# Do the moons of Mars have enough gravity to colonize?

## Context

NASA commissioned some awesome, vintage-style prints a while back advertising the colonization of Mars.

Three of them stood out to me because they depicted colonization of the moons of Mars - Phobos and / or Deimos.

(The rest of those prints are here, and they're free)

I would like to have colonies on these moons in my world, but the moons are extremely small relative to our moon (or even Mars) so they have very low gravity. Knowing NASA, there must be a scientific basis for these images, but intuitively, this does not seem possible.

Is there any scientific basis behind strong, gravity-like effects in these locations? If not, what can be engineered to allow "gravity" in such open spaces?

Note: "Enough gravity" means that people won't escape the planet with everyday movement; low gravity should also not be extremely inhibitive (ex. flying 10 feet into the air when you take a step up)

• Phobos' surface gravity is 0.0006g, Deimos is .0003g. We know their magnitude surface gravity, so there is no scientific basis for enough gravity to walk around in. What is the question? – kingledion Dec 16 '16 at 14:28
• The prints are of Mars itself, not the moons of Mars. – John Feltz Dec 16 '16 at 14:29
• @JohnFeltz Check the background... – MichaelK Dec 16 '16 at 14:31
• @Zxyrra We already know how to work in zero gravity without drifting into the big black yonder. So what more do you need? Really short answer to your question: yes, because of personal maneuvering thrusters and tethers. en.wikipedia.org/wiki/Manned_Maneuvering_Unit – MichaelK Dec 16 '16 at 14:34
• @Mołot I mean without being inhibited by jumping 10 feet into the air, etc. but I will edit this information in so it's clear – Zxyrra Dec 16 '16 at 15:01

# How to calculate surface gravity

Surface gravity ($\hat{g}$) is a function of the mass ($M$) and radius ($r$) of the planet:

$$\hat{g} = \frac{G\cdot M}{r^2},$$

where $G$ is the universal gravitation constant $6.67\times10^{-11}\,\frac{\text{N}\cdot\text{m}^2}{\text{kg}^2}$. If you assume your planet is in hydrostatic equilibrium (a good assumption for any planet with noticable surface gravity), then mass is in turn a function of radius and density ($\rho$):

$$M = \rho\frac{4}{3}\pi r^3.$$

Put these together and you get:

$$\hat{g} = \frac{4}{3}\pi G\rho r.$$

Proof. The radius of earth is 6371 km; the density is 5515 kg/m$^3$.

$$\hat{g}_{earth} = \frac{4}{3}\pi \left(6.67\times10^{-11}\,\frac{\text{N}\cdot\text{m}^2}{\text{kg}^2}\right) \left(5515 \frac{\text{kg}}{\text{m}^3}\right) \left(6371000 \text{m}\right) = 9.81 \frac{\text{m}}{\text{s}^2}.$$

# Surface gravity of Phobos and Deimos

To calculate surface gravity of Phobos and Deimos, we need the density and radius. Phobos has a mean radius of 11.3 km and density 1876 kg/m$^3$; Deimos is 6.2 km and 1471 kg/m$^3$. Since both objects are irregular (not perfect spheres) there is variable gravity on different points of its surface, but surface gravity at mean radius for Phobos is 0.0003g and Deimos is 0.0002g.

# How to stay on the surface

The escape velocities for Phobos and Deimos are 8 m/s and 5 m/s respectively. That is obviously very low. If you can jump half a meter (as in a box jump), your initial velocity is about 3 m/s. So Michael Jordan could definitely jump off these moons, and I probably could have too, back in high school. In order to stay on to something you have no business staying on, we should use the same thing people use on Earth: ropes. It wouldn't be easy to just walk around, but if you had work to do on the surface, get there in a space-suit with micro-thrusters to keep you from drifting away accidentally, then attach your harness to a secure point on the moon's surface and get to work.

Astronauts use bungee cords on tread-mills to give a more Earth-like sensation of being pulled down. Some thing like that could be used as well, in addition to the safety harness, to give you more traction with the ground in your immediate work area.

• Don't forget the tide, which makes apparent gravity at the in/out poles less than it would otherwise be. I haven't found a source that says clearly how much this effect is; some can be read as saying that the tide exceeds the gravity, so only the strength of the rock holds it together. – Anton Sherwood Jan 27 '17 at 5:11

Colonizing either of Mars' moons would be no different than colonizing a large asteroid. Essentially, that is what they are. There are lots of concepts for asteroid colonization, including encasing the entire thing in a gigantic dome that would allow you to pressurize the environment and would contain objects and people that would otherwise go flying off into space.

Without going into the vast cost and size involved in totally encasing the surface of one of these moons with a dome, we could still have a very large dome covering some significant part of the moon, which would allow us to use the moon basically as a really large space station with fairly cheap (in terms of fuel cost) docking/launching for ships. It might make a good repair depot for vessels of different types, with custom built ship cradles and a nice dome town with a lot of velcro and "nerf" like surfaces that keep people from giving themselves a concussion on the top of the dome when they jump too high.

Unlike repair docks for a ship in high orbit, tools would not just float away forever if dropped, but would collect at the floor of the "hangar" area. It would be easier to scoop up loose pieces and parts at the end of the day.

• I remember serious plans from NASA for asteroid 'terraforming' that involved using parabolic mirrors to focus sunlight on the body until the core liquified, and then ballooning it up. Let it cool, seal it, pressurize the interior, and spin it for centrifugal gravity. Everybody would live on the inside. – kbelder Dec 16 '16 at 18:11
• That is one of those nerd projects (like taking a virtualization blade server and trying to use it as a gaming PC, or putting a gas turbine engine in your muscle car) that sound both: extremely cool and EXTREMELY difficult/probably not worth it. ;-) – JBiggs Dec 16 '16 at 18:16

I don't know what walking in microgravity would be like. But in the second image the worker appears to be standing on a metal platform; magnetic boots would secure her to the platform. In the third image the farm floor could also be made of metal.

The only other way to gain a gravity on these moons would be to hollow out some tunnels and spin the moons quite quickly. You could then walk on the sides of the tunnels closest to the surface (almost like a reverse gravity).

• I did calculate that on Phobos a tool dropped from waist height (1m) would take about 30 minutes to hit the floor. – Slam Dec 17 '16 at 2:26