MELBOURNE, Fla. — To outside observers, SpaceX’s Falcon 9 rocket launch Thursday evening might not have looked particularly remarkable — no streaking contrail through the sky, the rumble muffled a bit by gusty winds.

But after the first stage completed its work and descended to a landing on an ocean platform, Elon Musk said he was rendered nearly speechless.

“This is a huge day,” the SpaceX founder and CEO said after the launch from Kennedy Space Center, joined by his five boys at a celebratory press conference. “My mind is blown, frankly.”

The launch of the commercial satellite marked the first time SpaceX had re-flown a Falcon booster, culminating 15 years of work to prove that large, orbital rockets can be reused.

It was a triumphant moment for Musk, the billionaire entrepreneur who since starting SpaceX in 2002 has faced doubters of his unconventional approach to spaceflight and grandiose ambitions to establish a city on Mars.