NEPTUNE

Neptune is the eighth planet from the sun in our solar system. This giant, frigid planet has a hazy atmosphere and strong winds. This gas giant is orbited by eight moons and narrow, faint rings arranged in clumps. Neptune's blue color is caused by the methane (CH₄) in its atmosphere; this molecule absorbs red light.
Neptune cannot be seen using the eyes alone. Neptune was the first planet whose existence was predicted mathematically (the planet Uranus's orbit was perturbed by an unknown object which turned our to be another gas giant, Neptune).

SIZE
Neptune is about 30,775 miles (49,528 km) in diameter. This is 3.88 times the diameter of the Earth. If Neptune were hollow, it could hold almost 60 Earths.

Neptune is the fourth largest planet in our Solar System (after Jupiter, Saturn, and Uranus).

MASS AND GRAVITY
Neptune's mass is about 1.02 x 10²⁶ kg. This is over 17 times the mass of the Earth, but the gravity on Neptune is only 1.19 times of the gravity on Earth. This is because it is such a large planet (and the gravitational force a planet exerts upon an object at the planet's surface is proportional to its mass and to the inverse of its radius squared).

A 100-pound person would weigh 119 pounds on Neptune.

LENGTH OF A DAY AND YEAR ON NEPTUNE
Each day on Neptune takes 19.1 Earth hours. A year on Neptune takes 164.8 Earth years; it takes almost 165 Earth years for Neptune to orbit the sun once.
Since Neptune was discovered in 1846, it has not yet completed a single revolution around the sun.

NEPTUNE'S ORBIT AND DISTANCE FROM THE SUN
Neptune is about 30 times farther from the sun than the Earth is; it averages 30.06 A.U. from the sun. Occasionally, Neptune's orbit is actually outside that of Pluto; this is because of Pluto's highly eccentric (non-circular) orbit. During this time (20 years out of every 248 Earth years), Neptune is actually the farthest planet from the Sun (and not Pluto). From January 21, 1979 until February 11, 1999, Pluto was inside the orbit of Neptune. Now and until September 2226, Pluto is outside the orbit of Neptune.

At aphelion (the point in Neptune's orbit farthest from the sun) Neptune is 4,546,000,000 km from the sun, at perihelion (the point in Neptune's orbit closest from the sun) Neptune is 4,456,000,000 km from the sun.

Neptune's rotational axis is tilted 30 degrees to the plane of its orbit around the Sun (this is few degrees more than the Earth). This gives Neptune seasons. Each season lasts 40 years; the poles are in constant darkness or sunlight for 40 years at a time.

TEMPERATURE
The mean temperature is 48 K.

DISCOVERY OF NEPTUNE
Neptune's existence was predicted in 1846, after calculations showed perturbations in the orbit of Uranus. The calculations were done independently by both J.C. Adams and Le Verrier. Neptune was then observed by J.G. Galle and d'Arrest on September 23, 1846.

SPACECRAFT VISITS
Neptune was visited by NASA's Voyager 2 in August, 1989. Before this visit, virtually nothing was known about Neptune.

NEPTUNE-EARTH COMPARISON



NEPTUNE'S NAME AND SYMBOL

This is the symbol of the planet Neptune.

Neptune was named after the mythical Roman god of the seas. Neptune's symbol is the fishing spear. 

INSIDE NEPTUNE

Neptune is composed mostly of ice, hydrogen and helium. It may have a small, rocky core, and an icy mantle that blends into the atmosphere.

Neptune radiates almost three times as much heat energy as it gets from the distant Sun. Some of this excess heat is probably left over from the formation of this planet and some is generated by the slow collapse of the surface because of the planet's own gravitational forces. 

NEPTUNE'S RINGS

Neptune is circled by a system of very thin, dark rings located in clumps. These rings are are composed of small rocks and dust. They almost circular (unlike Uranus' elliptical ring system). There are three distinct rings, named Adams, Le Verrier and Galle (after the discovers of Neptune) plus a wide plateau of dust adjacent to the Le Verrier ring (this wide sheet of orbiting dust is co-orbital with the moon Galatea).

These rings have been very difficult to detect because they are not uniform in thickness and density. The thicker part of the rings are called ring arcs; these are the parts of the rings that are more easily detected. The Adams ring has three prominent arcs (named Liberty, Equality and Fraternity). The gravitational pull of Neptune's moons may cause the unevenness of the rings. Some of Neptune's smaller moons may 'shepherd' the inner rings with their gravitational forces.

NEPTUNE'S ATMOSPHERE AND THE GREAT DARK SPOT

THE ATMOSPHERE
Neptune's thick atmosphere consist of 74% hydrogen, 25% helium and about 1% methane. The atmosphere merges into the planet's mantle.

Neptune's atmosphere has icy clouds and enormous storms. These features change rapidly because of tremendous winds that whip around the planet. Neptune has the fastest winds in our Solar System.

Icy particles of methane in the outer parts of its atmosphere give Neptune its deep blue color; methane absorbs red light.

THE GREAT DARK SPOT
Neptune's Great Dark Spot is an Earth-sized hurricane in the thick methane atmosphere of Neptune. The size, shape, and location of the spot vary greatly over time; it even disappears and reappears occasionally. The storm spins counterclockwise.

Horrendous winds near the spot were measured by Voyager 2 to be about 1,500 miles per hour (2,400 kph). These are the strongest recorded winds in our solar system.

NEPTUNE'S MOONS

Neptune has three main moons, Nereid, Proteus, and Triton, plus five smaller moons. Triton and Proteus orbit close to Neptune; Nereid is in a distant orbit.

Triton is the only moon in our Solar System with a retrograde orbit (orbiting in the opposite direction than its primary, Neptune, is rotating).

The moon Triton is the coldest measured object in our Solar System, and Nereid is the Solar System object with the most eccentric orbit.

The moons of Neptune in order of distance from Neptune.

Moon	Photo	Discovery	Distance from the center of Neptune (km)	Diameter (km)	Orbital period (in Earth days)
Naiad		NASA's Voyager 2 mission, 1989	48,230	58	0.29
Thalassa		NASA's Voyager 2 mission, 1989	50,070	80	0.3
Despina		NASA's Voyager 2 mission, 1989	52,530	148	0.33
Galatea		NASA's Voyager 2 mission, 1989	61,950	158	.43
Larissa		NASA's Voyager 2 mission, 1989	73,550	104 x 89	0.5
Proteus		NASA's Voyager 2 mission, 1989	117,650	218 x 208 x 201	1.1
Neptune's second largest moon. This dark moon has a circular orbit just over the cloud-tops of Neptune. Proteus is dotted with impact craters and is irregularly shaped.
Triton		Wm. Lassell, 1846	354,760	2,704	Synchronous
The largest of the 8 moons of Neptune. Unlike all other large planetary moons, Triton has a retrograde orbit (it rotates opposite to Neptune's rotation) and is in synchronous orbit. It also has a highly inclined axis. Triton is the coldest object that has been measured in our Solar System, with a temperature of -235° C (-391° F). This rocky moon has a polar ice cap at its south pole and many other varied geologic features including volcanoes, huge cracks in the surface, and geysers of gaseous nitrogen. It has a very thin, hazy atmosphere (mostly nitrogen) and a windy surface covered with nitrogen ice. Triton is slowly spiraling in towards Neptune.
Nereid		G. Kuiper, 1949	5,513,400	340	360.1
Neptune's outermost (and third largest) moon is Nereid, which has an irregular shape. It is the smaller of Neptune's 2 larger moons. It has a VERY elliptical orbit, going as close as 867,000 miles from Neptune and as far as 6 million miles from it; it may be a captured asteroid. It takes almost one Earth year for Nereid to orbit Neptune in this extreme orbit (360.1 days). Nereid's has the most eccentric orbit in the solar system. The distance from Nereid to Neptune varies from about 1,353,600 kilometers (841,100 miles) to over seven times as far, 9,623,700 kilometers (5,980,200 miles).