Walks about the Solar system

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 among the other moons of the Solar System. (Our Moon consists 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 does “walking” mean? With 1/10 of our Moon’s gravity, is it walking?

The surface gravity of a body depends on both its mass and on its radius: it is proportional to the mass divided by the square of the radius. Thus, a smaller body of denser material may have a higher surface gravity than one of lighter material.

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.

your own home and back yard

Earth requires no space suit, on dry land. Convenient, but terribly mundane. Deep ocean covers two thirds of Earth's surface, most of it too deep for any current diving suit. Only a minor part of the surface of Earth is walkable.

The Moon is a good, sturdy romp with a low-tech pressure suit, just one heavenly body away. But it has been done.

planets etc.

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 that provides some protection from radiation and smaller meteorites. Seasonal great views of the Earth and Jupiter. It can be a little chilly even in daytime in Summer, and the dust can pose difficulties, though.

Asteroids Ceres and Vesta both have borderline-walkable surface gravity, and they’re also fairly nearby. Another asteroid, 16 Psyche, is the densest asteroid known, and although it's much smaller than Ceres, has a similar surface gravity.

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 find Pluto acceptable, you might also consider trans-Neptunian dwarf planet Eris, which is more massive and denser than Pluto, so that its surface gravity is about half that of the Moon’s.

moons of Jupiter

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 volcanos 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 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.

moons of Saturn

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, it would require a suit to support the atmospheric pressure. Titan also has liquid methane rain, and maybe other surprises. It’s not clear whether a very special pressure suit would suffice — we just don’t know enough about surface conditions there.

Three medium-sized moons Dione, Rhea, Iapetus, have surface gravity around 1/4 of our Moon’s, so should provide a very low-exertion walk.

Enceladus is an interesting smaller moon, but with surface gravity 1/14 that of our Moon, whatever one did there wouldn’t be walking. Curiously, the moon Tethys, which twice as big, has surface gravity only slightly more than this — evidently it consists mostly of ices.

moons of Uranus

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.

moons of Neptune

Triton, the single big moon of Neptune, with surface gravity a bit less than half our Moon’s, should be walkable. This is a very odd moon orbiting an odd planet. We have pictures only of one side of it, from a single quick fly-by of a Voyager spacecraft, but that photo shows lots of interesting terrain, including volcanos. Don’t forget your camera!

best bets

Besides the Earth, the quite walkable places are

• Mercury (at night, with caution)
• the Moon
• Mars
• Ganymede
• Callisto
• Titan (unless surface conditions rule it out)
• Triton
• Pluto
• Eris

others

Depending on what you want to allow as “walking”, there are the biggest asteroids,
Ceres, 16 Psyche and Vesta,
whose surface gravities, 17%, 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,
Charon
and the several smaller known trans-Neptunian objects
Sedna, Quaoar, Makemake, Haumea, 2007 OR, Orcus.

dry land

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, add Mercury’s 15%. Altogether, the dry, walkable surfaces of the Solar System amount to 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.

missions

Of the bodies mentioned here, only a few have received visits by space missions.

Of course, besides the Earth, only the Moon has ever been trod upon. (And it was great!)

Many spacecraft of various kinds have visited Mars since the 1970s. Several of them are still operating.

The MESSENGER spacecraft visited Mercury in several fly-by passes, then went into orbit of the planet. It made extensive maps of the surface chemistry, the magnetic field, and gaseous outflow of the planet, and was finally crashed into it in 2025.

Only one moon besides Earth’s has ever been landed upon: that was Saturn’s Titan, in 2004 by the Huygens lander part of the Cassini-Huygens mission. Huygens’ mission was rather limited, but it transmitted images on its descent, and for a short time, images of the surface of the moon, as well as some physical conditions, and some information about the chemistry of the atmosphere.

Jupiter’s moons have enjoyed several fly-by passes, and from these, we have very extensive photographic surveys of their surfaces. Currently, the Juno spacecraft is orbiting Jupiter, and has also made several passes of its major moons. Likewise, the Cassini mission of 2004–2017 made very detailed surveys in of many of Saturn’s moons.

A few other bodies have only seen spacecraft fly-by’s: the moons of Uranus (by Voyager 2 in 1986) and Neptune (by Voyager 2 in 1989), and the Pluto system (by the New Horizons in 2015).

Only the largest asteroids have ever been visited, primarily, by the Dawn spacecraft, which orbited Vesta for a while in 2012, then went on to Ceres in 2015, where it continues to orbit to this day. Currently, the Psyche spacecraft is underway to the asteroid 16 Psyche, and expected to arrive in 2030.

xkcd

The illustrating author of xkcd hit on a similar idea in Surface Area, Space without the space.