What is the scope of aerospace engineering in NASA?

There is no specific entry scheme to get into NASA but you need to be exceptionally good to get into NASA.
Make sure you finish tops in your group during your degree program in aerospace engineering. Publish quality technical research papers in journals. Focus and devote yourself to your research programs. Take up fellowship programs.

It is quite tough to get to NASA especially if you are not an American citizen but nothing is impossible. Join some NASA competition or reach out to some one that is already working in there. Be hopeful and devote yourself to your work.