Neil deGrasse Tyson may be a astrophysics genius, but he tells TMZ he ain’t boarding Elon ’s SpaceX rocket ... unless Musk sends his own bloodline up first ... as proof it won’t go kaboom! We ran into Neil outside CBS Studios in NYC Friday... and he cracked Elon’s gotta blast his mom Maye off first to prove the rocket’s safe -- making it crystal clear ... he’s down to explore the cosmos, but only if there’s a guaranteed round trip. Catch the full clip ... 'cause Neil also laid it out ... he’s