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[17.0] Missions To Neptune

v2.0.0 / chapter 17 of 20 / 01 jul 16 / greg goebel

* The planet Neptune was unknown to the ancients, only being discovered in the 19th century. For decades, this remote world remained mysterious, with the first, and so far only, close-up provided by the Voyager 2 space probe in 1989. This chapter outlines what is known about Neptune.

Neptune from Voyager 2


[17.1] THE DISCOVERY OF NEPTUNE
[17.2] NEPTUNE BEFORE THE SPACE AGE
[17.3] VOYAGER 2 AT NEPTUNE
[17.4] CLOSEUP ON TRITON
[17.5] NEW MOONS OF NEPTUNE
[17.6] RINGS OF NEPTUNE
[17.7] NEPTUNE STATISTICS

[17.1] THE DISCOVERY OF NEPTUNE

* Sir William Herschel's discovery of the planet Uranus in 1781 hinted to astronomers that there might be other planets hidden in the far reaches of the solar system, just waiting to be discovered. However, even though a planet is a very big thing, space is vast and empty, and simply searching the skies for a faint point of light is an exhausting process.

It was Uranus itself that provided a clue. In 1820, a French astronomer named Alexis Bouvard, assistant to the well-known "celestial mechanic" Pierre Simon de Laplace, began to compile data on the orbit of Uranus. He compiled the data that had been accumulated since the discovery of the planet, and using the information on its orbit then began to search older records to see if it had been observed before 1781 and mistaken for a star. Bouvard found that it had been observed at least 15 times before its formal discovery, and in a few cases logged in stellar cues. However, when he tried to incorporate the new data in his cue, he found that it didn't fit very well. The positions of Uranus did not precisely track the path that would have been expected from contemporary knowledge of its orbit.

Bouvard suspected that there had been errors in logging the positions of the planet. However, he also considered the possibility that the orbit of Uranus had been influenced by some cosmic body. After he published his findings, there were suggestions that Uranus had been hit by a large comet, but Bouvard investigated and found that the discrepancies could not be explained by a single event. They had to be the result of some long-term influence. Notions that the discrepancies were due to an unseen moon didn't work out either.

Some astronomers suspected that there might be some variation in the law of gravity far away from the Sun, while others suspected there was some error in the calculation of the masses of Jupiter and Saturn. Neither of these ideas proved productive, but there was another possibility: the orbit of Uranus was being influenced by a more distant unseen planet.

* In 1843, a brilliant young English mathematician named John Couch Adams, a fellow at Saint John's College, performed an analysis of the orbit of Uranus that assumed the deviations were due to the presence of such a "mystery planet". His analysis not only showed that the mystery planet could account for the deviations, but also pointed to where the planet might be found in the sky. Adams informed John Challis, director of the Cambridge Observatory, of his plan, and Challis expressed interest. In the fall of 1845, Adams provided Challis with the possible location of the new planet in the sky. Challis passed the information up to Sir George Airy, the British Astronomer Royal. This began a confused sequence of miscommunications that is now beyond all untangling. Nothing was done in response, and Adams was not forward enough to press the matter or seek alternative contacts.

Unknown to Adams at the time, a French mathematician named Urbain Jean Joseph Le Verrier had been pursuing the same investigations and had come to the same conclusions. Le Verrier began to contact various astronomers, including Airy, in the summer of 1846, but unlike Adams, Le Verrier was not content to wait very long for a response from any of them. Le Verrier finally wrote Johann Galle, an astronomer at the Berlin Observatory, giving him the coordinates of the unknown planet. Galle received the letter on 23 September 1846. Luckily, the observatory's director, Johann Encke, was taking the night off to celebrate his birthday with his family, leaving Galle free use of the observatory's telescope that night.

Galle and a colleague, Heinrich d'Arrest, turned the telescope on the predicted coordinates, and found a faint pinpoint of light that had not been cued before. On 25 September 1846, Galle sent a letter to Le Verrier proclaiming: "The planet whose position you have pointed out actually exists."

The finding was reported in the TIMES OF LONDON on 1 October 1846. Le Verrier found himself a celebrity, and as was his right gave the planet its name: "Neptune", the god of the seas. He then changed his mind and wanted to call it "Le Verrier" instead, but few besides himself were enthusiastic, and the planet remained "Neptune". Interestingly, later analysis of astronomical records showed that Neptune had been observed by Galileo Galilei on 28 January 1613, but Galileo had mistaken the planet for a star.

* At this point, John Herschel published an article that claimed Adams had discovered the planet first. The result was a flaming controversy, in which Challis and Airy were bitterly criticized by the British astronomical community. Airy was made out by his critics to be a fool at best and a scoundrel at worst who had thrown away a grand opportunity and let France upstage Britain. The controversy became so hot and angry that in hindsight it is impossible to determine if the whole fiasco was due to any misconduct on Airy's part, or if it had just been a series of fumblings between the reticent Adams and Airy, who was apparently very busy in both his professional and personal lives at the time.

The French and their partisans saw the whole furor as an attempt to rob le Verrier of his rightful credit. If so, it didn't work. It was and remains an entirely well-established tradition in the sciences, as well as many other fields, that the priority of discovery rests on precedence of publication, and Le Verrier formally remains the discoverer of Neptune. Adams did try to make his work known and it is now a clear part of the historical record. He never tried to claim priority over Le Verrier, and in fact the two men would become good friends. After all, they had a great deal in common.

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[17.2] NEPTUNE BEFORE THE SPACE AGE

* Neptune, the eighth planet from the Sun, orbits at an average distance of about 4.5 billion kilometers, or 30 astronomical units. It takes 164.79 years to orbit the Sun. Due to its great distance, very little was known about it before the Space Age. Although it has a high albedo of 41%, it is much too far away to be seen by the naked eye, with a maximum visual magnitude of 7.8.

Neptune is a gas giant planet, and observations showed it had a diameter about four times that of Earth. Its mass could be calculated from its orbit and showed that it is about 17 times that of Earth, implying that its density is about a quarter that of Earth. It is clearly composed mostly of hydrogen and helium, with a rocky core. There is some quantity of methane in the atmosphere that give it a blue-green color.

Observations showed that Neptune had a small equatorial bulge, which suggested a rotation rate of about 15 hours and an axial tilt relative to the plane of its orbit roughly the same as the Earth's 23 degree tilt. However, spectroscopic and photometric observations gave an estimate of roughly 17 hours. No telescopic observations of the planet ever revealed any details that could be duplicated. Radio observations indicated that Neptune emits twice as much energy as it receives from the Sun, though admittedly it receives very little solar energy, with the intensity of sunlight being 900 times less than it is at the orbit of Earth.

* A moon of Neptune was discovered on 10 October 1846, less than a month after the discovery of the planet itself, by the energetic William Lassell, who named the body "Triton". Triton is small and far away, and so its exact size was difficult to determine. Astronomers eventually placed it as being about 3,500 kilometers in diameter, but this was little more than a ballpark guess.

Spectroscopic studies of Triton performed in 1975 showed the presence of methane (CH4) and molecular nitrogen (N2) ices on the world's surface. The surface temperature was determined to be about 55 degrees Kelvin, and at that temperature some of the methane and molecular nitrogen would evaporate, giving Triton an atmosphere of sorts.

Triton is an irregular moon, the biggest of all the irregulars in the Solar System. It has a retrograde orbit at a high inclination, roughly 150 degrees. The irregular orbit suggested to astronomers that Triton had been captured by Neptune after the planet's formation. Triton's orbit was observed to be circular, with an orbital radius of about 350,800 kilometers from the center of Neptune, and with a period of 5.875 Earth days. Triton is slowly spiraling in towards the planet. Within 10 million to 100 million years, not all that long by cosmic standards, it will be torn apart by tidal forces, creating a great ring around the planet to rival or exceed the ring system of Saturn.

A second moon, "Nereid", was discovered by the Dutch-American astronomer Gerard P. Kuiper in 1949. It has a diameter of a few hundred kilometers. It is also an irregular, with a high inclination, highly eccentric orbit, ranging from about a million to nine million kilometers from the planet, and with a period of about 360 Earth days.

* The 1977 discovery of a ring system around Uranus during a stellar occultation suggested that a ring system might be found around Neptune using the same approach. However, Neptune is so far away and presents such a small target that occultations were less common than they were with Uranus. In addition, Uranus orbits the Sun tilted on its side, presenting the ring system in a "plan view" during part of its orbit that made it relatively easy to spot during an occultation. Neptune orbits the Sun at a relatively modest inclination, meaning that if there were a ring system, it would be seen more or less edge-on, making it difficult to spot.

Astronomers tried anyway and got very confusing results. Harold Reitsma and his colleagues observed a partial occultation in 1981. The star winked out once before the occultation but not after the occultation, which implied that it might have been blocked out by an unknown moon of Neptune but not a ring. A full occultation in 1983 gave no hint of a ring system.

In 1984, Andre Brahic of the University of Paris observed another occultation, with the star winking out on one side of the planet but not the other. However, to confound matters, the same behavior had been observed from another site at a distant location. That made no sense, because a moon too small to be observed could not have cast a shadow over such a distance. Some astronomers concluded that Neptune had partial rings or "ring arcs".

The notion that Neptune had ring arcs instead of complete rings only added to the mysteries of the planet. Although the introduction in the 1970s of sensitive CCD cameras for ground-based telescopes did provide improved estimates for various gross features of the planet, such as its diameter and day length, and even hinted at surface features, resolution of the puzzles had to await a visit to the planet.

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[17.3] VOYAGER 2 AT NEPTUNE

* Only a single space probe has visited Neptune. On 24 January 1989, the Voyager 2 spacecraft, after having observed Jupiter, Saturn, and Uranus, passed by Neptune at a closest approach of 81,000 kilometers. The probe then flew out of the solar system, having completed its "Grand Tour" of the Outer Planets with breathtaking success. At $865 million USD, spread over a 17 year mission, it was arguably the most cost-effective space science mission ever performed.

After Voyager 2's flyby, all the fuzzy details about Neptune suddenly fell into much sharper focus, with the spacecraft providing unarguable values for many parameters that had been the subject of debate, and also providing entirely new details.

Neptune's diameter is 49,400 kilometers, about 3.8 times the diameter of the Earth. Its day is 16 hours 3 minutes long and the inclination of its axis of rotation is 29 degrees. Triton's diameter is 2,705 kilometers, its orbital inclination is 156 degrees from Neptune's equator, and its orbital radius is 354,800 kilometers. Nereid has a diameter of about 340 kilometers, and its orbit ranges from 1.4 million kilometers to 9.7 million kilometers, with an inclination of 29 degrees.

These things were nice to know, but what Voyager 2 revealed that hadn't been known before was much more interesting. For one thing, although Voyager 2 had shown Uranus to be a featureless sphere, Neptune proved to be lively, with bands, clouds, and two large "spots" that were apparently big cyclonic storms similar to Jupiter's Great Red Spot. Neptune's "Big Dark Spot" was of the roughly the same size, relative to planetary dimensions, and at roughly the same longitude as Jupiter's Great Red Spot. At its largest, its diameter was about that of the Earth, though it changed in size over time, and it was located at about 22 degrees south latitude. There was a dot of white cirrus clouds of frozen methane above the eye of the storm.

Neptune also featured a "Little Dark Spot" at 55 degrees south latitude and with a diameter about that of Earth's moon. Like the Big Dark Spot, it had a dot of cirrus clouds above its eye. The two spots moved with planetary winds, the Great Dark Spot rotating around the planet every 16 hours, the Little Dark Spot every 17 hours. That implied the existence of winds of well more than 1,000 kilometers per hour, in some cases even reaching gusts of 2,000 kilometers per hour. Given the feeble energy the planet receives from the Sun, the weather activity on the planet was very surprising.

The methane that gives the planet its color seems to play a strong role in Neptune's atmosphere, which is mostly hydrogen and helium. Voyager 2 was able to use filters to determine the abundance of methane at different altitudes. Methane seems to circulate in an elaborate cycle from the lower regions of the atmosphere, the troposphere, to the upper regions, the stratosphere.

At the top of the troposphere, methane freezes into ice crystals, forming the white clouds observed by Voyager 2. When the methane rises higher into the atmosphere, it is photochemically turned into heavier hydrocarbons, primarily ethane and acetylene, forming smog layers that were also observed. The ethane and acetylene descend back down into the lower troposphere, where they are reconverted by some process into methane.

* Although Voyager 2 had discovered an unusual asymmetrical magnetic field at Uranus, the magnetic field of Neptune was even more bizarre. It was not only tilted 47 degrees from the planet's axis of rotation, but its axis didn't even go through the planet's center, missing it by 13,600 kilometers. At no point does Neptune's magnetic field go closer to the core of the planet than it does the surface, and it also features many irregularities. The Earth's magnetic field is generated by our planet's molten metal core. Neptune is a cold gas planet with a rocky core, and its magnetic field appears to be generated by a layer of water under high pressure, with turbulent mixing with adjacent layers setting up the field.

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[17.4] CLOSEUP ON TRITON

* When Voyager 2 made its flyby of Neptune, Nereid was far out on its elliptical orbit, 4.7 million kilometers from Neptune. The spacecraft took a single image of Nereid, covering only 20 pixels. The moon was dark gray, with an albedo of about 14%, and appeared to be irregular in shape. Its was lighter in color than other objects found by Voyager 2 around Neptune, helping confirm the assumption of astronomers from the moon's wildly elliptical orbit that it had been captured by Neptune.

In contrast, Voyager 2 was able to obtain many detailed images of Triton. The probe's trajectory took it only 38,000 kilometers from the moon's surface. Triton proved to be smaller than expected and much more reflective, apparently due to the ices on its surface. Its albedo ranges from 70% to close to a very shiny 100%. Its surface temperature is only about 30 degrees Kelvin, making it a real icebox.

Triton from Voyager II

Despite the extremely cold temperatures, Triton is more than a simple ball of ice. It has highly varied terrain -- for one thing, it is crisscrossed by fractures, ranging up to 35 kilometers wide and a thousand kilometers long. The fractures appear to be due to the fact that Triton's mantle contains a great deal of water ice. When the moon was captured, tidal strains cause the planet to melt and refreeze, from the outside in. Unlike other substances, water expands when it freezes, and this apparently created the cracks. The fractures are not broken by impact craters, which implies they were created no more than about a billion years ago, possibly not long on cosmic terms after the capture of the moon.

Another interesting feature of Triton's surface is its dimpled or "cantaloupe" terrain, consisting of a collections of more-or-less round depressions with raised rims. The biggest are about 25 kilometers in diameter and have rims about 300 meters high. They are not impact craters, since they are also more-or-less regularly spaced and of a bounded range of sizes. They are younger than the fractures, since they interrupt them in places. Nobody's sure what formed them, but they may be due to the upwelling of viscous fluids under the moon's surface.

Triton also features giant flooded and frozen basins, up to 200 kilometers across. These are possibly comet-impact craters that filled up and then froze solid. The frozen material is very likely water, since the regions feature prominences and rims about a kilometer higher than other terrain. Nitrogen and methane ices are mushy at the temperatures found on Triton and could not support such structures, but water is solid as rock. In addition, there are dark patches near Triton's seasonal icecap that appear to be frozen lakes of nitrogen, presumably upwelled from the planet's interior.

Triton terrain close-up

* Intriguingly, Triton appears to be geologically active, since it shows little evidence of cratering, meaning the craters have been generally wiped out by geological activity. This is puzzling because Triton is small, cold, and under no great tidal stress.

Planetary astronomers believe that Triton's geologic activity is due to its unusual chemistry. Most moons of the Outer Planets are essentially iceballs and have densities about the same as those of water, a gram per cubic centimeter. Triton, in contrast, has a density of about 2.05 grams per cubic centimeter, meaning that the moon is about 70% rock. The capture and melting event apparently segregated the moon's material, sorting the rock into a core about 2,000 kilometers across, with a mantle of water ice about 350 kilometers thick.

However, the core generates a small amount of heat due to radioactive materials in the rocks. This raises the lower regions of the mantle to a temperature of about 175 degrees Kelvin. That is still far below the freezing point of water, but the water is mixed with small amounts of ammonia and traces of methane, which act as anti-freeze. This material could upwell to the surface to form some of the terrain structures observed.

* Triton has geological activity due to its seasons as well. Since Neptune takes almost 165 years to orbit the Sun, the seasons on the planet and its moons are about 41 years long. In addition, given that Neptune has an axial tilt relative to the plane of its orbit of 29 degrees, and the inclination of Triton's orbit to the equator is about 23 degrees (ignoring the nicety of retrograde motion, which technically makes it 157 degrees), the Sun can appear at a maximum of 52 degrees north or south of the equator.

During a hemisphere's winter season, much of it will be in night all the time. During its summer season, much of it will be in daylight, such as it is so from the Sun, most of the time. The summer season was just beginning in the southern hemisphere during the Voyager 2 flyby, with sunlight falling on the south pole for the previous 30 years.

In the winter season, a polar icecap of frozen nitrogen forms over the dark pole of Triton. In the summer season, the icecap begins to melt. The nitrogen ice sublimates and creates a thin atmosphere of nitrogen and trace amounts of methane, with a pressure of about 1/60,000th of the Earth's at sea level. As the nitrogen melts at one pole, it freezes again at the other, being deposited as nitrogen snow and covering up surface features.

Mission scientists were then surprised when Voyager 2 observed long dark streaks as long as 160 kilometers near the moon's equator that should have been snowed over during the winter, implying that they had been created in less than one Neptunian year. They were apparently deposits of hydrocarbons that had been created by sunlight from methane. But where did they come from? Laurence Soderblom of the Voyager 2 science team discovered the answer when he examined one image that showed a dark eruption or geyser on Triton, spewing out materials to an altitude of 8 kilometers and leaving a plume 150 kilometers long.

The mission science team then started looking for other such geysers. They found another, plus three sets of plumes of what seemed to be geysers hidden underneath. Voyager 2 took images as it left the Neptune system and also spotted five more sets of what seemed to be plumes. What was particularly interesting was that this activity was taking place in regions of the moon where the Sun was directly overhead at the time. A few decades previously, it was directly overhead the equatorial regions, where the streaks were.

The geysers were almost certainly due to solar heating. Given the feeble light levels at Neptune's distance, the heating is faint, and the appearance of the geysers is clearly due to a trick of the composition of the surface of Triton. The moon has a surface layer of nitrogen ice and snow a few meters thick, overlaying water ice mixed with methane and other organic compounds. The Sun's radiation penetrates the ices, heating up the dark underlayer, eventually producing a vapor of nitrogen, water, and organics. The pressure set up by this vapor is very slight, but so is Triton's atmospheric pressure. Eventually the "hot spot" erupts and the vapor climbs high into the sky, with the winds carrying off the dark material.

* Triton's irregular orbit has long been something of a puzzle. Nobody has any serious doubt that the moon was captured by Neptune, but determining a workable process by which this could have been done has been tricky. To be trapped into an orbit around Neptune, Triton would need to have lost energy. That could have been the result of a collision with a pre-existing moon of Neptune, but that hypothetical moon would have had to be just the right size: too small and it wouldn't have done the job, too big and it would have shattered Triton.

Current models are much more plausible, suggesting that Triton was part of a binary system of bodies. The capture process broke up the binary system, with Triton's lost partner flying off into space and carrying off the needed energy. This line of thinking was provoked by the discovery, discussed in detail later, that the outer Solar System contains many free-orbiting bodies, and it is not unusual to find them organized as binaries.

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[17.5] NEW MOONS OF NEPTUNE

* Voyager 2 did discover six new moons of Neptune, bringing the total to eight. All are well inside the orbit of Triton, which was expected since Triton's unusual retrograde orbit would have caused the big moon to collide with or eject any other smaller moons in its path.

The six new moons include, from the outermost inward:

A total of five new irregular moons was spotted by telescope in 2002 and 2003, to be announced in 2004, giving 13 moons. In order of their distance from the planet, they were named:

   Halimede
   Sao
   Laomedeia
   Psamathe
   Neso

A fourteenth moon was provisionally identified in 2013 from 2004 telescope data, this moon being given the preliminary designation of "S/2004 N1". The new moons were typical irregulars: moonlets a few tens of kilometers in diameter, with wide, high-inclination orbits, some of them prograde and some of them retrograde.

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[17.6] RINGS OF NEPTUNE

* Although 1989N2 did account for the 1981 partial occultation event, it did not account for the others that had followed. As it turned out, Voyager 2 did in fact detect a number of dark rings around the planet. None are very bright, having about half the reflectivity of soot. Their dark color clearly shows they were created by a collision of Neptune's inner moons, which have a similar dark color.

Ring systems tend to be a bit tricky to categorize, since it may be hard or arbitrary to determine where one ring stops and another begins. For simplicity this document uses four. The rings include, from outward to inward:

They were seen to be complete rings, not ring arcs -- or were they? The outermost ring, Adams, proved to have five "clumps", with the density of material in the clumps up to ten times greater than in other parts of the ring. The clumps were actually given names, including "Courage", "Liberte", "Egalite 1", "Egalite 2", and "Fraternite". The likelihood of this uneven distribution happening by chance is very slight, since if the ring were under no constraints, random motions of the materials would quickly spread them evenly around the ring. However, planetary astronomers have not been able to identify any particular reason to account for the clumping. No moonlets of any size have been observed within the rings.

Some of the rings are also very narrow. This is puzzling because studies of other planetary ring systems suggested they were kept in their narrow state by "shepherd moons" that straddled them, but none of the narrow rings were flanked by two moons. The peculiar configuration of the rings suggests they may be transitory, with ongoing telescopic observations suggest that they are fading away at a rapid rate, at least by planetary standards.

* There are currently no plans to revisit Neptune. A closer look at the planet and particularly a detailed inspection of Triton would be very interesting, but the worldwide budget for space probes is relatively modest and a second mission to Neptune is not likely to happen for decades. It is lucky that we were able to take advantage of the rare alignment of the planets that permitted the "Grand Tour" to perform at least one close-up inspection of Neptune and its moons.

For now, further observations are being performed by Earth-based and spaced-based telescopes. Observations taken by the Hubble Space Telescope from Earth orbit in 1994 show that the Great Dark Spot and the DS2 dark spot had disappeared, demonstrating that such storm systems had long but still limited lifetimes. There did, however, seem to be a pattern, since a new Dark Spot appeared less than a year later.

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[17.7] NEPTUNE STATISTICS

* Statistics for Neptune:

  __________________________________________________________________________

  mean distance from Sun              30.06 AU (4496.6 x 10^6 kilometers)
  orbital period (sidereal)           164.79 years
  orbital eccentricity                0.047
  orbital inclination                 1.8 degrees
  equatorial diameter                 49,520 km (3.88 Earth)
  mass (relative to Earth)            17.131
  mean density (relative to water)    1.64
  gravity (relative to Earth)         1.18
  escape speed                        23.8 kilometers per second
  rotation period                     0.67 days
  oblateness                          1/40
  inclination of equator              29.6 degrees
  albedo                              0.41
  max surface temperature             -214 degrees Celsius (cloud tops)
  atmosphere (major constituents )    H, He, CH4, clouds of CH4
  atmospheric pressure at surface     not applicable
  number of moons > 10 km in size     8, plus a handful of known moonlets
  __________________________________________________________________________


Neptune scale diagram

* Moons of Neptune, from outermost to innermost, with pronunciations in parenthesis. Radii are measured from the center of Neptune. The abbreviation "RN" stands for the radius of Neptune, while "RM" stands for the radius of the orbit of the Earth's Moon, and "M" stands for the diameter or mass of the Earth's Moon. Densities are given relative to water, which is equivalent to grams per cubic centimeter. Orbital and rotation periods are given in days and fractions of days, with days-hours-minutes added for periods under two days.

  __________________________________________________________________________

  NEREID ("NEER-ee-ed") / Neptune II:

  mean distance from planet           5,513,400 km / 223 RN / 14.34 RM
  orbital period (sidereal)           360.13 days
  orbital eccentricity                0.75
  orbital inclination                 29 degrees
  equatorial diameter                 340 km / 0.10 M
  mass & density                      UNKNOWN
  albedo                              0.14
  year of discovery                   1949 (Kuiper)
  __________________________________________________________________________

  TRITON ("TRY-ton") / Neptune I:

  mean distance from planet           354,800 km / 14.33 RN / 0.92 RM
  orbital period (sidereal)           5.875 days
  orbital eccentricity                0.00
  orbital inclination                 157 degrees (retrograde)
  equatorial diameter                 2,705 km / 0.78 M
  mass                                0.293 M
  mean density                        2.1
  albedo                              0.8
  year of discovery                   1846 (Lassell)
  __________________________________________________________________________

  PROTEUS ("PROH-tee-us") / Neptune 1989N1 / Neptune VIII:

  mean distance from planet           117,600 km / 4.75 RN / 0.31 RM
  orbital period (sidereal)           1.121 days / 1 day 2 hours 54 minutes
  orbital eccentricity                0
  orbital inclination                 0 degrees
  equatorial diameter                 400 km / 0.12 M
  mass & density                      UNKNOWN
  albedo                              0.06
  year of discovery                   1989 (Voyager 2)
  __________________________________________________________________________

  LARISSA ("LA-ree-suh") / Neptune 1989N2 / Neptune VII:

  mean distance from planet           73,600 km / 2.97 RN / 0.19 RM
  orbital period (sidereal)           0.554 days / 13 hours 18 minutes
  orbital eccentricity                0
  orbital inclination                 0 degrees
  equatorial diameter                 190 km / 0.05 M
  mass & density                      UNKNOWN
  albedo                              0.056
  year of discovery                   1981 (Reitsma, et al)
  __________________________________________________________________________

  GALATEA ("gal-eh-TEE-uh") / Neptune 1989N4 / Neptune VI:

  mean distance from planet           62,000 km / 2.5 RN / 0.16 RM
  orbital period (sidereal)           0.429 days / 10 hours 18 minutes
  orbital eccentricity                0
  orbital inclination                 0 degrees
  equatorial diameter                 180 km / 0.05 M
  mass & density                      UNKNOWN
  albedo                              0.054
  year of discovery                   1989 (Voyager 2)
  __________________________________________________________________________

  DESPINA / Neptune 1989N3 / Neptune V:

  mean distance from planet           52,600 km / 2.12 RN / 0.14 RM
  orbital period (sidereal)           0.333 days / 8 hours
  orbital eccentricity                0
  orbital inclination                 0 degrees
  equatorial diameter                 150 km / 0.05 M
  mass & density                      UNKNOWN
  albedo                              0.06
  year of discovery                   1989 (Voyager 2)
  __________________________________________________________________________

  THALASSA ("tuh-LASS-uh") / Neptune 1989N5 / Neptune IV:

  mean distance from planet           50,000 km / 2.02 RN / 0.13 RM
  orbital period (sidereal)           0.313 days / 7 hours 31 minutes
  orbital eccentricity                0
  orbital inclination                 0 degrees
  equatorial diameter                 80 km
  mass & density                      UNKNOWN
  albedo                              0.06
  year of discovery                   1989 (Voyager 2)
  __________________________________________________________________________

  NAIAD ("NAY-ed") / Neptune 1989N6 / Neptune III:

  mean distance from planet           48,200 km / 1.94 RN / 0.12 RM
  orbital period (sidereal)           0.296 days / 7 hours 6 minutes
  orbital eccentricity                0
  orbital inclination                 0 degrees
  equatorial diameter                 54 km
  mass & density                      UNKNOWN
  albedo                              0.06
  year of discovery                   1989 (Voyager 2)
  __________________________________________________________________________

Irregular moons of Neptune:

  __________________________________________

                                preliminary
  moon                          designation
  __________________________________________

  HALIMEDE      Neptune IX       S/2002 N1 
  SAO           Neptune XI       S/2002 N2 
  LAOMEDEIA     Neptune XII      S/2002 N3 
  PSAMATHE      Neptune X        S/2003 N1 
  NESO          Neptune XIII     S/2002 N4 
  -             -                S/2004 N1
  __________________________________________


Rings of Neptune, given with orbital radius and width:

  __________________________________________________________________________

  ADAMS                 62,933 km / 2.54 RN / 0.16 RM                 50 km 
  ARAGO                 57,200 km / 2.31 RN / 0.15 RM                100 km
  LEVERRIER-LASSELL     53,200 km / 2.15 RN / 0.14 RM              4,000 km
  GALLE                 41,900 km / 1.69 RN / 0.11 RM              1,000 km
  __________________________________________________________________________
  
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