Where in our solar system can a person reasonably walk with a space suit?
Certainly not the Sun, nor any of the big gas planets Jupiter, Saturn, Uranus, or Neptune. Walking, as we know it, relates to a solid surface; in these places, it’s not clear there’s any surface to walk on at all (other practical concerns aside.)
The strength of the gravitational field of the body also plays a role: too weak a gravitational field rules out walking, in the sense of taking strides. We know what walking was like on the Moon. It was funny-looking, but definitely walking. But our Moon, with only 1/6 Earth’s surface gravity, has the strongest surface gravity of any of the other moons of the Solar System. (Our Moon is composed mostly of rock, while all the other moons are primarily ices, and therefore much less dense.) With a weaker gravity, walking would be even stranger, and slower, but at some point, nothing like steps could happen. Where to draw the line? What is meant by “walking”? With 1/10 of our Moon’s gravity, is it walking?
On most asteroids and smaller moons, the gravity is just too low—it wouldn’t be walking. Some medium-sized moons may be border cases though.
Earth requires no space suit. Convenient, but terribly mundane.
The Moon has been done. It’s a good, sturdy romp with a low-tech pressure suit, just one heavenly body away.
Mercury is a possibility, at least at night — but not advised. Daytime temperature and radiation would necessitate an extremely robust space suit—it might be hard to call such equipment a “space suit”. Nights however are 88 Earth days long, so a night visit doesn’t require precise planning. The rotational velocity of the planet at the equator is only about 10 km/h, so in principle, one could remain on the night side just by gently loping along. But even then, there is the worry that Mercury’s weak magnetic field might deflect radiation from a solar flare all the way to the night side. Most sun worshipers prefer their deity at a greater distance.
Venus requires more than a space suit as we know them — pressure too great, far too hot, sulfuric acid etc. The survival equipment required would preclude a walk, as we know it.
Mars is convenient, and the best extraterrestrial walking with a conventional space suit. Its surface gravity is 1/3 of the Earth’s, but twice that of our Moon. Moreover, Mars boasts almost as much dry land as Earth. Other attractions: An atmosphere providing some protection from radiation and smaller meteorites. Seasonal great views of the Earth and Jupiter.
Asteroids Ceres and Vesta both have borderline-walkable surface gravity, and they’re also fairly nearby.
Pluto presents a real get-away, for those who like a brisk, nippy walk. Its surface gravity is only about 40% that of our Moon, so you can comfortably pack a big picnic.
But if you’re going to accept Pluto, you might also be interested in trans-Neptunian object Eris, which is even bigger, and whose surface gravity is about half that of the Moon’s.
Jupiter has four big moons. Any of these affords either a great view of the big planet, or none at all, depending on which side of the moon you walk. Each has surface gravity somewhat less than our Moon’s (but not a lot less.) Gravity-wise, they are walkable. But there are other concerns.
Io has many volcanoes that are constantly erupting, throwing material far into space, which mostly falls back to the surface—not slowed by an atmosphere. If that weren’t enough, the radiation intensity of the Jovian radiation belts here is several times a lethal dose per day. This is an immediately lethal environment.
Europa is these days much talked about, but it also orbits deep within Jupiter’s radiation belts; a human would receive a lethal dose in days without thick radiation shielding. Not a great bet for a walk.
Ganymede, the biggest moon in the Solar System, has surface gravity slightly less than our Moon’s, and should be quite walkable. (The temperature would be nippish...)
Callisto likewise should be walkable.
Saturn has one big moon, several medium-sized ones, and a bunch of little ones.
Titan, the largest moon of Saturn, is a very interesting place, as well as a walkability question. It is the only moon in the solar system that has a substantial atmosphere — and it’s a doozy, 50% denser at the surface than Earth’s. Instead of a pressure suit, a suit to support the atmospheric pressure is needed. Titan also has liquid methane rain, and maybe other surprises. A very special suit would be required, at least, but it’s not clear whether that would suffice — we just don’t know enough about surface conditions there.
Four medium-sized moons Tethys, Dione, Rhea, Iapetus, have surface gravity less than 16% our Moon’s. Enceladus is an interesting smaller moon, but with surface gravity 1/16 that of our Moon, it wouldn’t be walking.
Uranus’ biggest moons aren’t very big. Titania has a surface gravity less than a quarter of our Moon’s. Walkable? Maybe barely — very very slowly — but not a great bet, and certainly not effective exercise.
Triton, the single big moon of Neptune, with surface gravity a bit less than half our Moon’s, should be walkable.
Besides the Earth, the quite walkable places are
Depending on what you want to allow as “walking”, there are the two biggest
Ceres and Vesta,
whose surface gravities, 17% and 15% of our Moon’s, might permit a very floaty interpretation of walking. And they’re relatively nearby.
You might also consider the medium-sized moons of Saturn
Dione, Rhea, Iapetus
and the larger moons of Uranus
Titania, Umbriel, Ariel, Oberon,
farther from the beaten path. For serious adventure-seekers, there’s the surprising little moon of Pluto,
and the several smaller known trans-Neptunian objects
Sedna, Quaoar, Makemake, Haumea, 2007 OR, Orcus.
It was mentioned above that Mars has almost as much dry land as does the Earth. The five largest moons of the Solar System together have more than double the Earth’s dry land. To that one might add Mercury’s 15%. Altogether, the dry, walkable surfaces of the Solar System comprises several times that of Earth. In fact, more than the total surface of the Earth, dry or not.
So while none of these places has breathable air, and they all have comfort issues, it’s a consideration that the bulk of the walkable land in the Solar System is not on the Earth.
The illustrating author of xkcd hit on a similar idea in Surface Area, Space without the space.