Q & A: How could you change your direction in space?

Q:
Hi, We had a heated discussion at work about the following scenario: a man is traveling in space and there is an object traveling at the exact opposite direction on a collision course. Man is in a light suit and have nothing else with him. Can he avoid the crash with the object? Can he move himself out of the way using his own muscles, eg. punching 'air', or punching himself to change the direction ever so slightly? Or is it that the only way to change a vector of movement is outside force? Thank you. Best Regards, Lucas
- Lucas Stolarczyk (age 33)
London
A:

Sounds like an interesting discussion! Your last statement is basically correct. A more precise way to phrase it would be the following: the total momentum (the product of mass and the velocity vector) of a system cannot be changed by forces inside the system. Only an external force can change the system's momentum.

Assume that our astronaut is in an empty universe, with no gas or dust floating around, and no stars or planets. Then there really is nothing he can do to change the total momentum of himself and his suit. However, he can change the momentum of things within the astronaut-and-suit system; for example, by tearing something off his suit and throwing it.

If he does this, he will move in the opposite direction with a velocity that depends on the mass and velocity of the thing he threw. The two momentum vectors will be equal and opposite and the total momentum will still be conserved. This is how rockets work--they blast out tons of mass in the opposite direction of the way they want to go. If you don't let him throw anything (or spit or sneeze outside the suit, or let air leak out) then he's out of luck. Punching himself or anything else won't help.

But... there is one thing he might be able to do which doesn't seem to be disallowed in your scenario. (This idea came from a discussion in my office. Thanks for giving us something to do other than work!)

Light has momentum. If he had a flashlight, the astronaut could point it in some direction and get a very small kick in the opposite direction (see for more). This would take a very long time to have an effect, because the momentum involved would be tiny, but I'm assuming the astronaut is arbitrarily far away from the incoming object and only needs a tiny change in direction to escape, and that he's somehow immortal and can survive that long.

Okay, he's not allowed to have a flashlight--but he's still emitting light because his body is warm. His body heat is warming the suit, causing it to radiate very long-wavelength light, like the light sensed by infrared cameras. This infrared light carries off some momentum, so, in theory, it could give the astronaut momentum in the opposite direction. The trick would be to make sure that there's more radiation in a particular direction, otherwise the kicks in many different directions will just cancel each other out. The astronaut could try to achieve this by changing the position of his limbs, or by twisting his body.

This change in momentum due to asymmetrical thermal radiation . They drifted slightly off course because of the heat from the radioisotope generators that power their electrical systems. People proposed all kinds of explanations before the thermal recoil effect was accepted, even speculating that we needed a new theory of gravity.

Rebecca Holmes

(published on 09/01/2014)