In some of the world's best science fiction films, starships and even the actors themselves
engage in massive space battles. While most of these battles are fought with
laser blasters or cannons rather than older black powder muskets or modern semiautomatic
pistols and rifles, it still makes you wonder if a modern gun was fired in outer space.
Modern firearms indeed use combustion. It is also true that outer space is essentially
an airless vacuum, so most would assume such a gun couldn't fire.
Instead, gunpowder contains its oxidizer, which allows guns to fire even in airless
environments, such as underwater or even in space.
In the oxygen-free vacuum of space, fire cannot burn, but guns can fire. Wherever you are
in the universe, ammunition contains an oxidizer, a chemical that triggers the explosion of
gunpowder, and thus the firing of a bullet. It does not require atmospheric oxygen.
The only difference between pulling the trigger on Earth and in space is the shape of the
smoke trail that results. During an impact, it would expand like a sphere of smoke from
the tip of the barrel. In space, gunfire opens up a wide range of
absurd scenarios. The gunshot will not be heard, since there is no air for the sound
waves to travel through. Furthermore, the smoke leaving the barrel
of a shotgun would coalesce into a sphere at the end of the barrel. This is similar
to how water forms into orbs on the International Space Station because of gravity.
What about the projectile? With an ordinary gun, the bullet would have no air resisting
its journey. Therefore, it would theoretically just keep moving through space at a constant
speed, forever, until it smashed into something. In the latter case, Newton's third law of
motion dictates that the force exerted on the bullet will impart an equal and opposite
force on the gun as well as on you since you are holding the gun.
If you don't have any intergalactic atoms to brace yourself against, you'll start moving
backward. Because you're much more massive than it is, when a bullet leaves the gun barrel
at 1,000 meters per second, it will head the other way at only a few centimeters per second.
The recoil of the firearm would also affect the shooter. In the vacuum of space, the force
emitted by the projectile igniting and leaving the barrel would cause the person to rotate
slowly. All of this is hypothetical, and firing a
gun in outer space would be reckless. It shows just how unrealistic some of those science
fiction guns and cannons are in light of actual physics and the dangers they would pose in
space-based shootouts. Imagine that you are floating freely between
galaxies with just your gun and a single bullet. Once fired, the bullet will continue to travel
indefinitely. The bullet will never stop because the universe is expanding faster than a serious
amount of mass can slow it down. If the universe were not expanding, the one
or two atoms per cubic centimeter encountered by the bullet in the near-vacuum of space
would bring it to a halt after 10 million light-years.
Actually, the universe expands at a rate of 73 kilometers per second per megaparsec. This
means matter that is 40,000 to 50,000 light-years away from the bullet would move away from
it at about the same speed at which it is traveling, and would thus be forever out of
reach. In the entire future of the universe, the
bullet will catch up only to atoms that are less than 40,000 or so light-years from the
chamber of your gun. Guns are carried into space, but not quite
to the void between galaxies. The standard survival pack for Russian cosmonauts has included
a gun for decades. It wasn't just any gun, but a deluxe all-in-one weapon with three
barrels and a folding stock that doubles as a shovel and has a swing-out machete.
Cosmonauts receive space guns just in case they need them back on Earth. This is to defend
themselves in case an emergency landing of their Soyuz spacecraft leaves them stranded
in a dangerous region. However, astronauts could theoretically fire their guns before
landing. Shooting from the hip should not be a problem
for the astronaut. Even if a bullet is badly targeted, Jupiter's enormous gravitational
field will absorb it. The gas giant is so large that it will catch the bullet and then
follow a curved path into the planet. As a result, it will generate a lot of steam.
Due to Jupiter's gravity, the bullet will travel at almost 60 kilometers per second
by the time it crosses Jupiter's threshold. It is cowardly to shoot someone in the back.
You could theoretically shoot yourself in the back in space. In orbit around a planet,
you could do it. Objects orbiting planets are actually in a
constant state of free fall, so the setup must be precise. For the bullet to circle
the planet and fall back to where it started, you'd have to shoot horizontally at just the
right altitude. In addition, you have to consider how much
you will be thrown backward if you fire. This scenario isn't as absurd as it sounds.
At one point, scientists considered setting up such a self-hit in space to investigate
the effects of high-speed impacts. It might be easier to commit space suicide
by standing on a mountain on the moon, considering all the math involved. When you shoot a bullet
at the horizon from the top of a lunar mountain at 1600 meters per second, you can shoot yourself
in the back in principle. If you adjust your aim so that you can account
for lumps and irregularities in the shape of the moon, the bullet will travel at a higher
altitude. While the speed of the bullet would be relatively
the same as that on Earth, the distance it travels would be very different. The force
of gravity from the Earth draws a bullet down to the ground and robs it of its energy to
continue flying. A bullet flying on Earth might travel a few
miles before hitting the ground because of gravity.
However, in space, the bullet will go on and on, possibly forever, since no strong force
will slow it down or pull it down. The vacuum of space also does not have friction to slow
down the speed of the bullet or alter its course.