This is actually a straightforward question to answer, I think: By conservation of momentum, the center of gravity of the entire ISS + astronaut system needs t

This is actually a straightforward question to answer, I think: By conservation of momentum, the center of gravity of the entire ISS + astronaut system needs to stay on the same path before and after your wall bounce. So, the momentum change for the ISS is equal in magnitude but opposite in direction to the momentum change for the astronaut, and the velocity change will be the velocity change for the astronaut times astronaut mass/ISS mass Getting actual numbers to plug in left as an exercise for the reader.