The most powerful launch system ever built blasted off from Starbase in Boca Chica, Texas at 7:50 am (1250 GMT). More than two million people followed along on a live stream on X.

Starship is vital to both NASA's plans for returning astronauts to the Moon later this decade, and for SpaceX CEO Elon Musk's hopes of one day colonizing Mars.

Three previous attempts have ended in its fiery destruction, all part of what the company says is an acceptable cost in its rapid trial-and-error approach to development.

"The payload for these flight tests is data," SpaceX posted on X. "Building upon what we achieved during Starship's third flight test, our primary goal today is to get through the extreme heat of reentry."

The flight path is similar to the third test, which took place in March and saw Starship fly for 49 minutes before it was eventually lost as it re-entered the atmosphere.

Since then SpaceX says it has made several software and hardware upgrades -- and for the first time ever on Thursday succeeded in a soft splashdown for the Super Heavy booster in the Gulf of Mexico, to massive applause from engineers at mission control in Hawthorne in California.

Next, it hopes to achieve its first "controlled entry" for the upper stage, which will glow a fiery red as it careens into the atmosphere at around 27,000 kilometers per hour (nearly 17,000 mph).

- Twice as powerful as Saturn V -

Designed to eventually be fully reusable, Starship stands 397 feet (121 meters) tall with both stages combined -- 90 feet taller than the Statue of Liberty.

Its Super Heavy booster produces 16.7 million pounds (74.3 Meganewtons) of thrust, about twice as powerful as the Saturn V rockets used during the Apollo missions -- though later versions should be more powerful still.

SpaceX's strategy of carrying out tests in the real world rather than in labs has paid off in the past.

Its Falcon 9 rockets have come to be workhorses for NASA and the commercial sector, its Dragon capsule sends astronauts and cargo to the International Space Station, and its Starlink internet satellite constellation now covers dozens of countries.

But the clock is ticking for SpaceX to be ready for NASA's planned return of astronauts to the Moon in 2026, using a modified Starship as the final vehicle to take astronauts from orbit down to the surface.

To accomplish this, SpaceX will need to first place a primary Starship in orbit, then use multiple "Starship tankers" to fill it up with supercooled fuel for the onward journey -- a complex engineering feat that has never before been accomplished.

At least one SpaceX fan has grown tired of waiting. Japanese billionaire Yusaku Maezawa announced this week he has canceled a planned trip around the Moon on Starship with a crew of artists, because he has no idea when it might actually happen.