Neptune Fact Sheet


Neptune

Neptune/Earth Comparison


Bulk parameters

   
   Neptune   
 
   Earth   
Ratio
(Neptune/Earth)
Mass (1024 kg) 102.413 5.9724 17.15
Volume (1010 km3) 6,254 108.321 57.74
Equatorial radius (1 bar level) (km) 24,764 6,378.1 3.883
Polar radius (1 bar level) (km) 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 0.39860 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) 1.508 1,361.0 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 (hrs) 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
Inclination of equator (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 = 357.85 + 52.316T degrees
T = Julian centuries from reference date 

Neptunian Magnetosphere

Model GSFC-O8
Dipole field strength: 0.142 Gauss-Rn3
Dipole tilt to rotational axis: 46.9 degrees
Longitude of tilt: 288 degrees (IAU convention)
Dipole offset: 0.485 Rn
Surface (1 Rn) field strength: ~0.1 - 0.9 Gauss

Rn denotes Neptunian model radius, 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 
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(?)

Notes on the factsheets - definitions of parameters, units, notes on sub- and superscripts, etc.
Neptunian Satellite Fact Sheet
Neptunian Rings Fact Sheet

Planetary Fact Table - metric units
Planetary Fact Table - U.S. units
Planetary Fact Table - Earth ratio

Neptune Home Page
Directory to other Planetary Fact Sheets


NASA
Author/Curator:
Dr. David R. Williams, dave.williams@nasa.gov
NSSDCA, Mail Code 690.1
NASA Goddard Space Flight Center
Greenbelt, MD 20771
+1-301-286-1258


NASA Official: Ed Grayzeck, edwin.j.grayzeck@nasa.gov
Last Updated: 19 May 2016, DRW