Neptune

File:Neptune thumb.jpg Neptune as photographed by NASA's Voyager 2 probe. In the summer of 1989, Voyager 2 became the first spacecraft to observe the planet Neptune, its final planetary target. It passed about 4,950 kilometers (3,000 miles) above Neptune's north pole. larger image

Orbiting so far from the sun, Neptune receives very little heat. Its 'surface' temperature is -218 degrees Celsius (below zero). However, the planet seems to have an internal source of heat. It is thought that this may be leftover heat generated by infalling matter during the planet's birth, now slowly radiating away into space. Neptune's atmosphere has the highest wind speeds in the solar system, up to 2000 km/h, thought to be powered by this flow of internal heat.

The internal structure resembles that of Uranus - a rocky core covered by an icy crust, buried deep under its thick atmosphere. The inner 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. Like Uranus, and unlike the uniform composition of Jupiter and Saturn, Neptune's internal structure is thought to consist of distinct layers. Like Uranus, Neptune's magnetic field is strongly tilted relative to its rotational axis at 47° and offset at least 0.55 radii (about 13,500 kilometers) from the planet's 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 Uranus' sideways orientation.

The exploration of Neptune

After the discovery of Uranus, it was noted that the orbits of Uranus, Saturn, and Jupiter were apparently being perturbed by an additional unknown mass in the outer solar system. John Couch Adams and Urbain Jean Joseph Le Verrier used calculations to predict the region of the sky where the mysterious additional planet was likely to be located, and it was subsequently independently discovered by the German astronomer Johann Gottfried Galle of the Berlin Observatory on September 23 1846, near the star δ Capricorni. Later it was found that Galileo had observed Neptune in early 1611 on the occasion of its conjunction with Jupiter. Evidence of the fact was found in his notes, but he had thought it was a star. Had he observed Neptune only a matter of days earlier, its orbital motion would have been far more obvious. With an orbital period of 165 years, Neptune will first return to the point in its orbit where Galle discovered it in 2011. Due to Pluto's eccentric orbit, Neptune is sometimes the farthest known planet from the Sun.

Neptune is never visible with the naked eye. With the use of a telescope it appears as a blue-green disk, similar in appearance to Uranus; the blue-green colour comes from the methane in its atmosphere. Neptune has been visited by only one spacecraft, Voyager 2, which flew by the planet on August 25 1989.

The moons of Neptune

Neptune has a faint planetary ring system of unknown composition. The rings have a peculiar "clumpy" structure, the cause of which is not currently known but which may involve gravitational interaction with small moons in orbit near them. 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 of which, Adams, contains three prominent arcs now named 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 center 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.

Neptune has eleven known moons.

Neptune's natural satellites
Name	Diameter (km)	Mass (kg)	Mean orbital radius (km)	Orbital period
Naiad	58	Unknown	48,200	0.294396 days
Thalassa	80	Unknown	50,000	0.311485 days
Despina	148	Unknown	52,600	0.334655 days
Galatea	158	Unknown	62,000	0.428745 days
Larissa	193 (208 × 178)	Unknown	73,600	0.554654 days
Proteus	418 (436 × 416 × 402)	Unknown	117,600	1.122315 days
Triton	2700	2.14×10²²	354,760	-5.87685 days
Nereid	340	Unknown	5,513,400	360.1362 days

Three additional moons are 29-39km in diameter.

Solar system:
Sun - Mercury - Venus - Earth - Mars - Asteroids - Jupiter - Saturn - Uranus - Neptune - Pluto - Comets