system
Given the following post, respond as if you're commenting on Reddit:
human
How can nuclear reactors work without steam?

There are spacecraft in space right now that are powered by onboard nuclear reactors. Surely they don't use steam to spin a turbine like a normal land-based nuclear power plant. That's a lot of extra weight to carry into space. Turbines are heavy.

So how do these reactors work? Have we found a way to convert heat *directly* into electricity?
gpt
Well there are no spacecraft in space **right now** that is powered by a nuclear reactor. All the nuclear powered spacecraft operational now are powered by RTGs (radioisotope thermoelectric generator). Other can explain the difference better but the basic idea is that a RTG is a purely passive device that produce heat from the natural decay of some radioactive material. In a nuclear reactor the fission is controlled and can be shut off completely. About 35 real nuclear reactors where ever launched in space between the 60 and late 80's (nearly all of them Soviet, 1 American). 

RTGs and nuclear reactors all use thermometric converters relying on the Seebeck effect. It's the same thing used in thermocouples (or Peltier devices). The idea is that two different metal at different temperature put in contact will produce a little bit of electricity. It is super convenient for space applications because it has no moving parts. The issue is that it is super inefficient, you usually get less than 5% of the thermal energy converted into electricity. It is also fairly expensive which is why it's not used much on earth.