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(?)
AEROSPACE and ASTRONAUTICS 