Neptune ♆
Neptune - Voyager 2 (29347980845) flatten crop.jpg
Photograph taken by NASA'sVoyager 2in 1989
Discovery
Discovered by
  • Johann Galle
  • Urbain Le Verrier
  • John Couch Adams
Discovery date23 September 1846
Designations
Pronunciation/ˈnɛptjn/(About this soundlisten)
Named after
LatinNeptunus, via FrenchNeptune
AdjectivesNeptunian (/nɛpˈtjniən/), Poseidean
Orbital characteristics
Epoch J2000
Aphelion30.33 AU (4.54 billion km)
Perihelion29.81 AU (4.46 billion km)
Semi-major axis
30.07 AU (4.50 billion km)
Eccentricity0.008678
Orbital period
  • 164.8 yr
  • 60,195 days
  • 89,666 Neptunian solar days
Synodic period
367.49 days
Average orbital speed
5.43 km/s
Mean anomaly
256.228°
Inclination1.770°to ecliptic
6.43° to Sun's equator
0.74° to invariable plane
Longitude of ascending node
131.783°
Time of perihelion
2042-Sep-04
Argument of perihelion
273.187°
Known satellites14
Physical characteristics
Mean radius
24,622±19 km
Equatorial radius
24,764±15 km
3.883 Earths
Polar radius
24,341±30 km
3.829 Earths
Flattening0.0171±0.0013
Surface area
7.6187×109 km2
14.98 Earths
Volume6.253×1013 km3
57.74 Earths
Mass1.02413×1026 kg
17.147 Earths
5.15×10−5 Suns
Mean density
1.638 g/cm3
Surface gravity
11.15 m/s2
1.14 g
Moment of inertia factor
0.23(estimate)
Escape velocity
23.5 km/s
Rotation period
0.67125 d
16 h 6 m 36 s
(synodic; solar day)
Sidereal rotation period
0.6713 day
16 h 6 min 36 s
Equatorial rotation velocity
2.68 km/s (9,650 km/h)
Axial tilt
28.32° (to orbit)
North poleright ascension
19h57m20s
299.3°
North pole declination
42.950°
Albedo0.290 (bond)
0.442 (geom.)
Surface temp.minmeanmax
1 bar level72 K (−201 °C)
0.1 bar (10 kPa)55 K (−218 °C)
Apparent magnitude
7.67 to 8.00
Angular diameter
2.2–2.4″
Atmosphere
Scale height
19.7±0.6 km
Composition by volume
  • Gases:
  • 80%±3.2%hydrogen (H2)
  • 19%±3.2%helium (He)
  • 1.5%±0.5%methane (CH4)
  • ~0.019% hydrogen deuteride (HD)
  • ~0.00015% ethane (C2H6)
  • Ices:
  • ammonia (NH3)
  • water (H2O)
  • ammonium hydrosulfide (NH4SH)
  • methane ice (?) (CH4·5.75H2O)