20 credit hours per semester - manageable?

It sounds like your going into this with a very cocky attitude. If you can stay cocky once you get that homework load I think you'll do fine. All the people I know who load up on credit hours like that seem to spend most of their time talking about how much work they have though, not necessarily complaining, more like bragging. But if you think you can do it, and then you do do it, I'm sure you will have a fantastic time telling your fellow astrophysicists how you did a bunch of extra work to get an extra degree.