One of the things that has always fascinated us about Jupiter is that it’s a microcosm of the solar system. Just as myriad planets orbit the sun, so too does Jupiter have an entourage of heavenly bodies accompanying it on its voyage. Even though Jupiter is a gas giant with no solid surface on which to land a spacecraft, it does have 79 rocky moons which we could explore.

Four of those moons are substantially larger than the others, with gravity similar to our moon. The first three — Io, Europa, and Ganymede — are too close to Jupiter and receive too much radiation to allow humans on their surface without lots of protection. But the fourth moon, Callisto, receives significantly less radiation than the other three. It’s also suspected that it has a subsurface ocean of liquid water which would make life much easier for any human visitors. (via Journal of the Washington Academy of Sciences).

In the early 2000s a study was done to assess the feasibility of a crewed mission to Callisto. It concluded that it wouldn’t happen until at least the 2040s and it would require new kinds of propulsion that haven’t been tested in space before. But, assuming everything falls into place, an eventual, round-trip mission could take anywhere between 652 and 1,661 days.

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