[Scarblac]

> Of course the energy requirements are insane and would require some kind of physicists nightmare propulsion system like antimatter rockets. Lose reactor containment and you are instantly converted into plasma and gamma rays. If you hit something the size of a dust particle it will destroy your spacecraft.

And, this one spacecraft needs to be able to slow down, have the telescopes and equipment needed to identify exoplanets and decide they are suitable, have the means to travel all the way there, and have everything aboard to start a complete new viable colony on some unknown planet. It probably needs to bring all the energy for the first few millenna in batteries. That must mean a huge spacecraft, and there also needs to be some way to land it all on the planet, hopefully with all the pieces close to each other. Et cetera.

The engine that got all that close to light speed is just a tiny piece of the puzzle.

[rewgs]

Ha! Funny, I've never considered it even though it's so incredibly obvious: the spacecraft needs to be able to slow down out of light speed. That might honestly be even harder than getting up to light speed, and that of course is almost certainly impossible.

Imagine the "runway" you would need. How many lightyears would it take just to slow down? And that's before you even consider how some sort of reverse propulsion mechanism would even begin to turn on without tearing the ship apart in a microsecond.

[hwc]

If you simply assume that your spacecraft uses its own engines to accelerate to cruising speed, then the Δv to decelerate is approximately equal to the Δv to accelerate.

Similarly, if the engines are operated at a constant acceleration, then the distance needed to decelerate is equal to the distance needed to accelerate.

Fuel(/reaction mass), on the other hand, us not equal. The fuel to be used to decelerate must also be accelerated first, meaning more fuel is used to accelerate than to decelerate.

[ncmncm]

Maybe count on somebody where you are going to help you slow down? They will see you coming a hundred thousand years ahead of time. They might decide it is safer to just park a planet in your way, though.

So for, what, two million years or five, the front of your spaceship is blasted with near-lightspeed intergalactic-medium molecules, plus trillions of dust particles and an unknown number of rocks.

"Erosion" is not an adequate word, but we don't have any that is.