Steam Engine Spaceship

What is the most primative propulsion system that could be used in a
spaceship to travel between asteroids? I am specifically thinking of
one that could refuel on asteroids.

Is it possible, for example, to make a steam engine that uses the
direct rays of the sun to vaporise the water? Is there any advantage
to using a steam engine in space? I suppose you could split the water
into hydrogen and oxygen and burn that, but doesnt that require
electricity?

regards,

Philip