Hi,

Main subjects should be physics. But choosing PCMC (Physics, Chemistry, Maths, Computer Science) would be more beneficial as astronauts will be dealing with computer systems as well in their career, though you will get a good basic in programming skills.

To become an astronaut you should choose Science stream. And to become an astronaut if NASA you must have an experience of 100 flight hours, so you will have to take aeronautical engineering and then do aerospace engineering.

Most astronauts have a master's degree, and a PhD will significantly improve your chances of reaching the stars.

Hope this helps

Wish you luck