Propulsion in space is an extremely tricky affair, one that’s
centred heavily on trade-offs. The engines we use to get into
space are woefully inefficient due to the large amount of
propellent that has to be taken along with them. The
faster/further you want to go the more propellent you need which
makes the rockets increasingly bigger, putting a soft upper
limit on what makes for a feasible craft. On the flip side once
you’re in space we have engines with efficiencies that are so
good that they can achieve incredible speeds with fractions of a
percent of the fuel that it takes to get them into orbit. It’s
no wonder that these engines were chosen for the Dawn mission to
Vesta and Ceres. There’s also engines that straddle the
boundaries of these two like the VASIMR which aren’t capable of
getting payloads off the surface of the earth but are quite
capable of performing the same tasks as chemical rockets in
space with a fraction of the required reaction mass (fuel). The
trade off here is that it requires a rath