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Neptune

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   CAPTION: Neptune Astronomical symbol of Neptune

   Neptune from Voyager 2
   Click image for description
                             Discovery
   Discovered by           Urbain Le Verrier
                           John Couch Adams
                           Johann Galle
   Discovered on           September 23, 1846
              Orbital characteristics ( Epoch J2000)
   Semi-major axis         4,498,252,900 km
                           30.068 963 48 AU
   Orbital circumference   28.263 T m
                           188.925 AU
   Eccentricity            0.008 585 87
   Perihelion              4,459,631,496 km
                           29.810 795 27 AU
   Aphelion                4,536,874,325 km
                           30.327 131 69 AU
   Orbital period          60,223.3528 d
                           (164.88 a)
   Synodic period          367.49 d
   Avg. Orbital Speed      5.432 km/s
   Max. Orbital Speed      5.479 km/s
   Min. Orbital Speed      5.385 km/s
   Inclination             1.769 17°
                           (6.43° to Sun's equator)
   Longitude of the
   ascending node          131.721 69°
   Argument of the
   perihelion              273.249 66°
   Number of satellites    13
                     Physical characteristics
   Equatorial diameter     49,528 km
                           (3.883 Earths)
   Polar diameter          48,681 km
                           (3.829 Earths)
   Oblateness              0.0171
   Surface area            7.619×10^9 km^2
                           (14.94 Earths)
   Volume                  6.254×10^13 km^3
                           (57.74 Earths)
   Mass                    1.0243×10^26 kg
                           (17.147 Earths)
   Mean density            1.638 g/cm^3
   Equatorial gravity
   (At 1 bar)              11.15 m/s^2
                           (1.14 g)
   Escape velocity         23.5 km/s
   Rotation period         16.11 h (16 h 6 min 36 s) ^1
   Rotation velocity       2.68 km/s = 9660 km/h (at the equator)
   Axial tilt              28.32°
   Right ascension
   of North pole           299.33° (19 h 57 min 20 s)
   Declination             42.95°
   Albedo                  0.41
   Surface temp.
                           min  mean  max
                           50 K 53 K N/A K
   Adjective               Neptunian
                    Atmospheric characteristics
   Atmospheric pressure    ≫100 kPa
   Hydrogen - H[2]         80% ±3.2%
   Helium - He             19% ±3.2%
   Methane - CH[4]         1.5% ±0.5%
   Hydrogen Deuteride - HD 192 ppm
   Ethane - C[2]H[6]       1.5 ppm

   Neptune ( IPA: /ˈnɛpt(j)uːn/) is the eighth and outermost planet in our
   solar system. It is the fourth-largest planet by diameter and the
   third-largest by mass; Neptune is 17 times the mass of Earth and is
   slightly more massive than its near twin Uranus which is 14 Earth
   Masses, but slightly smaller due to its higher density. The planet is
   named after the Roman god of the sea. Its astronomical symbol (♆,
   Unicode U+2646) is a stylized version of the god's trident.

   Neptune's atmosphere is primarily composed of hydrogen and helium, with
   traces of methane that account for the planet's blue appearance.
   Neptune's blue colour is much more vivid than that of Uranus, which has
   a similar amount of methane, so an unknown component is presumed to
   cause Neptune's intense colour. Neptune also has the strongest winds of
   any planet in the solar system, with estimates as high as 2,500 km/h or
   1,500 mph. At the time of the 1989 Voyager 2 flyby, it had in its
   southern hemisphere a Great Dark Spot comparable to the Great Red Spot
   on Jupiter. Neptune's temperature at its cloud tops is usually close to
   −210℃ (−346°F), one of the coldest in the solar system, due to its long
   distance from the sun. Neptune's center is about 7,000℃ (13,000°F),
   however, hotter than the sun's surface. This is due to extremely hot
   gases and rock in the centre.

   Faint dark colored rings have been detected around the blue planet, but
   are much less substantial than those of Saturn. When these rings were
   discovered by a team led by Edward Guinan, it was thought that they
   might not be complete but this was disproved by Voyager 2. Neptune
   possesses thirteen confirmed moons. Neptune's largest moon, Triton, is
   notable for its retrograde orbit, extreme cold (38 K), and extremely
   tenuous (14 microbar) nitrogen/methane atmosphere.

   Discovered on September 23, 1846, Neptune is notable for being the
   first planet discovered based on mathematical prediction rather than
   regular observations. Perturbations in the orbit of Uranus led
   astronomers to deduce Neptune's existence. It has been visited by only
   one spacecraft, Voyager 2, which flew by the planet on August 25, 1989.
   In 2003, there was a proposal to NASA's "Vision Missions Studies" to
   implement a " Neptune Orbiter with Probes" mission that does
   Cassini-level science without fission-based electric power or
   propulsion. The work is being done in conjunction with JPL and the
   California Institute of Technology.

Discovery

   Galileo's astronomical drawings show that he had first observed Neptune
   on December 27, 1612, and again on January 27, 1613; on both occasions
   Galileo had mistaken Neptune for a fixed star when it appeared very
   close (in conjunction) to Jupiter in the night sky. Believing it to be
   a fixed star, he cannot be credited with its discovery. At the time
   Galileo first observed Neptune on December 28, 1612, it was stationary
   in the sky because it had just turned retrograde that very day; because
   it was stationary in the sky and only beginning the planet's yearly
   retrograde cycle, its motion was far too slight to be detected with
   Galileo's small telescope.
   Size comparison of Neptune and Earth
   Enlarge
   Size comparison of Neptune and Earth

   In 1821, Alexis Bouvard published astronomical tables of the orbit of
   Uranus. Subsequent observations revealed substantial deviations from
   the tables, leading Bouvard to hypothesize some perturbing body. In
   1843, John Couch Adams calculated the orbit of an eighth planet that
   would account for Uranus' motion. He sent his calculations to Sir
   George Airy, the Astronomer Royal, who asked Adams for a clarification.
   Adams began to draft a reply but never sent it.

   In 1846, Urbain Le Verrier, independently of Adams, produced his own
   calculations but also experienced difficulties in encouraging any
   enthusiasm in his compatriots. However, in the same year, John Herschel
   started to champion the mathematical approach and persuaded James
   Challis to search for the planet.

   After much procrastination, Challis began his reluctant search in July
   1846. However, in the meantime, Le Verrier had convinced Johann
   Gottfried Galle to search for the planet. Though still a student at the
   Berlin Observatory, Heinrich d'Arrest suggested that a recently drawn
   chart of the sky, in the region of Le Verrier's predicted location,
   could be compared with the current sky to seek the displacement
   characteristic of a planet, as opposed to a fixed star. Neptune was
   discovered that very night, September 23, 1846, within 1° of where Le
   Verrier had predicted it to be, and about 10° from Adams' prediction.
   Challis later realized that he had observed the planet twice in August,
   failing to identify it owing to his casual approach to the work.

   In the aftermath of the discovery, there was much nationalistic rivalry
   between the French and the British over who had priority and deserved
   credit for the discovery. Eventually an international consensus emerged
   that both Le Verrier and Adams jointly deserved credit. However, the
   issue is now being re-evaluated by historians with the rediscovery in
   1998 of the "Neptune papers" (historical documents from the Royal
   Greenwich Observatory), which had apparently been misappropriated by
   astronomer Olin Eggen for nearly three decades and were only
   rediscovered (in his possession) immediately after his death. After
   reviewing the documents, some historians now suggest that Adams does
   not deserve equal credit with Le Verrier.

Naming

   Internal structure of Neptune
   Enlarge
   Internal structure of Neptune

   Shortly after its discovery, Neptune was referred to simply as "the
   planet exterior to Uranus" or as "Le Verrier's planet." The first
   suggestion for a name came from Galle. He proposed the name Janus. In
   England, Challis put forth the name Oceanus, particularly appropriate
   for a seafaring people. In France, Arago suggested that the new planet
   be called Leverrier, a suggestion which was met with stiff resistance
   outside France. French almanacs promptly reintroduced the name Herschel
   for Uranus and Leverrier for the new planet.

   Meanwhile, on separate and independent occasions, Adams suggested
   altering the name Georgian to Uranus, while Leverrier (through the
   Board of Longitude) suggested Neptune for the new planet. Struve came
   out in favour of that name on December 29, 1846, to the Saint
   Petersburg Academy of Sciences. Soon Neptune became the internationally
   accepted nomenclature. In Roman mythology, Neptune was the god of the
   sea, identified with the Greek Poseidon. The demand for a mythological
   name seemed to be in keeping with the nomenclature of the other
   planets, all of which, except for Uranus, were named in antiquity.

   The planet's name is translated literally as the sea king star in the
   Chinese, Korean, Japanese, and Vietnamese languages (海王星 in Chinese
   characters, 해왕성 in Korean).

Physical characteristics

   The Great Dark Spot, as seen from Voyager 2.
   Enlarge
   The Great Dark Spot, as seen from Voyager 2.

Relative size

   At 1.0243×10^26 kg Neptune is an intermediate body between Earth and
   the largest gas giants: it is seventeen Earth masses but just 1/18th
   the mass of Jupiter. It and Uranus are often considered a sub-class of
   gas giant termed "ice giants", given their smaller size and important
   differences in composition relative to Jupiter and Saturn. In the
   search for extra-solar planets Neptune has been used as a metonym:
   discovered bodies of similar mass are often referred to as "Neptunes"
   just as astronomers refer to various extra-solar "Jupiters."

Composition

   Orbiting so far from the sun, Neptune receives very little heat with
   the uppermost regions of the atmosphere at −218 °C (55 K). Deeper
   inside the layers of gas, however, the temperature rises steadily. It
   is thought that this may be leftover heat generated by infalling matter
   during the planet's birth, now slowly radiating away into space.

   The internal structure resembles that of Uranus. There is likely to be
   a core consisting of molten rock and metal, surrounded by a mixture of
   rock, water, ammonia, and methane. There is no solid surface and the
   atmosphere, extending perhaps 10 to 20 percent of the way towards the
   center, is mostly hydrogen and helium at high altitudes (80% and 19%,
   respectively). Increasing concentrations of methane, ammonia, and water
   are found as the dark, hotter and lower regions atmosphere approaches
   and finally blends into the superheated liquid interior. The pressure
   at the centre of Neptune is millions of times more than that on the
   surface of Earth. Comparing its rotational speed to its degree of
   oblateness indicates that it has its mass less concentrated towards the
   centre than does Uranus.

Magnetic field

   Neptune also resembles Uranus in its magnetosphere, with a magnetic
   field strongly tilted relative to its rotational axis at 47° and offset
   at least 0.55 radii (about 13,500 kilometres) from the planet's
   physical centre. Comparing the magnetic fields of the two planets,
   scientists think the extreme orientation may be characteristic of flows
   in the interior of the planet and not the result of Uranus' sideways
   orientation.

Weather

   Great Dark Spot (top), Scooter (middle white cloud), and the Wizard's
   eye (bottom).
   Enlarge
   Great Dark Spot (top), Scooter (middle white cloud), and the Wizard's
   eye (bottom).

   One difference between Neptune and Uranus is the level of
   meteorological activity. Uranus is visually quite bland, while
   Neptune's high winds come with notable weather phenomena. Neptune's
   atmosphere has the highest wind speeds in the solar system, thought to
   be powered by the flow of internal heat, and its weather is
   characterized by extremely violent hurricanes, with winds reaching up
   to 2000 km/h.

   In 1989, the Great Dark Spot, a cyclonic storm system the size of
   Eurasia, was discovered by NASA's Voyager 2 spacecraft. The storm
   resembled the Great Red Spot of Jupiter. However, on November 2, 1994
   the Hubble Space Telescope did not see the Great Dark Spot on the
   planet. Instead, a new storm similar to the Great Dark Spot was found
   in the planet's northern hemisphere. The reason for the Great Dark
   Spot's disappearance is unknown. Many scientists believe heat transfer
   from the planet's core disrupted the atmospheric equilibrium and
   disrupted existing circulation patterns. The Scooter is another storm
   described as a white cloud south of the Great Dark Spot. The Wizard's
   eye (Great Dark Spot 2) is a southern hurricane, the second most
   intensive hurricane on the planet.

   Unique among the gas giants is the presence of high clouds casting
   shadows on the opaque cloud deck below. Though Neptune's atmosphere is
   much more dynamic than that of Uranus, both planets are made of the
   same gases and ices. Uranus and Neptune are not strictly gas giants
   similar to Jupiter and Saturn, but are rather ice giants, meaning they
   have a larger solid core and are also made of ices. Neptune is very
   cold, with temperatures as low as -224°C (-372°F) recorded at the cloud
   tops in 1989.

Exploration of Neptune

   Voyager 2 image of Neptune
   Enlarge
   Voyager 2 image of Neptune

   The closest approach of Voyager 2 to Neptune occurred on August 25,
   1989. Since this was the last major planet the spacecraft could visit,
   it was decided to make a close flyby of the moon Triton, regardless of
   the consequences to the trajectory, similarly to what was done for
   Voyager 1's encounter with Saturn and its moon Titan.

   The probe also discovered the Great Dark Spot, which has since
   disappeared, according to Hubble Space Telescope observations.
   Originally thought to be a large cloud itself, it was later postulated
   to be a hole in the visible cloud deck.

   Neptune turned out to have the strongest winds of all the solar
   system's gas giants. In the outer regions of the solar system, where
   the Sun shines over 1000 times fainter than on Earth (still very bright
   with a magnitude of -21), the last of the four giants defied all
   expectations of the scientists.

   One might expect that the farther one gets from The Sun, the less
   energy there would be to drive the winds around. The winds on Jupiter
   were already hundreds of kilometres per hour. Rather than seeing slower
   winds, the scientists found faster winds (over 1600 km/h) on more
   distant Neptune.

   Scientists now know why this is the case —if enough energy is produced,
   turbulence is created, which slows the winds down (like those of
   Jupiter). At Neptune however, there is so little energy, that once
   winds are started, they meet very little resistance, and are able to
   maintain extremely high velocities.

Planetary rings

   Neptune has a faint planetary ring system of unknown composition. The
   rings have a peculiar "clumpy" structure, the cause of which is not
   currently understood but which may be due to the gravitational
   interaction with small moons in orbit near them.
   Neptune's rings
   Enlarge
   Neptune's rings

   Evidence that the rings are incomplete first arose in the mid- 1980s,
   when stellar occultation experiments were found to occasionally show an
   extra "blink" just before or after the planet occulted the star. Images
   by Voyager 2 in 1989 settled the issue, when the ring system was found
   to contain several faint rings. The outermost ring, Adams, contains
   three prominent arcs now named Liberté, Egalité, and Fraternité
   (Liberty, Equality, and Fraternity). The existence of arcs is very
   difficult to understand because the laws of motion would predict that
   arcs spread out into a uniform ring over very short timescales. The
   gravitational effects of Galatea, a moon just inward from the ring, are
   now believed to confine the arcs.

   Several other rings were detected by the Voyager cameras. In addition
   to the narrow Adams Ring 63,000 km from the centre of Neptune, the
   Leverrier Ring is at 53,000 km and the broader, fainter Galle Ring is
   at 42,000 km. A faint outward extension to the Leverrier Ring has been
   named Lassell; it is bounded at its outer edge by the Arago Ring at
   57,000 km.

   New Earth-based observations announced in 2005 appeared to show that
   Neptune's rings are much more unstable than previously thought. In
   particular, it seems that the Liberté ring might disappear in as little
   as one century. The new observations appear to throw our understanding
   of Neptune's rings into considerable confusion.

        Name of ring      Radius (km) Width (km)             Notes
   1989 N3R ('Galle')     41,900      15         Named after Johann Galle
   1989 N2R ('Leverrier') 53,200      15         Named after Urbain Le Verrier
   1989 N4R ('Lassell')   55,400      6          Named after William Lassell
   Arago Ring             57,600      -          Named after François Arago
   Liberté Ring Arc       62,900      -          "Leading" arc
   Égalité Ring Arc       62,900      -          "Equidistant" arc
   Fraternité Ring Arc    62,900      -          "Trailing" arc
   Courage Ring Arc       62,900      -
   1989 N1R ('Adams')     62,930      <50        Named after John Couch Adams

Natural satellites

   Neptune has 13 known moons. The largest by far, and the only one
   massive enough to be spheroidal, is Triton, discovered by William
   Lassell just 17 days after the discovery of Neptune itself. Unlike all
   other large planetary moons, Triton has a retrograde orbit, indicating
   that it was captured, and probably represents a large example of a
   Kuiper Belt object (although clearly no longer in the Kuiper Belt). It
   is close enough to Neptune to be locked into a synchronous orbit, and
   is slowly spiraling inward and eventually will be torn apart when it
   reaches the Roche limit. Triton is the coldest object that has been
   measured in the solar system, with temperatures of 38.15K (-235°C,
   -392°F).

   Triton, compared to Earth's Moon
   Name

   ( Pronunciation key)
   Diameter
   (km) Mass
   (kg) Orbital radius (km) Orbital period (days)
                              Triton trye'-tən
                              ˈtraɪtən 2700
                           (80% Luna) 2.15×10^22
                             (30% Luna) 354,800
                              (90% Luna) -5.877
                                 (20% Luna)

   Neptune's second known satellite, the irregular moon Nereid, has one of
   the most eccentric orbits of any satellite in the solar system.

   From July to September 1989, Voyager 2 discovered six new Neptunian
   moons. Of these, the irregularly shaped Proteus is notable for being as
   large as a body of its density can be without being pulled into a
   spherical shape by its own gravity. Although the second most massive
   Neptunian moon, it is only one quarter of one percent of the mass of
   Triton. Neptune's innermost four moons, Naiad, Thalassa, Despina, and
   Galatea, orbit close enough to be within Neptune's rings. The next
   farthest out, Larissa was originally discovered in 1981 when it had
   occulted a star. This was attributed to ring arcs, but when Voyager 2
   observed Neptune in 1989, it was found to have been caused by the moon.
   Five new irregular moons discovered between 2002 and 2003 were
   announced in 2004. As Neptune was the Roman god of the sea, the
   planet's moons have been named after lesser sea gods.

Appearance and visibility from Earth

   Neptune is never visible with the naked eye. The brightness of Neptune
   is between magnitudes +7.7 and +8.0, so a telescope or binoculars are
   required to observe it. With the use of a telescope it appears as a
   small blue-green disk, similar in appearance to Uranus; the blue-green
   colour comes from the methane in its atmosphere. Its small apparent
   size has made it almost impossible to study visually; even observatory
   data was fairly poor until the advent of adaptive optics.

   With an orbital period of 165 years, Neptune will soon return to the
   approximate position in the sky where Galle discovered it. This will
   happen three different times. These are April 11, 2009, when it will be
   in prograde motion; July 17, 2009, when it will be in retrograde
   motion; and finally for the last time for the next 165 years, on
   February 7, 2010. This is explained by the concept of retrogradation.
   Like all planets in the solar system beyond Earth, Neptune undergoes
   retrogradation at certain points during its synodic period. In addition
   to the start of retrogradation, other events within the synodic period
   include astronomical opposition, the return to prograde motion, and
   conjunction to the Sun.

   In its orbit around the Sun, Neptune will return to its original point
   of discovery in August 2011.

Voyager flyby

   In 1989, Voyager II flew by Neptune and the images relayed back to
   Earth became the basis of a PBS all-night program called Neptune All
   Night.

Special characters

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