory that took longer to reach gas giants Jupiter and Saturn but enabled further encounters with ice giants Uranus and Neptune.

Five hours after Voyager 2 made its closest approach to Neptune, it performed a close fly-by of Triton, the larger of Neptune's two originally known moons, passing within about 40000 km.

William Lassell

English merchant and astronomer.

English merchant and astronomer.

In 1846, Lassell discovered Triton, the largest moon of Neptune, just 17 days after the discovery of Neptune itself by German astronomer Johann Gottfried Galle, using his self-built instrument.

Retrograde orbit: the satellite (red) orbits in the direction opposite to the rotation of its primary (blue/black)

Retrograde and prograde motion

Object in the direction opposite the rotation of its primary, that is, the central object .

Object in the direction opposite the rotation of its primary, that is, the central object .

Retrograde orbit: the satellite (red) orbits in the direction opposite to the rotation of its primary (blue/black)
The orange moon is in a retrograde orbit.

Retrograde satellites are generally small and distant from their planets, except Neptune's satellite Triton, which is large and close.

Nereid imaged by Voyager 2 in 1989

Nereid (moon)

Nereid imaged by Voyager 2 in 1989
Nereid's highly eccentric orbit around Neptune.

Nereid, or Neptune II, is the third-largest moon of Neptune.

The unusual orbit suggests that it may be either a captured asteroid or Kuiper belt object, or that it was an inner moon in the past and was perturbed during the capture of Neptune's largest moon Triton.

Photograph of Uranus in true colour (by Voyager 2 in 1986)

Uranus

Seventh planet from the Sun.

Seventh planet from the Sun.

Photograph of Uranus in true colour (by Voyager 2 in 1986)
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Simulated Earth view of Uranus from 1986 to 2030, from southern summer solstice in 1986 to equinox in 2007 and northern summer solstice in 2028.
Size comparison of Earth and Uranus
Diagram of the interior of Uranus
Uranus's atmosphere taken during the Outer Planet Atmosphere Legacy (OPAL) program.
Aurorae on Uranus taken by the Space Telescope Imaging Spectrograph (STIS) installed on Hubble.
The magnetic field of Uranus
(animated; 25 March 2020)
The first dark spot observed on Uranus. Image obtained by the HST ACS in 2006.
Uranus in 2005. Rings, southern collar and a bright cloud in the northern hemisphere are visible (HST ACS image).
Major moons of Uranus in order of increasing distance (left to right), at their proper relative sizes and albedos (collage of Voyager 2 photographs)
Uranus's aurorae against its equatorial rings, imaged by the Hubble telescope. Unlike the aurorae of Earth and Jupiter, those of Uranus are not in line with its poles, due to its lopsided magnetic field.
Crescent Uranus as imaged by Voyager 2 while en route to Neptune

Uranus is similar in composition to Neptune, and both have bulk chemical compositions which differ from that of the larger gas giants Jupiter and Saturn.

The Uranian satellite system is the least massive among those of the giant planets; the combined mass of the five major satellites would be less than half that of Triton (largest moon of Neptune) alone.

Pictured in 2012 in natural color. The thick atmosphere is orange due to a dense haze.

Titan (moon)

Largest moon of Saturn and the second-largest natural satellite in the Solar System.

Largest moon of Saturn and the second-largest natural satellite in the Solar System.

Pictured in 2012 in natural color. The thick atmosphere is orange due to a dense haze.
Christiaan Huygens discovered Titan in 1655.
Titan's orbit (highlighted in red) among the other large inner moons of Saturn. The moons outside its orbit are (from the outside to the inside) Iapetus and Hyperion; those inside are Rhea, Dione, Tethys, Enceladus, and Mimas.
True-color image of layers of haze in Titan's atmosphere
Trace organic gases in Titan's atmosphere—HNC (left) and HC3N (right).
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Methane clouds (animated; July 2014).
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False-color Cassini radar mosaic of Titan's north polar region. Blue coloring indicates low radar reflectivity, caused by hydrocarbon seas, lakes and tributary networks filled with liquid ethane, methane and dissolved . About half of the large body at lower left, Kraken Mare, is shown. Ligeia Mare is at lower right.
Mosaic of three Huygens images of channel system on Titan
Rimmed lakes of Titan (artist concept)
Near-infrared radiation from the Sun reflecting off Titan's hydrocarbon seas
Radar image of a 139 km-diameter impact crater on Titan's surface, showing a smooth floor, rugged rim, and possibly a central peak.
Ligeia Mare – SAR and clearer despeckled views.
Near-infrared image of Tortola Facula, thought to be a possible cryovolcano
False-color VIMS image of the possible cryovolcano Sotra Patera, combined with a 3D map based on radar data, showing 1000-meter-high peaks and a 1500-meter-deep crater.
Sand dunes in the Namib Desert on Earth (top), compared with dunes in Belet on Titan
Titan - three dust storms detected in 2009–2010.
Voyager 1 view of haze on Titan's limb (1980)
Cassini's Titan flyby radio signal studies (artist's concept)
The balloon proposed for the Titan Saturn System Mission (artistic rendition)
Global map of Titan – with IAU labels (August 2016).
Titan – infrared views (2004–2017)
Titan's North Pole (2014)
Titan's South Pole (2014)

It is second in terms of relative diameter of moons to a gas giant; Titan being 1/22.609 of Saturn's diameter, Triton is larger in diameter relative to Neptune at 1/18.092.

Neptune Odyssey

Neptune Odyssey is an orbiter mission concept to study Neptune and its moons, particularly Triton.

Neptune Moon Triton - Proposed Trident Mission (16 June 2020)

Trident (spacecraft)

Space mission concept to the outer planets proposed in 2019 to NASA's Discovery Program.

Space mission concept to the outer planets proposed in 2019 to NASA's Discovery Program.

Neptune Moon Triton - Proposed Trident Mission (16 June 2020)
alt=Venus-real color|Venus (flyby)
alt=Earth Western Hemisphere|Earth (3x flybys)
alt=Jupiter and its shrunken Great Red Spot|Jupiter (flyby)
alt=Io highest resolution true color|Io (flyby)
alt=Neptune from Voyager 2 (5278071676)|Neptune (targeted flyby)
alt=Triton moon mosaic Voyager 2 (large) - non-edit version|Triton (targeted flyby)

The concept includes flybys of Jupiter and Neptune with a focus on Neptune's largest moon Triton.

The James Webb Space Telescope launched in December 2021. By the time it launched, JWST cost about US$10 billion.

Large strategic science missions

NASA's large strategic science missions or large strategic missions, formerly known as Flagship missions or Flagship-class missions, are the costliest and most capable NASA science spacecraft.

NASA's large strategic science missions or large strategic missions, formerly known as Flagship missions or Flagship-class missions, are the costliest and most capable NASA science spacecraft.

The James Webb Space Telescope launched in December 2021. By the time it launched, JWST cost about US$10 billion.

The targets of Flagship missions may include complex missions to the atmosphere and surface of Venus, the lower atmosphere and surface of Titan, the surface and subsurface of Europa, the stormy atmosphere of Jupiter, the dusty surface of Mars, the ring systems of Saturn, the deep atmospheres of the ice giants Neptune and Uranus, the surface of the moon Triton, the plumes of Enceladus, the surface and magnetosphere of Mercury, and the surface of a comet nucleus in the form of cryogenically preserved samples.