Neil deGrasse Tyson Says Space Flight Won't Mess Up William Shatner Despite Age

William Shatner's space flight won't take a toll on his body despite his age ... so says Neil deGrasse Tyson who explained why, by dropping some serious science knowledge.

The famed astrophysicist tells TMZ ... he's not worried about the fact Capt. Kirk's all of 90 years old and off to space Wednesday morning. He says the kind of space flight Blue Origin's designed to take is not like the old days where astronauts needed strenuous physical training.

So, what's changed? Neil explains it's not even about advances in the spacecraft itself ... it's more about where the New Shepard capsule ends up.

He says the bottom line is ... Captain Kirk should have a cakewalk of a "mission" to space. As for being weightless up there -- Neil says it's not the lack of gravity Shatner needs to worry about, but the opposite.

TMZ broke the story ... William will be part of the second crew to take the space flight ... making him the oldest person ever to be launched into space.

He'll be joined by Chris Boshuizen, founder of Earth-observation company Planet Labs, microbiologist Glen de Vries and Blue Origin's Audrey Powers. They're scheduled to launch at 10 AM ET from West Texas ... and TMZ will be streaming live.