I mean as long as it doesn't stray into the atmosphere of another celestial body or straight up hit something then it could theoretically go on forever.
This, recruits, is a 20-kilo ferrous slug. Feel the weight! Every five seconds, the main gun of an Everest-class Dreadnought accelerates one to 1.3 percent of light speed. It impacts with the force of a 38-kiloton bomb. That is three times the yield of the city buster dropped on Hiroshima back on Earth. That means: Sir Isaac Newton is the deadliest son-of-a-bitch in space! I dare to assume you ignorant jackasses know that space is empty! 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!"
There's an easter egg in Stellaris where one of your science ships gets hit by "an ancient projectile traveling at a fraction of lightspeed that seems to have come from a distant galaxy."
It seems someone didn't wait for their firing solution.
Imagine that kinetic strike
Edit: also wouldn’t it still be travelling at the same speed, making it the fastest ‘non-self repelled’ (or whatever) space craft in human history?
It comes right in the face of an alien, and also destroys a lot of the planet around him. But life goes on. Tens of millions years later, another alien species find some clues of what happened, but the manhole has disappeared upon impact. Life goes on. One day, this new species sets a camera to record a nuclear explosion...
Just to clarify, it wasn't a traditional manhole cover. It was a 4 inch thick, 1000 lb cap that was welded shut to cover the access tunnel of the 400 foot deep shaft dug for the underground nuclear bomb test.
It seems someone didn't wait for their firing solution.
Edit: also wouldn’t it still be travelling at the same speed, making it the fastest ‘non-self repelled’ (or whatever) space craft in human history?