Spotting a "small" dark object against bright stars - which will necessarily be at a fantastic distance - isn't going to be easy. Even if it turns out to be closer to the size of Neptune than Pluto.

(I got the "Europa is off limits" line, from Arthur C. Clarke. Nice.) Callisto is not entirely within the protective shield of Jupiter's magnetosphere, which might mean it is bombarded by the sorts of things that are unfriendly to living creatures like us. So it could be very difficult. Mercury, which does have a magnetosphere, is too close to the Sun, whose solar wind overpowers it at that range. So the Solar System is not very cooperative in this regard.

I suggest placing Mercury in orbit around Mars (or Venus). Mercury is supposed to have a magnetic field as strong as Earth's, but for the effect of the Sun's nearby solar wind. If it was instead in orbit around Mars or Venus, and much farther out from the Sun, you'd have the strong magnetosphere of its moon to help protect your main planet. Hey look, it's the only other planet with a magnetosphere in the Solar System -

Einstein's General Theory of Relativity shows that time is a physical thing. It is part of space-tme. Space-time manifests itself as a gravitational field created by the mass of objects. The presence of mass will warp, or deform, space-time. As a consequence, time passes more slowly near to a massive object such as a planet. This has been demonstrated to be true by comparing super-accurate clocks in orbit to clocks on the surface of the Earth. The clocks on the surface run just a little bit slower. (This is a paraphrase from other people's explanations)

Getting to Mars will be tough, might take 10-20 years. The real problem is getting people off the planet. That will require much more force and energy than it took to get off the moon. We'll have to send quite an elaborate ship that can get out of there. And no Lockheed Martin up the road for spare parts.