In recent years there has been much discussion about human space exploration venturing beyond the earth-moon system to take on Mars and possibly other planets. In light of this, I decided to do a little research to determine exactly what celestial bodies are suitable for human exploration. Certainly, humans can fly by most any part of the solar system, should they have an advanced enough spacecraft. But there are a limited number of places where surface exploration is possible. What qualifies these places is the ability to survive in any current or foreseeable space suite or any kind of reasonable habitation module.
Since we are far from having any kind of faster than light travel (which, if possible at all, requires warping space or using some kind of artificial wormhole) and we are not ready for multi-generational spacecraft, the solar system is pretty much what we are stuck with.
It seems the places we can actually send humans are pretty limited. There is definitely Mars, but after that, what comes next? Possibly some of the moons of Jupiter, assuming its worth our while to send humans there in the future. Mars appears to be the best candidate for any kind of permanent or semi-permanent colonization or station.
Places humans could visit with reasonable habitat modules and/or spacesuits:
Mercury - Possibly on the side that faces away from the sun, but it’s questionable whether it would be worth visiting.
Venus - The temperature and pressure on the surface are far too high for a spacesuit. Manned flybys, however, have been considered in the past.
Earth’s Moon - Yes, obviously, since it has been done. The environment is certainly harsh, but well within the capabilities of a spacesuit.
Ceres - A dwarf planet that is the largest member of the asteroid belt. It could be visited by humans in spacesuits for surface study, but it is so small that it would be possible to jump off it into space. The gravity is not sufficient to allow walking around on it. Therefore, it would be more like clinging to the surface and floating around it than it would be “landing” on it in the normal sense.
Other asteroids - Again, lack of gravity makes surface exploration in the sense of walking impossible. It’s possible human exploration of an asteroid would be worthwhile. Some asteroids may have orbits that make them easier to get to than mars or other planets. The scientific value of this may be questionable. An asteroid does not seem like a good place to position any kind of manned outpost or colony.
Mars - The environment on mars is certainly within the capabilities of a spacesuit. The gravity is more than sufficient for relatively normal movement. Mars is also close enough to earth to make a trip to and from Mars practical for a crew. This is probably the best place for exploration beyond the earth-moon system, although asteroids have been suggested as well.
Phobos - The largest moon of mars, but still much smaller than our own moon and more similar to Ceres in size. There is no atmosphere and it should be within the capabilities of spacesuits, but again, hard to really walk around on because of the small size and lack of strong gravity. It has the advantage of being easier to take off from than the surface of mars, due to such little gravity.
Jupiter - No. The gas giants are out of the question. Not only is it a massive ball of gas, with nothing to stand on, but the pressure is far too high for survival, not to mention the crushing gravity. Probes that visited the area around Jupiter discovered that it has powerful radiation belts, which could be a problem for even a manned flyby.
Io (Moon of Jupiter) - Quite possible. It is only slightly larger than our own moon, so it has a fair amount of gravity. Radiation might or might not be an issue. The distance from the sun would make it very cold, necessitating heated space suits.
Europe (Moon of Jupiter) - Also possible. Good size, but the surface characteristics are less well known. It is believed to be covered with either ice or a cold brittle rock. The surface therefore may or may not be suitable for exploration. Again, radiation and cold are issues.
Ganymede (Moon of Jupiter) - Similar to Io, but larger and thus more gravity on the surface, but still much smaller than earth. Possible, but cold and radiation are concerns.
Callisto - Possible, rocky moon similar to Io and Ganymede
Saturn - No. Again, as with Jupiter, the gas giant has massive gravity and no place to stand.
Titan (moon of Saturn) - It’s hard to say but it might be possible. It’s larger than our own moon. It has its own dense atmosphere, which is unusual for a moon. It would be very cold and harsh, but maybe within the capabilities of future space suits and habitats. With Saturn and its moons, the distance of the travel and thus the time exposed to cosmic radiation and weightlessness become an issue, although this could be overcome with a powerful enough rocket, such as a nuclear pulsed propulsion system.
Other moons of Saturn - Saturn has dozens of moons, with Titan being the largest. Most of the moons are small and unappealing for manned exploration.
Uranus - No. It is a gas giant, though smaller than Jupiter and Saturn.
Moons of Uranus - Some might be possible, but the extreme distance becomes a concern. None appear especially appealing.
Neptune – As with the others, no landing on this gas giant.
Moons of Neptune - Only one moon is of substantial size, Triton. It might be possible, but cold, distance and radiation are issues.
Pluto and Satellites - Though no longer considered a planet, it could be a target worth investigating. Probably not worth human exploration. Not only is it far enough from the sun to be super cold, but the distance would necessitate many years in transit to and from it. The same is true with other Kuiper belt objects.