What are the things I can specialize in Astronautics/Aerospace Engg. ?

What are the areas of specialization in this field? I am planning on pursuing an MS graduate degree in this field in the US. So I looked through the work done by various professors in many universities.

I want to work on something which will help me contribute to human spaceflight. So far I have identified a couple of areas I find interesting. Propulsion ( Electric and the other new forms of propulsion like VASIMR seem interesting ) and Orbital Mechanics ( calculation of the trajectory etc. ).

What other fields are there which I can focus on?

As for background, I am currently in the final year of an undergraduate course in Mechanical Engg. I am also good at computer programming.