If I got 20,000 rank in jee mains. Then which is best college to join aerospace engineering in India?

Well, to join NASA, you will need to get a US citizenship, so I would recommend to go and study there, because you need to live there for a long time (8–10 years) before you even become eligible for an American citizenship.

As far as Indian colleges are concerned, apart from IITs, not a lot of colleges provide a quality education in this field. My institute, Indian Institute of Space Science and Technology (IIST) in Thiruvananthapuram is the other option. But, they also take students through JEE advanced, so I don’t know if that’s an option for you. There are some colleges in south India, like Anna University, SRM and similar that are also good. Punjab Engineering college (PEC) is another good one. The link below gives a list of all the colleges that provide aerospace engineering.

Hello,

Your rank is good but if you want to aerospace engineering in good colleges like IITs you need to work hard to improve your rank  because doing BTech from IIT gives  good impact  to others .so here i am sharing you the cutoff of various IITs which you can aim , Your rank should be under 2k under general category in JEE advance examination.With this rank you can secure seats in IIT KANPUR,IIT MADRAS,IIT KHARAGPUR.For such rank you need to score at least 240 out of 366 in advance examination.To know cuttoff for each category if aerospace engineering stream of IITs visit below mention link:

Https://www.google.com/url?sa=t&source=web&rct=j&url=https://engineering.careers360.com/articles/iits-cutoff-for-b-tech-aerospace-engineering/amp&ved=2ahUKEwir65nd46vhAhW463MBHaYECNIQFjAAegQIAxAB&usg=AOvVaw3mWytoLfNFEAdiDzvUfumK&ampcf=1.

Best of luck!!