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Moon of Neptune

Proteus

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One of the darkest objects in the Solar System. Artist's impression: it has been photographed, but not mapped here. This look is generated from many factors as a best guess at what it might look like, and will be updated as more data comes in.

At Neptune.

Measured

Mean radius
210 km
From the Earth today
29.3 AU (4.38 billion km)
Goes round once every
1.12 days

How far, from the Earth

29.3 AU. To cross it:

  • Light4 hours 3 minutes
  • Voyager 1's speed8.22 years
  • Driving at 100 km/h4,990 years

Voyager 1 at the speed it moved outward over the year to 2026-10-25 (JPL Horizons). Driving: nonstop, never refuelling.

Facts

  • Within 2° of where JPL Horizons has it round its planet, 2024-2031. Further from those years it is less sure.
  • Its map is turned by the rule for a moon that keeps one face to its planet: longitude 0 towards the planet, on the planet's pole. Which way it really faces can differ by a few degrees, and the published rotation formulas differ from this by as much (Titan 5° now and 16° by 2050, Enceladus 6°).

Where this comes from

  • Planets, Pluto, the Moon and Jupiter's big moons: astronomy-engine (Don Cross, MIT). Other moons: JPL Planetary Satellite Mean Elements. Ceres, Eris, Haumea, Makemake: JPL Small-Body Database. NASA/JPL-Caltech.

Every catalogue Ouranos draws is credited on the credits page.