I appreciate your concern for Mother Nature and it quite applaudable that you willing to go to such lengths to save the environment. Yes, you should definitely opt for environmental engineering if that is what you want. But, it won't help you become an astronaut in ISRO,India. Compared to NASA where al  different types of technical bachelor degrees are allowed, ISRO is a very technical organization with it's main concern on Rocket and Satellite technologies. So, as far as ISRO is concerned your degree won't be of any help. This is not it, there are many eligibility requirements for you to become an astronaut:-

1.)You need to be physically fit, there are examinations, to check if you are mentally strong and have great stamina.

2.)You should qualify the criterion of height, a particular level of height is required for people to become an astronaut.

3.)You should possess great mathematical skills, and not simple addition, multiplication etc.

4.)You should have good communication skills, you must be good at English because there are very high chances that people you will be working with will not even be from your own country.

5.)You should have a good academic career and team spirit because all the space missions are done in teams, and you should be able to work under high pressure and in cramped environment.

Furthermore, to join ISRO one needs to clear the Joint Entrance Exam which is held by the IITs.

I hope these points provide you the relevant information and are able to clear your query.