I know my gf's dream's won't work out but I won't tell her.

So my (M19) gf(F19) is going to start school in the spring to be an astronaut. I love this for her and want to push her, but I know she will not be able to unless there is a significant change or scientific advancement on the part of Nasa. My gf has heart problems, irregular heart beat, POTS, as well as other psychological disorders which I will not name but are restricted against in NASA's astronaut selection process. I know she hasn't looked into this yet because she doesn't show any concern besides the amount of work that she will have to do. I don't have it in me to be the one to explain to her that she won't be able to be an astronaut because of her health problems, I don't want to be the bad guy. But also, I don't want her to get too far into it that she only finds out on the brink of reaching her dream.