Last night, this question popped into my head and I wrote it down.
This morning, I looked it up.
“Fires can’t burn in the oxygen-free vacuum of space, but guns can shoot. Modern ammunition contains its own oxidizer, a chemical that will trigger the explosion of gunpowder, and thus the firing of a bullet, wherever you are in the universe. No atmospheric oxygen required.
The only difference between pulling the trigger on Earth and in space is the shape of the resulting smoke trail. In space, “it would be an expanding sphere of smoke from the tip of the barrel,” said Peter Schultz an astronomer at Brown University who researches impact craters.
The possibility of gunfire in space allows for all kinds of absurd scenarios.
Once shot, the bullet will keep going, quite literally, forever. “The bullet will never stop, because the universe is expanding faster than the bullet can catch up with any serious amount of mass” to slow it down, said Matija Cuk, an astronomer with joint appointments at Harvard University and the SETI Institute. (If the universe weren’t expanding, then the one or two atoms per cubic centimeter encountered by the bullet in the near-vacuum of space would bring it to a standstill after 10 million light-years.)
In space, “theoretically you could shoot yourself in the back,” Schultz said.
You could do it, for example, while in orbit around a planet. Because objects orbiting planets are actually in a constant state of free fall, you have to get the setup just right. You’d have to shoot horizontally at just the right altitude for the bullet to circle the planet and fall back to where it started (you). And you’d also have to consider how much you’ll get kicked backwards (and consequently, how much your altitude will change) when you fire.
“The aim has to be perfect,” Schultz said.
Such a scenario isn’t as absurd as it sounds. In fact, Schultz said scientists at one point were considering setting up such a self-hit in space in order to investigate the effects of high-speed impacts.
However, considering all the math involved, Cuk suggests it might be easier to commit space suicide by standing on a mountain on the moon. “‘Shooting yourself in the back’ works in principle if you shoot a bullet at horizon from the top of a lunar mountain, at 1600 meters per second or so,” he said. He thinks it just might work as long as you adjust your aim to account for lumps and irregularities in the shape of the moon, which would affect the altitude of the bullet as it travels.”