Why do rockets go in space instead of jets?

Answer 1

Both engines types work on a similar principle of compressing a working fluid and ejecting it very fast to create thrust, but the main reason for why we currently use rockets in space is simply because of the way Jet engines work. Virtually all Jet engines breathe air and use fuel to heat/compress it. This works great in-atmosphere and can even become extremely efficient at higher speeds, but obviously does not work in the vacuum of space where there is no air. The lack of air and pressure would also cause other indirect problems like overheating and extreme structural damage in jet engines.

On the other hand, rockets carry everything they need (fuel, gas pockets, oxidizer, etc) so that they are fully self-contained without needing to suck anything in. The fact that rockets have to lift all of their own fuel and require so much of it complicates things a lot and decreases their range/economic performance but it's really the best we can do right now. There are ways to blur the line between jet and rocket engines, such as by using nearly-hypersonic ramjet variants to put rocket-powered spacecraft into orbit to save a bunch of fuel while requiring far smaller rockets, but in the end, it's still only the rockets that can operate in space.