Neptune/Earth Comparison

Bulk parameters

                                   Neptune      Earth   Ratio (Neptune/Earth)
Mass (1024 kg)                     102.43      5.9736        17.147
Volume (1010 km3)                    6,254   108.321         57.74
Radius (1 bar level) (km)
    Equatorial                     24,766      6,378          3.883
    Polar                          24,342      6,356          3.830
Volumetric mean radius (km)        24,624      6,371          3.865
Ellipticity                         0.0171     0.0034         5.03
Mean density (kg/m3)                1,638      5,520          0.297
Gravity (eq., 1 bar) (m/s2)        11.00       9.78           1.125
Escape velocity (km/s)             23.5       11.19           2.10
GM (x 106 km3/s2)                   6.8351     0.3986        17.15
Bond albedo                         0.82       0.385          2.13
Visual geometric albedo             0.41       0.367          1.12
Visual magnitude V(1,0)            -6.87      -3.86             -
Solar irradiance (W/m2)             1.47       1,380          0.00107
Black-body temperature (K)         33.2      247.3            0.134
J2 (x 10-6)                      3411.      1082.63           3.151    

Orbital parameters

                                   Neptune      Earth   Ratio (Neptune/Earth)
Semimajor axis (106 km)           4,498.3       149.6        30.07
Sidereal orbit period (days)     60,189.        365.256     164.79
Tropical orbit period (days)     59,799.9       365.242     163.73
Perihelion (106 km)               4,458.0       147.1        30.31
Aphelion (106 km)                 4,535.2       152.1        29.82
Synodic period (days)               367.49        -             -
Mean orbital velocity (km/s)          5.48       29.79        0.184
Orbit inclination (deg)               1.769       0.00          -
Orbit eccentricity                    0.00859     0.0167      0.514
Sidereal rotation period (hours)     16.11*      23.9345      0.673
Obliquity to orbit (deg)             29.56       23.45        1.261

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

Neptune Observational Parameters

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

Apparent diameter from Earth
        Maximum (seconds of arc)    2.4
        Minimum (seconds of arc)    2.2
Distance from Earth
        Minimum (106 km)         4305.9
        Maximum (106 km)         4687.3
Apparent magnitude at opposition    7.8      

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

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: >>100 bars
Average temperature: ~58 K
Temperature at 1 bar: ~72 K
Density at 1 bar: ~0.45 kg/m3
Wind speeds: 0-200 m/s
Scale height: 19.1 - 20.3 km
Mean molecular weight: 2.53 - 2.69 g/mole
Atmospheric composition (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(?)