CNN--  Thousands of onlookers in Florida could be heard cheering on the company's livestream, which was viewed by about 3 million people.

In the run up to launch, it wasn't at all clear that the rocket would work.

"People [came] from all around the world to see what will either be a great rocket launch or the best fireworks display they've ever seen," Musk said in an interview with CNN's Rachel Crane Monday.

The rocket's smooth takeoff wasn't the only stunning thing about this launch.

In a never-before-seen feat, SpaceX also managed to guide at least two of the Falcon Heavy's first-stage rocket boosters to land upright back on Earth. They cut back through the Earth's atmosphere and landed in unison at a Kennedy Space Center landing pad.

"That was probably the most exciting thing I've ever seen -- literally ever," Musk said.

The third booster was supposed to land on a sea-faring platform called a droneship -- but just as it was about to land, the livestream cut out. Musk confirmed after the launch that the booster made a crashed.

On board the rocket that's now headed deeper into space is Musk's personal Tesla (TSLA)roadster. At the wheel is a dummy dressed in a spacesuit. Musk said in December the car would play David Bowie's "Space Oddity" on repeat. Cameras on board the car show it cruising by Earth, which appears as a big blue orb in the background. Musk plans to send the car into orbit around the sun.

He announced last year he planned to put his car on the inaugural Falcon Heavy flight. When asked on Twitter why he wanted to throw away a $100,000 vehicle, he replied, "I love the thought of a car drifting apparently endlessly through space and perhaps being discovered by an alien race millions of years in the future."