Period of rotation (equatorial): 16 hr. 3 min. or 19 hr. 12 min.
Mean orbital velocity: 3.4 miles (5.4 km) per second
Inclination of axis: 29.6
Orbital eccentricity: 0.009
Orbital inclination: 1.77
Diameter at equator: 30,780 miles (49,528 km)
3.96 x Earth's diameter
Oblateness: 0.017
Mass: 1.02 x 10^26 kg
17.2 x Earth's mass
Volume: 6.18 x 10^13 km^3
54 x Earth's volume
Density: 1.64 g/cm^3
0.32 x Earth's density
Surface gravity: 11.0 m/s^2 (Earth = 1): 1.12
Escape velocity: 14.5 miles (23.3 km) per second
Mean surface temperature: -364 F / -220 C / 53 K (at cloud tops)
Maximum apparent diameter as seen from Earth (seconds of arc): 2.2"
Albedo: 0.4
Number of known satellites: 8
Mean diameter of Sun as seen from the planet: 1' 04"
Satellite statistics
Naiad (N3)
Mean distance from center of Neptune: 29,960 miles (48,200 km)
Inclination of axis: approx. 0
Rotation period: unknown
Orbital period: 0.296 days
Orbital eccentricity: 0
Orbital inclination to Neptune's equator: 0
Diameter: 31 miles (50 km)
Mass: unknown
Density: unknown
Escape velocity: approx. 0.02 mile (0.03 km) per second
Maximum apparent diameter as seen from Neptune: ---
Thalassa (N4)
Mean distance from center of Neptune: 31,100 miles (50,000 km)
Inclination of axis: approx. 0
Rotation period: unknown
Orbital period: 0.312 days
Orbital eccentricity: 0
Orbital inclination to Neptune's equator: 4.5
Diameter: 56 miles (90 km)
Mass: unknown
Density: unknown
Escape velocity: approx. 0.025 mile (0.04 km) per second
Maximum apparent diameter as seen from Neptune: ---
Despina (N5)
Mean distance from center of Neptune: 32,600 miles (52,500 km)
Inclination of axis: approx. 0
Rotation period: unknown
Orbital period: 0.333 days
Orbital eccentricity: 0
Orbital inclination to Neptune's equator: 0
Diameter: 112 miles (180 km)
Mass: unknown
Density: unknown
Escape velocity: approx. 0.055 mile (0.09 km) per second
Maximum apparent diameter as seen from Neptune: ---
Galatea (N6)
Mean distance from center of Neptune: 38,500 miles (62,000 km)
Inclination of axis: approx. 0
Rotation period: unknown
Orbital period: 0.429 days
Orbital eccentricity: 0
Orbital inclination to Neptune's equator: 0
Diameter: 100 miles (160 km)
Mass: unknown
Density: unknown
Escape velocity: approx. 0.045 mile (0.075 km) per second
Maximum apparent diameter as seen from Neptune: ---
Larissa (N7)
Mean distance from center of Neptune: 45,700 miles (73,600 km)
Inclination of axis: approx. 0
Rotation period: unknown
Orbital period: 0.554 days
Orbital eccentricity: 0
Orbital inclination to Neptune's equator: 0
Diameter: 124 miles (200 km)
Mass: unknown
Density: unknown
Escape velocity: approx. 0.06 mile (0.095 km) per second
Maximum apparent diameter as seen from Neptune: ---
Proteus (N8)
Mean distance from center of Neptune: 73,090 miles (117,600 km)
Inclination of axis: approx. 0
Rotation period: unknown
Orbital period: 1.121 days
Orbital eccentricity: 0
Orbital inclination to Neptune's equator: 0
Diameter: 260 miles (420 km)
Mass: unknown
Density: unknown
Escape velocity: approx. 0.12 mile (0.20 km) per second
Maximum apparent diameter as seen from Neptune: ---
Triton (N1)
Mean distance from center of Neptune: 220,000 miles (354,800 km)
Inclination of axis: 0
Rotation period: 5.877 days
Orbital period: 5.877 days
Orbital eccentricity: 0.0
Orbital inclination to Neptune's equator: 157
Diameter: 1,690 miles (2,720 km)
Mass: 2.14 x 10^22 kg
Density: 2.07 g/cm^3
Escape velocity: 0.9 mile (1.45 km) per second
Maximum apparent diameter as seen from Neptune: 26' 24╙
Nereid (N2)
Mean distance from center of Neptune: 3,500,000 miles (5,510,000 km)
Inclination of axis: unknown
Rotation period: unknown
Orbital period: 360.16 days
Orbital eccentricity: 0.75
Orbital inclination to Neptune's equator: 29
Diameter: 210 miles (340 km)
Mass: unknown
Density: unknown
Escape velocity: approx. 0.11 mile (0.17 km) per second
Maximum apparent diameter as seen from Neptune: 19╙ (average)
Object composition
The internal structure of Neptune is thought to be very much like that of Uranus. At the center of Neptune lies a core of rock and metal, with a thick layer of water, methane, and ammonia ices above it. Surrounding the ice is a layer of hydrogen, helium, and methane that reaches to the top of the atmosphere. Compared to Uranus, a larger proportion of the material in Neptune is dense rock; consequently Neptune, although its diameter is only 96% that of Uranus, is actually 18% more massive than its neighbor.
Core: radius of 8000 km
Ice and Water Layer: 7500 km thick
Gas Layer: 9000 km thick
Topography
Neptune has no distinct topographical features.
Atmosphere
Like the other giant planets, Neptune has a gaseous hydrogen and helium upper layer over a liquid interior. Its blue appearance is due to the presence of methane, which absorbs light in the red and near-infrared wavelengths. Neptune's atmosphere also appears to contain a haze of aerosol particles or ice crystals, the exact composition of which is unknown.
The highest wind speeds of any planet 2,400 km (1,500 miles) per hour occur on Neptune. The mean temperature at the cloud tops is -364 F / -220 C / 53 K .
The atmosphere on Neptune contains a feature known as the Great Dark Spot, a circular storm the size of Earth that resembles Jupiter's Great Red Spot. The spot spins counter-clockwise and moves west at almost 1,200 km (745 miles) per hour.
Magnetic fields and gravity
Neptune's magnetic field is tilted 47 from the planet's rotation axis, and is offset at least 0.55 radii (about 13,500 kilometers or 8,500 miles) from the physical center. The dynamo electric currents produced within the planet, therefore, must be relatively closer to the surface than for Earth, Jupiter or Saturn. The field strength at the surface varies, depending on which hemisphere is being measured, from a maximum of more than 1 gauss in the southern hemisphere to a minimum of less than 0.1 gauss in the northern. (Earth's equatorial magnetic field at the surface is 0.32 gauss.) Because of its unusual orientation and the tilt of the planet's rotation axis, Neptune's magnetic field goes through dramatic changes as the planet rotates in the solar wind. Neptune's magnetic field polarity is the same as those of Jupiter and Saturn, and opposite to that of Earth.
The surface gravity of Neptune is 1.22 times the gravity of the Earth.
Orbit and rotation
Neptune takes 164.8 years to complete one journey around the Sun. Though a lack of surface detail hinders precise calculation, the equatorial rotation period is believed by some astronomers to be 16 hours. Others believe it to be closer to 22 hours.
Moons and/or rings
For many years, there were only two known satellites: Triton and Nereid. In 1987, Voyager 2 provided proof of the existence of 6 more.
Triton, by far the largest moon, has a diameter between 6,000 and 7,000 km. (Nereid, the next largest, has a diameter of approximately 500 km.) Triton circles Neptune in a retrograde (backward) orbit and a has tenuous methane atmosphere. The ice caps on Triton are the coldest surface yet found in the solar system (-391 F / -235 C / 2 K). Researchers note that tidal forces on Triton are causing it to spiral toward Neptune. They predict that in 10 to 100 million years, the moon will be so close that Neptune's gravity will tear it apart.
Voyager 2 confirmed the existence of the arcs of metallic material surrounding the planet. It was later determined that the arcs were, in fact, thin rings surrounding the planet.
Mythology
Neptune is named for the Roman god of the sea, and its astronomical symbol is a trident.
Discovery and exploration
Neptune was the first planet to be discovered through mathematical calculation. Its position was predicted in 1845 by English astronomer John Couch Adams and Urbain Jean Joseph LeVerrier of France. In 1846, the planet was identified by Johanne Galle and Heinrich D'Arrest in Berlin. (Both Adams and LeVerrier, however, are credited with the discovery.) Triton was discovered by William Lassell in 1846. Nereid was discovered by G. P. Kuiper in 1949. Modern-day exploration of Neptune was conducted in 1989 by Voyager 2, which came within 4,850 km (3,010 miles) of the planet on August 25.
