William Shatner's Going to Space on Jeff Bezos' Blue Origin Rocket Ship

Decades after he last played the role, William Shatner's going to become a real-life Captain Kirk ... because he's going to space ... on Jeff Bezos' rocket ship ... TMZ has learned.

Though he won't be boldly going where no man's gone before -- because Bezos and his Blue Origin crew already did it -- sources with direct knowledge tell TMZ ... the 90-year-old actor is slated to be part of the second crew to take the space flight in the New Shepard capsule. That would make him the oldest person ever to be launched into space.

We're told Shatner will be on board in October for the 15-minute civilian flight -- similar to the last launch. What we don't know -- BUT WHAT WOULD BE AWESOME -- is if he wears his Capt. Kirk getup.

Our sources say the mission will be filmed for a documentary. We're told Shatner's people were talking to Discovery about the special, but that didn't materialize ... but our sources say Shatner and Co. have taken the project elsewhere and are in negotiations.

It's unclear who will be joining William on the flight, but they'll join elite company as some of the first few amateur astronauts in space. You'll recall, when Bezos went up, up and away in July ... he was joined by just his brother, Mark Bezos, test pilot Wally Funk and recent HS grad Oliver Daemen.

That crew reached an altitude of 66.5 miles, floated around for a few minutes in zero-gravity while playing with ping pong balls ... then made their way back down for safe landing.