Hello aspirant, IIST Thiruvananthapuram is a government aided institute and one of the best institutes in India for space research . It takes admission on the basis of JEE advanced or IIST exam. You need a very good score in JEE advanced for admission into this institute. The cut off percentage changes rapidly every year but to be on the safe side for general category you should get atleast around 35-40 percentage of total marks. For 2020 closing cutoff for aerospace engineering was 29.8 while for 2018 it was 34 % . But for 2017 it was 53.8%.

Check out - IIST B.Tech Cutoff 2021 - Check Category & Rank-Wise Cut-off here (careers360.com)

BEST OF LUCK!!