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Above: The Greek sign for Neptune.
In Roman mythology Neptune (Greek: Poseidon) was the god of the Sea.
After the discovery of Uranus, it was noticed that its orbit did not abide by Newton's laws. It was then predicted that another more distant planet must be influencing Uranus' orbit. Neptune was first observed by Galle and d'Arrest on Sept 23, 1846 very near to the locations independently predicted by Adams and Le Verrier from calculations based on the observed positions of Jupiter, Saturn and Uranus. An international dispute arose between the English and French (but not between Adams and Le Verrier personally) over the right to name the new planet. Subsequent observations have shown that the orbits calculated by Adams and Le Verrier diverge from Neptune's actual orbit fairly quickly. Had the search for the planet taken place a few years earlier or later it would not have been found anywhere near the predicted location.
Neptune has been visited by only one spacecraft, Voyager 2 on Aug 25, 1989. Much of we know about Neptune comes from this planetary probe. But fortunately, recent ground-based and HST observations have added a great deal, as well.
Most of the mass in the planetary system is found in the outer solar system. Jupiter alone exceeds the mass of all the other planets combined. The chemistry of Neptune is also different, with hydrogen rather than oxygen dominating. When the planets and the Sun were forming, it was cooler in the parts of the solar nebula farther from the Sun. This allowed water ice and other volatile compounds to condense, whereas these materials remained as gas in the inner solar system.
As a result, the forming stages of the outer planets, which includes Neptune, included a rather bigger mass of solid material to begin with. This means that the core bodies for the giant planets had more gravity to capture the gases hydrogen and helium, which were very abundant. So, the outer planets were able to grow significantly bigger than the ones closer to the Sun.
Neptune's composition is probably similar to Uranus': various "ices" and rock with about 15% hydrogen and a little helium. Like Uranus, but unlike Jupiter and Saturn, it may not have a distinct internal layering but rather to be more or less uniform in composition. But there is most likely a small core (about the mass of the Earth) of rocky material. Its atmosphere is mostly hydrogen and helium with a small amount of methane.
 Neptune's blue color (right) is largely the result of absorption of red light by methane in the atmosphere but there is some additional as-yet-unidentified chromophore which gives the clouds their rich blue tint.
Like a typical gas planet, Neptune has rapid winds confined to bands of latitude and large storms or vortices. Neptune's winds are the fastest in the solar system, reaching 2000 km/hour.
Like Jupiter and Saturn, Neptune has an internal heat source -- it radiates more than twice as much energy as it receives from the Sun.
 At the time of the Voyager encounter, Neptune's most prominent feature was the Great Dark Spot (left) in the southern hemisphere. It was about half the size as Jupiter's Great Red Spot (about the same diameter as Earth). Neptune's winds blew the Great Dark Spot westward at 300 meters/second (700 mph). Voyager 2 also saw a smaller dark spot in the southern hemisphere and a small irregular white cloud that zips around Neptune every 16 hours or so now known as "The Scooter" (right). It may be a plume rising from lower in the atmosphere but its true nature remains a mystery.
 However, HST observations of Neptune (right) in 1994 show that the Great Dark Spot has disappeared! It has either simply dissipated or is currently being masked by other aspects of the atmosphere. A few months later HST discovered a new dark spot in Neptune's northern hemisphere. This indicates that Neptune's atmosphere changes rapidly, perhaps due to slight changes in the temperature differences between the tops and bottoms of the clouds.
|
Satellite
|
Distance (km)
|
Radius (km)
|
Mass (kg)
|
Discoverer
|
Date
|
|
Naiad
|
48
|
29
|
?
|
Voyager 2
|
1989
|
|
Thalassa
|
50
|
40
|
?
|
Voyager 2
|
1989
|
|
Despina
|
53
|
74
|
? |
Voyager 2
|
1989
|
|
Galatea
|
62
|
79 |
? |
Voyager 2
|
1989
|
|
Larissa
|
74 |
96 |
? |
Voyager 2
|
1989
|
|
Proteus
|
118 |
209 |
? |
Voyager 2
|
1989
|
|
Triton
|
355 |
1350 |
2.14e22 |
Lassell
|
1846
|
|
Nereid
|
5509 |
170 |
? |
Kuiper
|
1949
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Neptune's Rings
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Ring
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Distance (km)
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Width (km)
|
aka
|
|
|
Diffuse
|
41900
|
15
|
1989N3R
|
Galle
|
|
Inner
|
53200
|
15
|
1989N2R
|
LeVerrier
|
|
Plateau
|
53200
|
5800
|
1989N4R
|
Lassell, Arago
|
|
Main
|
62930
|
< 50
|
1989N1R
|
Adams
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Here you will find all the stats and facts for Neptune. This includes everything from the escape velocity needed to leave its surface to surface temperatures! All these figures will be compared with Earth...
Discoverer: Johann Gottfried Galle (based on predictions by John Couch Adams and Urbain Leverrier)
Discovery Date: September 23, 1846
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Neptune
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Earth
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Ratio (Neptune/Earth)
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| Mass (1024 kg) |
102.43
|
5.9736 |
17.147
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| GM (x 106 km3/s2) |
6.8351
|
0.3986 |
17.15
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| Volume (1012 km3) |
6,254
|
108.321 |
57.74
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| Equatorial radius (km) |
24,764
|
6,378.1 |
3.883
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| Polar radius (km) |
24,341
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6,356.8 |
3.829
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| Volumetric mean radius (km) |
24,624
|
6,371.0 |
3.865
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| Mean density (kg/m3) |
1,638
|
5,515 |
0.297
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| Surface gravity (eq.) (m/s2) |
11.00
|
9.78 |
1.12
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| Escape velocity (km/s) |
23.5
|
11.19 |
2.10
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| Ellipticity (Flattening) |
0.01708
|
0.00335 |
5.10
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| Visual magnitude V(1,0) |
-6.87
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-3.86 |
-
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| Bond albedo |
0.290
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0.306 |
0.95
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| Visual geometric albedo |
0.41
|
0.367 |
1.12
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| Solar irradiance (W/m2) |
1.51
|
1367.6 |
0.0011
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| Black-body temperature (K) |
46.6
|
254.3 |
0.183
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| J2 (x 10-6) |
3411
|
1082.63 |
3.151
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| 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
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| 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
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| Max. orbital velocity (km/s) |
5.50
|
30.29 |
0.182
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| Min. orbital velocity (km/s) |
5.37
|
29.29 |
0.183
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| Orbit inclination (deg) |
1.769
|
0.000 |
-
|
| Orbit eccentricity |
0.0113
|
0.0167 |
0.0113
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| Sidereal rotation period (hrs) |
16.11
|
23.9345 |
0.673
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| Length of day (hrs) |
16.11
|
24.0000 |
0.671
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| Obliquity to orbit (deg) |
28.32
|
23.45 |
1.208
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| Min. distance from Earth (106 km) |
4305.9
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| Max. distance from Earth (106 km) |
4687.3
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**Don't understand these measurements? Click here to get help with the definitions and notes**
Atmospheric composition
Molecular hydrogen (H2) - 80.0% (3.2%)
Helium (He) - 19.0% (3.2%)
Methane (CH4) 1.5% (0.5%)
Trace Amounts of: (ppm)
Hydrogen Deuteride (HD) - ~192
Ethane (C2H6) - ~1.5
- Neptune's magnetic field is off center and at a large angle to its rotation axis. What processes in the interior generate this oddly shaped field?
- What accounts for the relative lack of hydrogen and helium in Neptune (and Uranus)?
- Why are Neptune's winds so strong in spite of the fact that it is so far from the Sun and has a relatively weak internal heat source?
- What happened to the Great Dark Spot?
- Can we design a useful Neptune orbiter mission cheap enough to be funded?
All nine photos below are of great quality and very large!! So download time will be slow, but well worth it...
Click on any image to view it full size!
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Neptune
