If you join IIST then There is some limited period of Time where you has to work For ISRO. since most of Your expenses are funded by the Govt of India so they want you In return to work for them. I think It is not a good decision to work for NASA after building your career on the Money of the People of our Country. You can go for NASA post period of minimum required work set by IIST. Now if you have better opportunities knocking on your door or you want to go for higher studies then you have to reject the job offer. On failing to accept the job offer, a student is liable to pay Rs.10 lakhs to IIST/DOS as per the Surety Bond signed by you at the time of admission.