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.
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.
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 42.8 out of 100, on 100% coverage of the axes.
Below the Armstrong limit (0.062 bar) body fluids boil at body temperature — the difference between needing a mask and needing a pressure vessel.
Delta-v sets what a launch costs; cruise time sets how much consumable, shielding and crew patience each kilogram of payload drags with it.
Past the asteroid belt solar panels stop being an option and a settlement needs fission, which means importing fuel.
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.
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 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.
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.
