orbit:    4,504,000,000 km (30.06 AU) from Sun
            diameter: 49,532 km (equatorial)
            mass:     1.0247e26 kg

    In Roman mythology Neptune (Greek: Poseidon) was the god of the Sea.

    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.

    Neptune has been visited by only one spacecraft, Voyager 2 on Aug 25 1989. Almost everything we know
    about Neptune comes from this encounter.

    Because Pluto's orbit is so eccentric, it sometimes crosses the orbit of Neptune. Since 1979 Neptune has
    actually been the most distant planet from the Sun; Pluto will again be the most distant in 1999.

    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 the result of absorption of red light by methane in the atmosphere.

    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.

    Like Jupiter and Saturn, Neptune has an internal heat source -- it radiates more than twice as much energy
    as it receives from the Sun.

  
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 white cloud that zips around Neptune every 16 hours or so now known as "The Scooter" (right). 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 has 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.
 

    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 coorbital with Galatea, then Leverrier
    (whose outer extensions are called Lassell and Arago), and finally the faint but broad Galle.

    Neptune's magnetic field is, like Uranus', oddly oriented and probably generated by motions of conductive
    material (probably water) in its middle layers.

    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.
 
 
 

Planet Profile
 

    Mass (kg)                                            102.43 x (10^24)
    Volume (km3)                                     6,254 x (10^10)
    Radius (1 bar level) (km)
        Equatorial                                        24,766
        Polar                                                 24,342
    Volumetric mean radius (km)            24,624
    Ellipticity                                               0.0171
    Mean density (kg/m^3)                       1,638
    Gravity (eq., 1 bar) (m/s^2)                 11.00
    Escape velocity (km/s)                       23.5
    GM (km^3/s^2)                                    6.8351 x (10^6)
    Bond albedo                                        0.82
    Visual geometric albedo                    0.41
    Visual magnitude V(1,0)                   -6.87
    Solar irradiance (W/m^2)                   1.47
    Black-body temperature (K)               33.2
    J2                                                          3411.x (10^-6)
 

Orbital parameters

    Semimajor axis (km)                                4,496.6 x (10^6)
    Sidereal orbit period (days)                    60,189
    Tropical orbit period (days)                     59,799.9
    Perihelion (km)                                          4,458.0 x (10^6)
    Aphelion (km)                                            4,535.2 x (10^6)
    Synodic period (days)                              367.49
    Mean orbital velocity (km/s)                     4.75
    Orbit inclination (deg)                               1.773
    Orbit eccentricity                                       0.00858
    Sidereal rotation period (hours)              16.11*
    Obliquity to orbit (deg)                              29.56

    * Magnetic coordinates (as determined by the Voyager 2 Radio Science experiment)
 
 

Neptunian Magnetosphere

    Goddard Space Flight Center OTD (O8) Model

    Dipole field strength:                                                            0.142 gauss-Rn^3
    Dipole tilt to rotational axis:                                                 46.9 degrees
    Longitude of tilt:                                                                    288 degrees (IAU convention)
    Dipole offset (planet center to dipole center) distance:   0.55 Rn

    Note: Rn denotes Neptunian radii, here defined to be 24,765 km
 
 
 

Neptunian Atmosphere

    Surface Pressure:                   >>100 bars
    Average temperature:               ~58 K
    Temperature at 1 bar:               ~72 K
    Density at 1 bar:                        ~0.45 kg/m^3
    Wind speeds:                               0-200 m/s
    Scale height:                                19.1 - 20.3 km
    Mean molecular weight:              2.53 - 2.69 g/mole
    Atmospheric composition
            Major:       Molecular hydrogen (H2) - 89%; Helium (He) - 11%

            Minor (ppm): Methane (CH4)

            Aerosols:    Ammonia ice, water ice, ammonia hydrosulfide, methane ice(?)
 
 
 

Neptune's Satellites
 

    Neptune has 8 known moons; 7 small ones and Triton.
 
 

Satellite	Distance  (km)	Raduis  (km)	Mass  (kg)	Discoverer	Date
Naiad	48,000	29	N/A	Voyager 2	1989
Thalassa	50,000	40	N/A	Voyager 2	1989
Despina	53,000	74	N/A	Voyager 2	1989
Galatea	62,000	79	N/A	Voyager 2	1989
Larissa	74,000	96	N/A	Voyager 2	1989
Proteus	118,000	209	N/A	Voyager 2	1989
Triton	355,000	1305	2.14e^22	Lassel	1846
Nereid	5,509,000	170	N/A	Kuiper	1949

 
 
 

Neptune's Rings
 
 
 

Ring	Distance  (km)	Width  (km)
Diffuse	41,900	15
Inner	53,200	15
Plateau	53,200	5800
Main	62,930	< 50

    (distance is from Neptune's center to the ring's inner edge)