system
Given the following post, respond as if you're commenting on Reddit:
human
If you were to set an object in motion in space in a random direction what are the chances it will continue on without ever hitting anything?
gpt
If it's faster than the escape velocity of the galaxy, then it would almost certainly never hit anything ever (unless it happens to start right next to a planet etc). This is quite fast - over 500 km/s - but note that we're already going around the Milky Way at over 200 km/s in our orbit. Milky Way escape velocity is <0.2% of the speed of light, and at that speed it would still take you thousands of years to reach Alpha Centauri, so while it's fast, it's not sci-fi ridiculous fast.

Once you escape the Milky Way, the odds of you ever reentering another galaxy are near zero. As the universe is expanding, you don't get infinite chances at this. Basically if you're not aimed straight at a nearby galaxy, you'll just drift through space forever, and never reach another galaxy ever. And even if you reach another galaxy, you will shoot right through with low odds of hitting anything.

If you're slower than the escape velocity of the galaxy, then you get to orbit around the Milky Way for a very long time, and you will *eventually* hit something. However, this is only over an extremely long time-scale.

There's a couple of ways to picture this. One is that a star is more likely to hit another star than a spaceship is to hit a star. Two stars have more gravity and pull each other together. A spaceship doesn't accelerate stars or planets significantly, so gravity is less of a help. So the time-scale over which you have a 50/50 chance that your drifting spaceship has hit a star or planet is *at minimum* the same time-scale over which *every star* has a 50/50 chance of hitting another star. Basically, by the time a drifting spaceship has hit a star, so many stars in the Milky Way will have had mergers or close encounters with other stars that the Milky Way will look very different. Many stars will be ejected from the Milky Way by close encounters or fall down to the centre. This very far in the future - thousands of times the current age of the universe.

Another way is to think of it in terms of Olber's Paradox. For unresolved distant stars - as in the Milky Way - they blur together into a faint wispy white cloudy band across the sky. This is of course not as bright as the Sun. The reason it's not as bright as the Sun is because there's big gaps between the stars. Basically, the brightness of the Milky Way is the "filling factor" of these stars, multiplied by the surface brightness of the stars (which is similar to the surface brightness of the Sun). Basically, if the Milky Way looks a million times dimmer than the Sun, that would be because the gaps between the stars are a million times bigger than the area filled by stars. The fact that the Milky Way is so dim tells you that there are huge gaps between stars, even if you add up all the stars along your line of sight, and any random ray will almost certainly miss every single star.

Note that this means that this popular quote from Mass Effect 2 is not accurate:

> Every five seconds, the main gun of an Everest-class Dreadnought accelerates one to 1.3 percent of light speed ... Once you fire this hunk of metal, it keeps going 'till it hits something! That can be a ship, or the planet behind that ship. It might go off into deep space and hit somebody else in ten thousand years. If you pull the trigger on this, you are ruining someone's day, somewhere and sometime!

Unless a high-speed projectile hits something immediately (i.e. a ship or a planet behind the ship), it will almost certainly never hit anything ever while drifting through deep space. It'll fly straight out of the galaxy and never be seen again.