Callisto
Fifty thousand times lower than Io, and lower than the Moon. Of the four Galilean moons, this is the only surface a human could work on for months.
Surface conditions on Callisto
Every figure below is tagged with where it came from. Measured means an instrument read it on or at the body; modeled means it was computed and never directly observed.
What it would take to settle Callisto
No world off Earth is habitable — the honest habitable fraction of every surface here is zero. What can be answered is how far each one sits from Earth along the seven axes a habitat has to buy its way out of. Callisto scores 44.6 out of 100, on 100% coverage of the axes. The single worst axis here is pressure (1e-12 bar (vacuum)); the one that most sets Callisto apart from the other thirteen is available energy.
Below the Armstrong limit (0.062 bar) body fluids boil at body temperature — the difference between needing a mask and needing a pressure vessel. Above 1 bar the problem inverts into containment, and Venus at 92 bar has never let a lander live two hours.
Delta-v sets what a launch costs; cruise time sets how much consumable, shielding and crew patience each kilogram of payload drags with it.
Both directions cost power continuously. A cold world burns reactor output to stay warm; a hot one has no cold sink to dump heat into at all.
Past the asteroid belt solar panels stop being an option and a settlement needs fission, which means importing fuel.
Who has been to Callisto
Never landed on. NASA's 2003 HOPE study picked Callisto as the target for the first crewed mission to the outer Solar System.
Unprotected survival: Seconds from the vacuum — but the radiation alone would not kill you.
Water ice across the surface, and magnetometer data implying a salty subsurface ocean 100+ km down.
The named places on Callisto
The IAU has approved 154 names on Callisto, each drawn from a theme the naming committee chose for this world. Every name in the scene above is one of them, placed at its catalogued coordinates.
Sources
- Voyager ISS / Galileo SSI — Voyager + Galileo global mosaic, 1 km/px (NASA Trek / USGS Astrogeology)
- IAU / USGS Gazetteer of Planetary Nomenclature
- NASA — Callisto
- NASA HOPE — Revolutionary Concepts for Human Outer Planet Exploration
Imagery is public-domain NASA/USGS mission data, streamed live from NASA Trek as a plate-carrée tile pyramid and draped on our own sphere — no third-party globe library and no cached copy of anyone else's map.