> takes the same amount of time to slow down as it did to speed up

Assuming you're going somewhere, you can use atmospheres and gravity to slow down. Decelerating should take less time than accelerating in practical contexts.

That is doable for "normal" speeds, not one where you accelerate 1G for twenty years, reach relativistic speeds that make you travel for hundreds of thousands of light-years in merely 20 years ship time

> doable for "normal" speeds, not one where you accelerate 1G for twenty years

It's still a lot of energy you can bleed off, particularly if you're aiming for a system with gas giants. I'm not suggesting one only rely on passive deceleration. But especially given it's fuel saved at the very end of the journey, fuel you no longer need to accelerate and decelerate for the entire duration of the trip, the savings could be sizeable.

If you are going really, really fast, you wouldn't just want gas giant planets. You'd want the outer layers of red giant stars.

the top of this thread is filled with a discussion of the almost unimaginably catastrophic consequences of a ship moving at 0.9C hitting atoms in interstellar space.

trying to decelerate by "braking" anywhere close to a gravitionally significant mass sounds like a guarantee to total destruction from the impact of "stuff" (even individual photons).

> a ship moving at 0.9C

You decelerate from 0.9C to 50 km/s conventionally, more if you can aerobrake or line up multiple slingshots, and that last 0.00001% with gravity assist.

It saves you more than that in fuel, because the fuel you'd have used on that last bit of deceleration needed to be accelerated and decelerated the entire way from 0 to 0.9C back to close to zero.

Right, because space is known to be full of atmospheres to use to slow down. ???

> space is known to be full of atmospheres to use to slow down

The places people talk about travelling to tend to have atmospheres and gravity, yes.

How many G's of deceleration are we talking here? I imagine even 2Gs of force wouldn't be tolerable for humans for more than an hour

> even 2Gs of force wouldn't be tolerable for humans for more than an hour

You can tolerate 2G for hours, particularly if everyone's oriented eyeballs in. That said, both aerobraking and gravity assists are intermittent accleerations.

so you've been traveling at some large speed for quite some time, and you are now proposing to come to a near stop to come into orbit around some far away planet? and what magical tech have you forgotten to tell us about that allows that sudden deceleration to not liquefy the bags of meat inside the ship?

We'd still be limited by however many Gs human body can handle right?