Neptune is the outermost planet of the gas giants. It has an equatorial
diameter of 49,500 kilometers (30,760 miles) and is the eighth planet from the
sun. If Neptune were hollow, it could contain nearly 60 Earth\'s. Neptune
orbits the Sun every 165 years. It has eight moons, six of which were found by
Voyager 2. A day on Neptune is 16 hours and 6.7 minutes. Neptune was
discovered on September 23, 1846 by Johann Gottfried Galle, of the Berlin
Observatory. Neptune got its named from the Roman God of the Sea. Much of what
is know today about Neptune was discovered in 1989 by the U.S Voyager 2
spacecraft during its 1989 flyby f Neptune. Neptune as compared to Earth is 3.9
times the diameter, 30 times the distance from the sun, 17 times as massive, and
0.3 times the density. Neptune travels around the Sun in an elliptical orbit at
an average distance of 4.504 billion km (2.799 billion miles). Neptune consists
largely of hydrogen and helium, and it has no apparent solid surface. The first
two thirds of Neptune is composed of a mixture of molten rock, water, liquid
ammonia and methane. The outer third is a mixture of heated gases comprised of
hydrogen, helium, water and methane. The atmospheric composition is 85%
Hydrogen, 13% Helium, and 2% methane. The planet\'s atmosphere, particularly
the outer layers, contains substantial amounts of methane gas. Absorption of
red light by the atmospheric methane is responsible for Neptune\'s deep blue
color. Neptune is a dynamic planet with several large, dark spots reminiscent of
Jupiter\'s hurricane-like storms. The largest spot, known as the Great Dark Spot,
is about the size of the earth and is similar to the Great Red Spot on Jupiter.
Neptune receives less than half as much sunlight as Uranus, but heat escaping
from its interior makes Neptune slightly warmer than Uranus. The heat liberated
may also be responsible for Neptune\'s stormier atmosphere, which exhibits the
fastest winds seen on any planet in the solar system. Most of the winds there
blow westward, opposite to the rotation of the planet. Near the Great Dark Spot,
winds blow up to 2,000 kilometers (1,200 miles) an hour. Voyager 2 found that
the winds averaged about 300 meters per second (700 miles/hour) in the planet\'s

Long bright clouds, similar to cirrus clouds on Earth, were seen high in
Neptune\'s atmosphere. At low northern latitudes, Voyager captured images of
cloud streaks casting their shadows on cloud decks below.

Feathery white clouds fill the boundary between the dark and light blue regions
on the Great Dark Spot. The pinwheel shape of both the dark boundary and the
white cirrus suggests that the storm system rotates counterclockwise. Periodic
small scale patterns in the white cloud, possibly waves, are short lived and do
not persist from one Neptunian rotation to the next. (Courtesy NASA/JPL)

Until the Voyager 2 encounter in 1989, the rings surrounding Neptune were
thought to be arcs. We now know that the rings completely circle the planet, but
the thickness of each ring varies along its length. Neptune has a set of four
rings which are narrow and very faint. The rings are made up of dust particles
thought to have been made by tiny meteorites smashing into Neptune\'s moons.
From ground based telescopes the rings appear to be arcs but from Voyager 2 the
arcs turned out to be bright spots or clumps in the ring system. The exact
cause of the bright clumps is unknown. The magnetic field of Neptune, like that
of Uranus, is highly tilted at 47 degrees from the rotation axis and offset at
least 13,500 kilometers or 8,500 miles from the physical center. 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
that planet\'s sideways orientation or of any possible field reversals within the
planet. Neptune also has eight known satellites. Only two of these, Triton and
Nereid, had been observed prior to the Voyager 2 flyby. Triton is the largest
of the eight satellites and is almost as big as the Earth\'s Moon. The other
Neptunian satellites range in diameter from 58 to 416 km (36 to 258 miles).
Apart from Triton, the moons of Neptune are irregularly shaped and have very
dark surfaces.

Triton is the largest moon of Neptune, with a diameter of 2,700 kilometers
(1,680 miles). It was discovered by William Lassell, a British astronomer, in
1846 scarcely a month after Neptune was discovered. Triton is colder than any
other measured object in the