the astrophysics option comes from the prelim topic on cosmic engine (which is studied by default) so yeah. If you are going to venture into astrophysics there is some theory lol
you would need to know some parts of cosmic engine to do the HSC space topic (not astrophysics). So far the Kepler's law is covered in prelim, but in HSC it is covered in more detail and stuff. I don't think I remember stuff on newton's law of gravitation being covered in prelim. But the HSC space topic is a little bit different than prelim's cosmic engine so it shouldn't affect you too much. But if you were to do astrophysics (which is covered really late.. as in term 3 year 12 so I doubt it would be of much use until then) you should study and master cosmic engine in prelim first.