# Which planets and moons can you walk on?

Which planets and moons have a surface where you could survive with a conventional or near-future spacesuit? Assume oxygen, water, and food supplies are accounted for, but roughly current limitations on handling heat, pressure, gravity, and so on.

For example, you might be able to survive on Mercury if you avoid going to the side facing the sun and stay on its cold side with your spacesuit on. Similarly a lot of moons of the gas giants like Ganymede might be survivable with technological aid.

What would other such heavenly bodies be in our solar system?

• Do you mean with an existing spacesuit? Or a theoretical spacesuit with currently available technology? Otherwise, I think this would be too speculative. – called2voyage Apr 29 '16 at 18:55
• Also, we might need to constrain the available supplies too or we might have similar shifting scope problems. – called2voyage Apr 29 '16 at 18:58
• @called2voyage currently available technology and enhanced current technology, in a nutshell say more advanced material but not full blown sci-fi quantum space suit – Allahjane Apr 29 '16 at 19:01
• So basically technology with theoretical confidence using known science? Please edit into the question. – called2voyage Apr 29 '16 at 19:03
• The asteroids too small to walk on can be hollowed out and spun up. You would walk, but on the inside surface. Might be the most habitable places, because there is no hostile atmosphere to contend with. Just bring your own air. – Anthony Stevens Jun 18 '20 at 10:56

Well, let's start with the minimum size of an object. The formula for escape velocity is:

$V_{escape} = \sqrt{2 G M/R}$

Okay, let's say that you don't want to accidentally jump off. Let's say that you can do that with about a 3 m/s jump. Let's give a density of $3 g/cm^3$, and let's just say a uniform sphere. Putting all of that gives:

$3=\sqrt{2*3000*G*(4/3*\pi*r^2)}=8000*G*\pi*r^2$

Solving for r gives about 2.3 km. So that's your absolute lower limit. Of course, you might want something that you could walk more upright. Let's say you wanted at least 50 m/s for an escape velocity, as that you would come down relatively quickly. That would come to about 38 km.

There is no object with a solid surface that is too massive to walk on, so that's okay. It must have a solid surface, so that takes out the gas giants. Aside from that, the only objects to be wary of are those with a toxic atmosphere, which would be Venus, and Titan. Venus would be difficult with any conceivable technology. Titan might be doable, but it would require some special work, and likely wouldn't work for long. The main problem with Titan is the atmosphere is flammable, which could cause some interesting complications if there was any kind of a leak.

Next, let's look at heat and cold. According to Wikipedia, a space suit can operate between -156 °C (−249 °F) to 121 °C (250 °F). Saturn is about -178, so you'd have to be inside of Saturn's orbit. The upper temperature range would be reached for an object at Venus's orbit, probably not that far.

Very last thing is extra hazards. The main hazard would be radiation, which only really affects Jupiter's closest moons, Io and Europa. The further ones should be okay.

Okay, that leaves hundreds of asteroids and NEOs, the moons of Mars, Mars, the Moon, Earth, and the further large moons of Jupiter, and the trojan asteroids of Jupiter, and probably some other stuff I'm missing. With some work, we could probably get to Mercury and the outer solar system. Venus, and the gas giants, well, not likely. Titan might be tricky, as it's atmosphere is literally flammable.

• Just because current suits don't go to Titan level cold doesn't mean we couldn't do it with today's tech. It's just NASA has had no reason to build such a suit. Venus is quite another matter, though! – Loren Pechtel May 1 '16 at 19:13
• Current space suits are made for vacuum. I don't think any suit could resist in any hot or cold environment if there is any form of atmosphere. – Antzi May 2 '16 at 1:16
• There are 4 bodies with solid surfaces with an appreciable atmosphere in the solar system, Venus, Earth, Titan, and Mars. Mars is too thin to matter. Venus and Titan I mentioned as being difficult. And in fact, space suits are worn on Earth for some kinds of testing. – PearsonArtPhoto May 2 '16 at 1:26
• We still didn't begin employing aerogel for spacesuits. Its truly wondrous thermoisolation properties could help a lot on Titan. – SF. May 2 '16 at 4:01
• @SF. Titan's issue isn't the insulation per say, but the fact that it's atmosphere is composed of flammable gases. We could probably build a space suit that would allow one to walk there, but all kinds of issues might result, especially when returning back to the spacecraft. – PearsonArtPhoto May 2 '16 at 11:37

Other than on the four gas giants you cannot walk in present-day-like spacesuits on Venus (due to the extreme heat) and Jovian moon Io (due to the extreme radiation and also some heat) either. But since you're including near-future spacesuits I think one could develop a spacesuit that would handle the radiation on Io for some time.

However, note that you could indeed walk on an airship or cloud city in the upper atmospheres of Venus, Saturn, Uranus and Neptune (not on Jupiter due its high gravity and radiation) and you wouldn't even need a pressurized suit at all, just an oxygen mask and on the gas giants protection from the cold. Same for the surface of Titan.

Let's split the question into parts.

where can you walk

The celestial body has to have enough mass, so you can walk/stand on it, without drifting away after the slightest movement. Should have landmass (no gas planets) / hard surface -> ice surface

time

The biggest issue is time, the acid rains on Venus are not that much of a problem, if you stay there only for a short period of time. The same goes for Pluto, you would need a lot of energy for keeping your body warm.

Even Mercury's hot side could be a destination, if you don't extend the visit too long, but landing there would probably be the bigger issue (gas eruptions and so on).

physics

Celestial bodies that are so close to other objects, that they interact in a bad way for things you don't want to be broken -> solar flares, quakes, gas eruptions and so on.

• Staying on Venus for a short time is not possible due to the high pressure of 92 bar and the very high temperature of 462 °C. – Uwe Nov 29 '17 at 10:27
• We currently build pressure suits that withstand 71 bars (en.wikipedia.org/wiki/Atmospheric_diving_suit). 92 bars isnt much of a stretch if you reeeaally wanted to go. The same goes for the temprature (en.wikipedia.org/wiki/Fire_proximity_suit) – Git Dec 18 '17 at 10:20