Space technology and astrophysics are rather different fields. The former sounds more like aerospace engineering. Since I have no experience here I can't really speak to that. But I am a first year astrophysics graduate student, so I can tell you something about this. Astrophysics, unlike technology-related fields, is more observational and experimental. It's basically the same as physics (i.e. experimental as opposed to applied), which is probably why most astro programs are done in combined physics and astronomy departments.
So, can you enter astro with a CivE background? As long as you have the physics background, yes! I was a physics and math major back in undergrad. Until my first semester at grad school, I'd never taken an astronomy course in my life. Most of the coursework requirements for my program are physics rather than astronomy courses (granted, my research is more physics-based than astronomy-based). It's OK to get a graduate degree in astronomy without having any prior background, but you need to have a strong physics background. As my advisor says, you can always read about the basic ideas in astronomy on your own time, but you can't learn classical mechanics or quantum on your own; for that you actually need to have the required coursework.
As for entrance exams, I don't know of any graduate schools that do this. But most grad schools do have you take a PhD qualifier, which you're required to pass within your first two years. At my school the qualifier has a very strong physics bent. There are two parts: a classical and modern section. The classical section covers advanced undergraduate physics except for the modern topics, and contains no astro. The modern section covers quantum mechanics, questions on specialized subjects like high energy, condensed matter, etc., and one or two astronomy questions. So there you see how important it is to have a good physics background.
Anyway, I hope this helps!