Pluto
Position on the route: 28/45 · level needed 300 · journey (Mk0 engine) 59.1 h
By the numbers
| Distance from Sun | 4.4–7.4 billion km |
|---|---|
| Diameter | 2,377 km |
| Gravity | 0.62 m/s² (0.06 g) |
| Day | 6.4 days |
| Year | 248 years |
| Temperature | −229 °C |
| Atmosphere | nitrogen, very thin, seasonal |
| Moons | Charon and four small ones |
The story
Discovered as a planet in 1930, reclassified as a dwarf planet in 2006; the discovery of Eris forced the decision. New Horizons flew past at 12,500 km in 2015 after a nine-and-a-half-year journey and, against expectations, found a geologically alive world: a glacier plain of nitrogen ice (Sputnik Planitia, the ‘heart’), 3 km mountains of water ice, possible ice volcanoes.
Charon is half Pluto’s size; the two orbit a common centre of mass and always show each other the same face. Pluto’s thin nitrogen atmosphere freezes and snows onto the surface as it moves away from the Sun.
Its orbit crosses Neptune’s, but a 3:2 resonance means they never collide. It is the best-known member of the Kuiper belt; thousands of similar ice worlds lie beyond.
Missions
- New Horizons (2015): the only flyby; it went on to Arrokoth in 2019
Colony note
Solid ground, nitrogen and water ice; very low gravity. Light is a thousandth of Earth’s: power must be entirely nuclear.
What probes bring back
- Sputnik Planitia: a moving glacier plain of nitrogen ice.
- Mountains of water ice 3 km high; possible ice volcanoes.
- A thin nitrogen atmosphere freezes and thaws with the seasons.