Neil's News

EVA Gunfire

4 November 2014

Thao's school gave her this exercise from NASA which asks one to identify which items would be useful for a stranded astronaut who needs to hike 200 miles across the Moon. Most of the list is fairly straight-forward: oxygen, water, maps, etc. But then they state the "two .45 calibre pistols" are a "possible means of propulsion". Er, what?

First of all, guns will fire in a vacuum. The propellant has the oxidiser included, so air is not needed to fire. That said, the lack of convection means that heat will become a problem, and the gun will jam after a few shots. But let's assume we can discharge both guns' entire load of 11 bullets (ten in the magazine, one in the chamber) before they jam.

The most energetic .45 calibre ammunition I could find is a Cor-Bon cartridge with JHP load. It has a mass of 12g and exits at a velocity of 350m/s. Twenty-two such shots fired in an instant produce a total linear momentum of 92kg m/s.

Opposing this is the astronaut's own mass. An astronaut on EVA has a mass of no less than 200kg (a 100kg space suit, a 60kg person, and 40kg of supplies). This is mass, not weight, so the fact that we are on the Moon is irrelevant. Dividing it out, we find that firing 22 bullets will impart a 0.46m/s motion on the astronaut in the opposite direction. It would be advisable to shoot from the hip (centre of mass), otherwise a spin will result.

Ok, so what happens if one is pushed at 0.46m/s? In space, you'd just float off at that speed indefinitely, and cover the required 200 miles in 8 days. Hope your aim was good. But on the Moon, you'd just fall over. You still weigh 33kg, and your boots are firmly planted on the ground. In order to get any horizontal displacement, you need to jump up, fire the guns sideways, then glide sideways until you land. Videos show that a good jump on the Moon will keep one aloft for two seconds. Thus in that time one will be able to travel nearly a meter.

So you've got a choice: (a) walk two steps, or (b) jump, let loose a hail of gunfire, and move at half the speed. I'll take (a), thank you.

Another option is firing straight down. Maybe you could scale a cliff or something? Well, no. Remember that the recoil from the gun gives us 0.46m/s. Compared against the lunar gravity of 1.62m/s², that won't even get us off the ground.

NASA needs to get a physicist to check their educational materials.

Speaking of NASA, here's my opinion of the best Halloween costume at Google this year: a Mars Rover. And speaking of Halloween, here are Guinan and Crusher exploring my pumpkin. And speaking of rats in pumpkins, here is Quynh's carving for this year.

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