After BTech in Aerospace Engineering from Sastra University, I'd give you few options, which would perhaps lead you to NASA or similar Space Research Organizations.
1. After BTech, pursue MTech from some of the Good/Top IITs or IISc Bangalore. Post which you may get a Job with ISRO or some Top Aerospace Manufacturing Company. After few years of experience, you may perhaps pursue a PhD in Aerospace & Avionics Abroad in US or maybe directly apply for Opportunities abroad. PhD options seems better as NASA recruits lot of PhD in Scientist Positions for various projects from Top Universities in US.
2. Other option maybe, directly moving to US for a MS/MS+PhD after BTech and hence opportunities in Industry, Research & Academia in the Aerospace, Avionics, Space Technology, Rocket Industry Abroad.
Yeah, secondly chances are bleak you'll be able to become an astronaut with Nasa. Usually they'll always prefer US Citizens. However you may be an excellent Flight Engineer or Rocket Systems Designer, who knows you maybe selected for a Mission as Specialist deep in future.
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