heat dissipation in space PCB
Thermal management in spacecraft is usually its own subsystem separate (but connected) to the electrical system. That means it's important for us to know where your space PCB is living. Is it inside a larger satellite like a cubesat? is it a sprite that's just floating around? These will inform the particulars of your solution.

Regardless, the only way you're going to get rid of heat is through radiating it away. This is normally done by having a flat plate mounted to your spacecraft that's painted white, which is then thermally connected to your amplifier. This connection can be made by simply bonding the plate directly to your amp with thermal paste (if its a sprite), or, if your pcb is inside a satellite, with something like a heat pipe, to connect the amp to the exterior structure. 

In the end, all you're doing is increasing the surface area of the amp with a piece of metal and heating that metal up so that the excess energy gets sent away as infrared radiation. It's not as dramatic as convective cooling here on Earth, but it will do the job.