Can I join NASA or ISRO after completing plain B.Sc?

You do not stand any chance of joining NASA or ISRO with B.Sc degree as a scientific officer. As a B.Sc graduate, you may get a reserach assistant job in ISRO.

Hello, Both of these are the top space agencies of the world. Simply graduating in aerospace/aeronautics etc. May not be enough, you might have to study further and go into research in a big way, M.Tech and Ph.D. would lead you to these organizations depending upon your research area. Pursuing these

You do not stand any chance of joining NASA or ISRO with B.Sc. Degree as a scientific officer. As a B.Sc. Graduate, you may get a research assistant job in ISRO.