The problem with space being cold is that it’s also empty. You know how a windy day at 40 degrees out feels much colder than when it’s 20 degrees out but with no wind? Take that to its logical extreme. Things do cool down in space, but not by convection or conduction so it’s very slow.

> nuclear-powered spacecraft, which would allow spacecraft to travel further distances without needing liquid fuel

Nuclear thermal rockets still use a liquid fuel. There still has to be some mechanism for momentum transfer, and that means shooting something out the back of the rocket really fast.

Specifically, NTR thrusters use hydrogen superheated from passing over a nuclear reactor. Exhaust velocity of tested variants from the 60s and 70s were up to roughly twice the exhaust velocity of typical chemical propulsion systems.

The disadvantages are the ones you’d expect: putting a nuclear reactor on top of a giant controlled explosion is risky, and mounting a nuclear reactor makes the engine weight significantly higher so thrust to weight ratios are much lower. Still, it’s expected they would provide a significant performance advantage if engineered to fruition.