Atlas · Neptune
The moons of Neptune
Neptune has 16 known moons. One of them, Triton, holds nearly all the mass, and it goes round backwards: the only large moon in the Solar System that does. It was almost certainly captured.
Drawing I
Triton and the inner moons
Neptune seen from above its north pole. Triton’s orbit is a nearly perfect circle, travelled backwards. It is tilted 23° to Neptune’s equator, so from here it looks slightly squashed. The small square is the area of the second drawing.
The edge of the drawing is 375,000 km from the centre of Neptune.
Drawing II
Close in: rings and inner moons
The rings and the seven inner moons. Galatea orbits just inside the outermost ring, and Hippocamp keeps close to Proteus.
The edge of the drawing is 125,000 km from the centre of Neptune.
Drawing III
Nereid and the outer moons
The eight outer moons, seen from above the plane of the planets. Nereid’s is the small, narrow loop nearest the middle. The small square in the middle is the whole of the first drawing.
The edge of the drawing is 61 million km from the centre of Neptune.
How to read it
The close-in drawings look straight down on Neptune’s north pole. The drawing of the outer moons looks down on the plane of the planets instead, because that is the plane those moons keep to. Neptune is drawn to scale with the orbits. The moons are not: at this scale most would be far too small to see, so each is a dot.
- Solid dot. A spacecraft has mapped the whole surface.
- Half dot. A spacecraft has seen part of it.
- Ring. Never seen as more than a point of light.
- Pale line. Goes round the usual way, the way the planets go round the Sun: anticlockwise on these drawings.
- Amber line. Goes round backwards. 5 moons do.
- Broken line. The moon can be more than 10° from where we draw it. 1 orbit is like this. The pull of the Sun and of other moons keeps changing these paths, and a simple orbit cannot follow them exactly.
Nobody has measured the size of 8 of the 16. Each orbit is a slowly turning ellipse fitted to positions from NASA’s Jet Propulsion Laboratory for the years 2000 to 2040. The list gives how far out each fit can be. Touch a row to pick out its orbit.
The inner moons
7 moons
Seven small moons close in, among and just outside the rings. Six were found by Voyager 2 in 1989, and tiny Hippocamp in 2013.
| Moon | Orbit size | One orbit | Tilt | Width | Found | See it in 3D |
|---|---|---|---|---|---|---|
| Naiad. Seen only as a point of light.drawn to within 0.70° | 48,300 km | 7.1 hours | 4.7° | 58 km | 1989 | Fly there |
| Thalassa. Seen only as a point of light.drawn to within 0.26° | 50,100 km | 7.5 hours | 0.5° | 80 km | 1989 | Fly there |
| Despina. Seen only as a point of light.drawn to within 0.16° | 52,600 km | 8 hours | 0.5° | 148 km | 1989 | Fly there |
| Galatea. Seen only as a point of light.drawn to within 0.13° | 62,000 km | 10.3 hours | 0.5° | 158 km | 1989 | Fly there |
| Larissa. Partly seen by spacecraft.drawn to within 0.32° | 73,600 km | 13.3 hours | 0.6° | 192 km | 1989 | Fly there |
| Hippocamp. Seen only as a point of light.drawn to within 0.18° | 105,300 km | 22.8 hours | 0.8° | not measured | 2013 | Fly there |
| Proteus. Partly seen by spacecraft.drawn to within 0.12° | 117,700 km | 26.9 hours | 1.0° | 416 km | 1989 | Fly there |
Triton
1 moon
Larger than Pluto. Voyager 2 saw plumes of dark material rising from its frozen surface.
| Moon | Orbit size | One orbit | Tilt | Width | Found | See it in 3D |
|---|---|---|---|---|---|---|
| Triton. Partly seen by spacecraft.drawn to within 0.01° | 354,800 km | 5.88 days | 157° | 2,705 km | 1846 | Fly there |
Nereid and the outer moons
8 moons
Nereid swings from 1.4 to 9.6 million km out. The furthest moons take more than 25 years to go round once, on the widest orbits of any known moons.
| Moon | Orbit size | One orbit | Tilt | Width | Found | See it in 3D |
|---|---|---|---|---|---|---|
| Nereid. Partly seen by spacecraft.drawn to within 0.80° | 5.52 million km | 360 days | 5.0° | 340 km | 1949 | Fly there |
| Halimede. Seen only as a point of light.drawn to within 2° | 16.6 million km | 1,879 days | 112° | not measured | 2002 | Fly there |
| Sao. Seen only as a point of light.drawn to within 0.93° | 22.3 million km | 2,915 days | 53° | not measured | 2002 | Fly there |
| S/2002 N 5. Seen only as a point of light.drawn to within 6° | 23.4 million km | 3,161 days | 43° | not measured | 2002 | Fly there |
| Laomedeia. Seen only as a point of light.drawn to within 5° | 23.5 million km | 3,178 days | 39° | not measured | 2002 | Fly there |
| Psamathe. Seen only as a point of light.drawn to within 8° | 47.8 million km | 9,291 days | 122° | not measured | 2003 | Fly there |
| Neso. Seen only as a point of light.drawn to within 17° | 50 million km | 10,119 days | 141° | not measured | 2002 | Fly there |
| S/2021 N 1. Seen only as a point of light.drawn to within 9° | 50.7 million km | 10,291 days | 135° | not measured | 2021 | Fly there |
Where this comes from
- NASA/JPL Solar System Dynamics, planetary satellites: orbits fitted to JPL Horizons positions, 2000 to 2040
- NASA/JPL, planetary satellite physical parameters
- NASA/JPL, planetary satellite discovery circumstances
- NASA NSSDC, Neptune rings fact sheet
- NASA, Neptune’s moons
Orbit size is the average distance from the centre of Neptune. Tilt is measured from Neptune’s equator for the close moons and from the plane of the planets for the far ones, and above 90° means the moon goes round backwards.