Precisely what I was thinking. Expelling liquid helium doesn't introduce any heat. Any active cooling needs to somehow expel the heat generated while compressin

Precisely what I was thinking. Expelling liquid helium doesn't introduce any heat. Any active cooling needs to somehow expel the heat generated while compressing the gas back into a liquid. In the vacuum of space, irradiating heat is the only way to remove it from the system, and that isn't going to be very efficient. Given the choices and the temperatures they needed to reach, liquid helium seems like a rational solution.