Neptune
Neptune ... The Blue Giant
Introduction to Neptune
Neptune was the final object visited by Voyager 2 in 1989, and since that    encounter, the Voyager 2 spacecraft has continued on its journey out of the    Solar System. You cannot see this planet with your naked eye, and it is barely    visible under the darkest sky with a pair of binoculars. Earth-bound telescopes    reveal some details, but almost everything we know comes from that single satellite    mission. Neptune may rival Saturn for beauty. Its blue color is unlike any other.    Neptune is the windiest planet in the Solar System and holds the only major    moon that orbits in a clockwise direction. Its discovery in 1849 is an interesting    story, similar to the serendipitous discovery of Pluto.
Planetary Data
Naiad, Thalassa, Despina, Galatea, Larissa, Proteus, Triton, Nereid,          5 new moons not yet given formal names
More Information on the Planet Neptune from the Nine Planets Website
Much of the information below is direct from the Nine    Planets Website . Some material has been altered by me for this course, while    other items and comments are directly copied. I hope to maintain a continuous    update of this material to keep up with the findings from space satellites and    telescopes.
Neptune is the eighth planet from the Sun and the fourth largest (by diameter). Neptune    is smaller in diameter but larger in mass than Uranus.
orbit: 4,504,000,000 km (30.06 AU) from Sun
diameter: 49,532 km (equatorial)
 
 
After the discovery of Uranus, it was noticed that its orbit was not as it should    be in accordance with Newton's laws. It was therefore predicted that another    more distant planet must be perturbing Uranus' orbit. Neptune was first observed    by Galle and d'Arrest on 1846 Sept 23 very near to the locations independently    predicted by Adams and Le Verrier from calculations based on the observed positions    of Jupiter, Saturn and Uranus. An international dispute arose between the English    and French (though not, apparently between Adams and Le Verrier personally)    over priority and the right to name the new planet; they are now jointly credited    with Neptune's discovery. Subsequent observations have shown that the orbits    calculated by Adams and Le Verrier diverge from Neptune's actual orbit fairly    quickly. Had the search for the planet taken place a few years earlier or later    it would not have been found anywhere near the predicted location.
More than two centuries earlier, in 1613, Galileo observed Neptune when it    happened to be very near Jupiter, but he thought it was just a star. On two    successive nights he actually noticed that it moved slightly with respect to    another nearby star. But on the subsequent nights it was out of his field of    view. Had he seen it on the previous few nights Neptune's motion would have    been obvious to him. But, alas, cloudy skies prevented obsevations on those    few critical days.
Neptune has been visited by only one spacecraft, Voyager    2 on Aug 25, 1989. Much of we know about Neptune comes from this single    encounter. But fortunately, recent ground-based and HST observations have added    a great deal of in formation, too.
Because Pluto's orbit is so eccentric, it sometimes crosses the orbit of    Neptune, making Neptune the most distant planet from the Sun for a few years.    This was actually the case between 1979 and 1999, but Pluto has since moved    back out farther from the Sun than any other planet.
Neptune's    composition is probably similar to Uranus': various "ices" and rock    with about 15% hydrogen and a little helium. Like Uranus, but unlike Jupiter    and Saturn, it may not have a distinct internal layering but rather to be more    or less uniform in composition. But there is most likely a small core (about    the mass of the Earth) of rocky material. Its atmosphere is mostly hydrogen    and helium with a small amount of methane.
 
 
 
Neptune's    blue color is largely the result of absorption of red light by methane in the    atmosphere but there is some additional as-yet-unidentified chromophore which    gives the clouds their rich blue tint.
Like a typical gas planet, Neptune has rapid winds confined to bands of latitude    and large storms or vortices. Neptune's winds are the fastest in the solar system,    reaching 2000 km/hour. This is hard for some students to grasp. So, consider    the smell of methane gas (you know the source of this gas since your study of    Uranus was completed earlier). Imagine the rotten smell of methane gas blowing    up your nose at 2000 km/hr! This makes the cloudtops of Neptune a less-than-exciting    place to visit.
 
 
 
Like Jupiter and Saturn, Neptune has an internal heat source -- it radiates    more than twice as much energy as it receives from the Sun. So, although the    temperatures are very cold (well below -200 C) the interior of the planet releases    heat that causes large convection within the planet's interior, and as this    heat wells up toward the upper layers of the gas giant, it makes Neptune warmer    at the cloudtops than it would otherwise be.
 
At    the time of the Voyager encounter, Neptune's most prominent feature was the    Great Dark Spot (left) in the southern hemisphere. It was about half the size    as Jupiter's Great Red Spot (about the same diameter as Earth). Neptune's winds    blew the Great Dark Spot westward at 300 meters/second (700 mph). Voyager 2    also saw a smaller dark spot in the southern hemisphere and a small irregular    and almost triangular-shaped white cloud that zips around Neptune every 16 hours    or so now known as "The Scooter" (below). It may be a plume rising    from lower in the atmosphere but its true nature remains a mystery.
 
 
 
However, HST    observations of Neptune (left) in 1994 show that the Great Dark Spot had disappeared!    It has either simply dissipated or is currently being masked by other aspects    of the atmosphere. A few months later HST discovered a new dark spot in Neptune's    northern hemisphere. This indicates that Neptune's atmosphere changes rapidly,    perhaps due to slight changes in the temperature differences between the tops    and bottoms of the clouds. Then, in 1999, the HST took additional photos of    Neptune and discovered that the Dark Spot had reappeared.
 
 
Neptune    has banded weather, as do the other gas planets, not as pronounced as that of    Jupiter, but more so than Uranus. The image to the left shows some of the clouds    in a weather band, and what is so striking in this photograph is the vertical    relief nature of these clouds. Earth's atmosphere has vertical clouds, but lacks    the tremenously high velocity winds of Neptune that take such clouds and blow    them into long bands instead of the pretty cumulus clouds of our home planet.
 
 
Neptune's Rings
Neptune    also has rings. Earth-based observations showed only faint arcs instead of complete    rings, but Voyager 2's images showed them to be complete rings with bright clumps.    One of the rings appears to have a curious twisted structure.
Like Uranus and Jupiter, Neptune's rings are very dark but their composition    is unknown.
Neptune's rings have been given names: the outermost is Adams (which contains    three prominent arcs now named Liberty, Equality and Fraternity), next is an    unnamed ring co-orbital with Galatea, then Leverrier (whose outer extensions    are called Lassell and Arago), and finally the faint but broad Galle.
 
MAGNETIC FIELD
Neptune's magnetic field is, like Uranus', oddly oriented and probably generated    by motions of conductive material (probably water) in its middle layers. The    orientation and location of the center of the magnetic field is a mystery, just    as it is within the planet Uranus. The magnetic field of Neptune is offset from    the center of the planet by almost two-thirds of its radius, and is tilted 47    degrees from the geographic pole.
Neptune can be seen with binoculars (if you know exactly where to look) but    a large telescope is needed to see anything other than a tiny disk. There are    several Web sites that show the current position of Neptune (and the other planets)    in the sky, but much more detailed charts will be required to actually find    it. Such charts can be created with a planetarium program such as Starry Night.
Neptune's Satellites
Neptune has 13 known moons; 12 small ones and Triton .    Most of the moons of Neptune are small and insignificant, but Triton is a fascinating    object whose surface temperature makes it the coldest object in the Solar System,    and whose orbit is retrograde. This means that it is orbiting clockwise instead    of counter-clockwise, as all of the major moons of the solar system do. Please    go to the Triton page to learn more    about its secrets.
moon
 
2002
Below are three images of Neptunian moons, Triton, Proteus, and Nereid, from    left to right as you look below. The images of Proteus and Nereid are really    fuzzy, but at least you can get a peek at these tiny moons. The reason Proteus    was discovered AFTER Nereid, even though it is larger is because Nereid orbits    farther away and Proteus is too close to Neptune to be seen with a large, Earth-bound    telescope.
 
During the latter part of 2002, a team of scientists led by Matthew Holman    of the Smithsonian Center for Astrophysics and J.J. Kavelaars of Canada's National    Research Council found three new moons orbiting Neptune. They are all very small    and are the first moons to be discovered around Neptune since the Voyager 2    flyby in 1989, and they are the first to be observed from a ground-based telescope    since 1949. What makes the discovery of these little moons so interesting is    the realization that each giant planet may have an irregular population of moons    caused by some ancient collision between a former moon and a passing comet or    asteroid. The collision would produce chunks of the original moon and eject    other parts.
Please go to the webpage that describes the unusual rings of Neptune . Below is a chart    that gives a statistical overview of the faint rings of Neptune. (distance is    from Neptune's center to the ring's inner edge)
Ring