Hello Ravi Kumar!

If you want to work at NASA there are several engineering branches you can take up

1. Aerospace engineering
2. computer hardware engineering
3. electronics engineering
4. mechanical engineering etc

Several aspects of engineering can be applied to NASA .Every domain has one or more application for sure.

I prefer going to engineering than choosing science degree because engineering gives you a lot more clear understanding.

After PhD you can take up many kinds of jobs like  post secondary physics teachers, Biophysicists ,physicists and astronomers ,Natural science managers, Petroleum engineers etc.

All these jobs have more than 6% job growth

Hope it helps!

All the best!!