The 22nd amendment to the US Constitution bars Trump as a viable candidate for the Office of the President. It would require an additional amendment to be possible and there's no way that 3/4ths of all US states will agree to that shit.
The only potential loophole that I can discern is that there's no clear consensus on if Trump would be allowed to run as Vice-President on someone else's ticket. If he can, then, theoretically, he could run for VP and then have the elected President immediately resign, making him the President again.
NASA gives SpaceX fat government paychecks. He doesn't want anything to happen to them.