CAPE CANAVERAL, Fla. — SpaceX is making another supply run to the International Space Station for NASA. But it’s the rocket’s return – not its takeoff – that has space fans talking.

Minutes after Tuesday morning’s planned launch from Cape Canaveral, the first stage of the unmanned Falcon rocket will aim for a vertical test landing on a barge in the Atlantic. The 14-story booster will soar close to 150 miles high, before flying back down and trying to settle upright on the floating platform – close to the size of a football field.

“When you look at it on the ground, I think it’s probably a very, very big platform, a big spaceport,” Hans Koenigsmann, vice president for mission assurance at SpaceX, said Monday. “But if you look at it from, I think, almost 150 or so miles up in suborbit, then it looks like a very, very small place to land on.”

“I’m going to be super-excited if this works,” Koenigsmann said. But he stressed that he did not want anyone losing sight of the main purpose of the Falcon’s mission: to deliver critical supplies to the space station and its six inhabitants.

This will be the sixth flight of a supply-filled Dragon capsule to the orbiting lab. It’s loaded more than usual because of an October launch explosion that wiped out another company’s delivery effort.This Dragon is stuffed with more than 5,000 pounds of goods, much of it replacing items lost on the Orbital Sciences Corp. flight.

As for the audacious landing experiment, once the first-stage booster has completed its primary job of hoisting Dragon, it’s entirely a SpaceX operation.

The California-based company has tried two such landings before on the open sea, both of them successful, but never on a platform like this. Normally, the boosters are discarded at sea.

SpaceX founder and chief Elon Musk predicts a 50-50 chance of success at best. He says that flying back boosters would allow them to be reused, speed up launches and save money.