# Can you ride a bicycle on Deimos?

Deimos is the small moon of Mars. The surface gravity is 0.003m/s2 (compare to Earth @ 9.807 m/s2). In theory a person with a bicycle could launch from and land on Deimos with a bicycle and a ramp.

A variation of the loop below, could in theory, be used increase traction on Deimos for take off and landing.

But would you actually be able to ride a bicycle on Deimos? With only human power would there be enough friction to move forward or turn?

• Do you mean on a specially constructed surface, or on the actual natural surface of the planet, craters and all? Normal inflated rubber tires, or "Deimos-special" tires? I mean the tires and ramps could have magnets or 22nd century nano-intelligent SpaceVelcro®. What are the constraints?
– uhoh
Jun 12 '16 at 11:02
• I had not thought of magnets nor Velcro (a tin or steel roadway, has lots of possibility now that you mention it. You can't use a power source to hold you down. Using rockets to give down pressure would be counter to the principal of human powered. I was thinking the natural surface primarily, but non-power consuming infrastructure would be fine. Loops, ramps, roads, trails, pretty much every place you ride a bike on Earth has had some infrastructure development. Bike modifications are unlimited, as long as you stay to human power only. Jun 12 '16 at 22:24

At that surface gravity, I don't see how it would be possible to ride a bicycle. The friction between the tires and the road is how the motion of the wheels is converted to motion of the bicycle and the rider.

0.3 milligee is practically no gravity at all. It would take 80 seconds to fall to the surface from a height of one meter - not so much a fall as a lazy drift. I don't think a person could even walk effectively on the surface of Deimos.

• JAXA's rovers agree. "Gravity on the surface of Ryugu is very weak, so a rover propelled by normal wheels or crawlers would float upwards as soon as it started to move. Therefore this hopping mechanism was adopted for moving across the surface of such small celestial bodies. The rover is expected to remain in the air for up to 15 minutes after a single hop before landing, and to move up to 15 m horizontally." Aug 6 '19 at 20:18
• @CamilleGoudeseune I'd like to read more on that, do you have a link to the article you took that from? Jun 22 '20 at 16:16
• @Speedphoenix The text appears in several news articles, but I think this is the origin: hayabusa2.jaxa.jp/en/topics/20180919e Jun 22 '20 at 16:20

Yes, with suitable modifications.

The loop must be ferrous. Your bike needs strong magnets in the wheels. You will start out with low friction and have to pedal very gently but as you build up your speed your friction will increase. Arrange the loop so you can go round and round as many times as you want before leaving it. You could build up substantial speed that way, plenty sufficient for an escape orbit.

If your guidance was good enough you could land the same way--enter the loop and gently apply your brakes. You wouldn't have much margin, though, I can't imagine such a landing system ever being actually used.

• To what extent do magnets (or velcro) holding you to the track resist the force you are applying to move to a different place on the track? Sep 5 '18 at 2:58
• @WGroleau Magnets, not velcro will have no effect on your movement around the track. If the magnets are on the rim the ones approaching the ground pull the wheel around exactly the same amount as the ones moving away retard you. If you have a fixed magnet behind the wheel it's not moving toward or away. Sep 5 '18 at 3:26
• But the one touching the track resists being removed from the track. When I've tried to pull two magnets apart, it seems the force required is not linearly proportional to the distance. Sep 5 '18 at 3:28
• @WGroleau But while you're removing one from the track you're putting another on the track. The two forces balance. Sep 5 '18 at 3:31
• @WGroleau You're right about that. Per Coulomb's law: the force exerted by magnets is proportional to the inverse square of the distance between them. Jun 22 '20 at 17:25

I think, if you assume a flat surface, nothing would physically prevent a bicycle being ridden, slowly, on Deimos.

In particular friction is not a problem: friction limits the amount of force the tyres can exert on the ground, but the force you need is extremely low if the acceleration you need is extremely low, which it is. Riding a bicycle is something where things scale with $$g$$: a bicycle can be ridden for any value of $$g > 0$$, you just have to ride it with acceleration (and, ultimately, speeds, if you don't want to reach escape velocity on the surface) which scale like $$g$$. This is all perfectly possible.

I think the problem would be getting a human to do this. We have spent a long time evolving for life on a planet where the $$g \approx 9.8\,\mathrm{m/s^2}$$ so we've evolved around the kinds of forces, powers and time constants to allow us to walk (and ride bikes) on Earth. To do it on Deimos you'd have to wind all this back by a huge factor, which would mean that a lot of things we do unconsciously in fractions of a second you would now have to do consciously over many seconds. My guess is humans would find that extremely hard. Perhaps robots could be taught to ride bikes on Deimos (pretty sure there's a science-fiction story here...).

On the other hand people can learn things pretty well, and there's plenty of time: the time taken for things to happen scales like $$\sqrt{1/g}$$, so if you assume you need to correct things on Earth in a fifth of a second to ride a bike, then on Deimos you'd have around ten seconds to think. The problem would be suppressing your reflexes. So I'm not willing to conclude humans couldn't do it, just that it might be hard.

Of course, in real life the surface is not smooth which would make things much more exciting. And also riding a bike would be really slow, especially on a non-smooth surface: if you wanted to get anywhere quickly better to jump.

One argument which is not true is that you couldn't ride a bike because bikes are stabilised by the gyroscopic forces from the wheels. They may be, but what is certainly the case is that humans can hold a bike up while it is effectively stationary (this is much easier, and may only be possible, if the bike is facing up a hill, or has no freewheel), so you can ride bikes with no gyroscopic help.

• @JamesJenkins: the existing answers, unfortunately, are wrong: friction is not a problem (I've added a paragraph explicitly stating that) as riding a bike is something which can be done for any value of $g > 0$. What is a problem is human reflexes: a human probably can't ride a bike, a machine certainly could.
– user21103
Jun 22 '20 at 12:45
• @JamesJenkins: I hadn't noticed the question was old. I always naively assume things which appear at the top of the list are new, which is obviously not true, but I don't understand what the mechanism is.
– user21103
Jun 22 '20 at 16:10
• @JamesJenkins Thanks, I agree, the first version was written too quickly!
– user21103
Jun 23 '20 at 10:48