What would happen if I peed on the moon?

17 January 2017

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Question

If, in some miraculous way, one were able to pee standing on the surface of the Moon, what kind of arc would it create?

Answer

Dr Chris Messenger from the University of Glasgow was on hand to help Tom Crawford with Michael's question.

Chris - The answer depends on the environment that you are in. If the astronaut is inside a pressurised and temperature controlled environment then the only difference between the moon and the Earth would be the gravity, which on the moon is 16% of the Earth’s. This gives the rather boring answer that the pee would simply follow a “straighter” arc, or, it would travel about 2 and a half times further than on earth depending on the trajectory.

Tom - So when I pee on earth, what shape is the arc that forms?

Chris - In a uniform gravitational field like on the surface of large objects such as the Earth and moon, the trajectory of an object (in our case each unit of pee) follows what’s known as a parabolic curve - sort of a u-shape. On Earth air resistance can cause objects to deviate from perfect parabolas but on the moon, since there’s essentially no atmosphere, the pee will follow a very accurate parabola. In fact, if you watch video of the lunar rover that the Apollo astronauts drove around the moon, you can see the dust being thrown into the air and following these exact curves.

Tom - Right, let’s assume we’re on the moon, or even better Neil Armstrong is there, back in 1969, and he needs to boldly go. Perhaps we can even ‘livestream’ the event as suggested by Stephen on Facebook… What would actually happen?

Chris - If we’re talking about somehow being able to pee directly out of your spacesuit while standing on the moon then the extremely low atmospheric pressure on the moon would immediately boil the pee. The steam would then fall to the floor under the moon’s weak gravity because there’s no atmosphere to carry the steam.

Tom - Hang on, I thought the moon was cold, why are things starting to boil?

Chris - There are number of different states that matter can be in, with the basic three being solid, liquid, and gas. The particular state that a particular substance is in depends only on the temperature and pressure. If you think about water (the main component of your pee) we know, for example, that when you climb a mountain, the air pressure drops and the boiling point of water starts to drop, so at the top of Mount Everest you can make a cup of tea at 71 degrees celsius. In the peeing on the moon scenario, the pee is at body temperature, around 37 degrees celcius and the pressure in the spacesuit will hopefully be at earth’s atmospheric pressure. As soon as it enters the moon environment the pressure drops by a factor of more than a thousand billion. The pee suddenly finds itself way above it’s own boiling point and has to immediately boil into steam.

Tom - If you thought the idea of peeing out steam sounds bad, it gets worse...

Chris - The steam molecules are able to react to the change in temperature between the spacesuit and the moon environment. Depending on the position of the sun in the lunar sky the temperature on the moon surface will range from -170 degrees Celsius to 120 degrees Celsius but since the pressure is so low, the freezing temperature of the pee also changes to a similar range, so in some circumstances the steam molecules will freeze into water ice crystals and you could get yellowish snow that falls to the ground under gravity.

Comments

If you where to piss on the moon would it float away ?

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