As you asking I am telling There are many ways by which one can join ISRO as Scientist or Engineer. After 12th or during 12th one can appear for IIST exam ( Indian Institute of Space Science and Technology (IIST), Trivandrum )

IIST Offers B.Tech Programmes In Three Streams

B.Tech in Aerospace Engineering
B.Tech in Avionics
B.Tech in Physical Science

IIST Admission Test

IIST Admission Test consists of two question papers of objective type; each paper is of two hours based on physics, chemistry and mathematics.

Eligibility
Students may only apply for the entrance test if they have possessed 12th or equivalent qualification with aggregate of at least 70% marks in Physics, Chemistry and Mathematics (PCM) from a recognized university / Institute

Selection procedure
Assortment of the candidates will be made up on the basis of their marks obtained in the written test.