Neptune Fact Sheet


Planet Neptune

Planet Neptune


Neptune/Earth Comparison


Bulk parameters

                                   Neptune      Earth   Ratio (Neptune/Earth)
Mass (1024 kg)                     102.42      5.9726        17.147
Volume (1010 km3)                    6,254   108.321         57.74 
Radius (1 bar level) (km)
    Equatorial                     24,764      6,378.1        3.883
    Polar                          24,341      6,356.8        3.829
Volumetric mean radius (km)        24,622      6,371.0        3.865
Ellipticity (Flattening)            0.01708    0.00335        5.10
Mean density (kg/m3)                1,638      5,514          0.297
Gravity (eq., 1 bar) (m/s2)        11.15       9.80           1.14
Acceleration (eq., 1 bar) (m/s2)   11.00       9.78           1.12
Escape velocity (km/s)             23.5       11.19           2.10
GM (x 106 km3/s2)                   6.8351     0.3986        17.15 
Bond albedo                         0.290      0.306          0.95
Visual geometric albedo             0.41       0.367          1.12
Visual magnitude V(1,0)            -6.87      -3.86             -
Solar irradiance (W/m2)             1.51       1,367.6        0.0011
Black-body temperature (K)         46.6      254.3            0.183
J2 (x 10-6)                      3411.      1082.63           3.151    
Number of natural satellites       14          1
Planetary ring system             Yes         No

Orbital parameters

                                   Neptune      Earth   Ratio (Neptune/Earth)
Semimajor axis (106 km)           4,495.06      149.60       30.047
Sidereal orbit period (days)     60,189.        365.256     164.79   
Tropical orbit period (days)     59,799.9       365.242     163.73
Perihelion (106 km)               4,444.45      147.09       30.216     
Aphelion (106 km)                 4,545.67      152.10       29.886
Synodic period (days)               367.49        -             -
Mean orbital velocity (km/s)          5.43      29.78         0.182   
Max. orbital velocity (km/s)          5.50      30.29         0.182       
Min. orbital velocity (km/s)          5.37      29.29         0.183      
Orbit inclination (deg)               1.769       0.000         -
Orbit eccentricity                    0.0113      0.0167      0.677
Sidereal rotation period (hours)     16.11*      23.9345      0.673
Length of day (hrs)                  16.11       24.0000      0.671
Obliquity to orbit (deg)             28.32       23.44        1.208

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


Neptune Observational Parameters

Discoverer:      Johann Gottfried Galle  (based on predictions by  
                 John Couch Adams and Urbain Leverrier)
Discovery Date:  23 September 1846

Distance from Earth
        Minimum (106 km)         4305.9
        Maximum (106 km)         4687.3
Apparent diameter from Earth
        Maximum (seconds of arc)    2.4
        Minimum (seconds of arc)    2.2
Mean values at opposition from Earth
        Distance from Earth (106 km)       4347.31    
        Apparent diameter (seconds of arc)    2.3
        Apparent visual magnitude             7.8      
Maximum apparent visual magnitude             7.78

Neptune Mean Orbital Elements (J2000)

Semimajor axis (AU)                 30.06896348  
Orbital eccentricity                 0.00858587   
Orbital inclination (deg)            1.76917  
Longitude of ascending node (deg)  131.72169   
Longitude of perihelion (deg)       44.97135  
Mean Longitude (deg)               304.88003

North Pole of Rotation

Right Ascension: 299.36 + 0.70 sin N
Declination    :  43.46 - 0.51 cos N
Reference Date : 12:00 UT 1 Jan 2000 (JD 2451545.0)
N = 359.28 + 549.308T degrees
T = Julian centuries from reference date 

Neptunian Magnetosphere

Goddard Space Flight Center OTD (O8) Model

Dipole field strength: 0.142 gauss-Rn3
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: >>1000 bars  
Temperature at 1 bar: 72 K (-201 C)
Temperature at 0.1 bar: 55 K (-218 C)
Density at 1 bar: 0.45 kg/m3
Wind speeds: 0-580 m/s
Scale height: 19.1 - 20.3 km
Mean molecular weight: 2.53 - 2.69 g/mole
Atmospheric composition (by volume, uncertainty in parentheses)
    Major:       Molecular hydrogen (H2) - 80.0% (3.2%); Helium (He) - 19.0% (3.2%);
                 Methane (CH4) 1.5% (0.5%)
    Minor (ppm): Hydrogen Deuteride (HD) - 192; Ethane (C2H6) - 1.5
    Aerosols:    Ammonia ice, water ice, ammonia hydrosulfide, methane ice(?)

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