American space company SpaceX has successfully launched its large Falcon Heavy rocket on a flight to deploy 24 satellites.

The rocket lifted off early Tuesday morning from Florida's Kennedy Space Center. The Falcon Heavy is considered the world's most powerful operating rocket.

It was the third successful flight of a Falcon Heavy rocket. The U.S. Department of Defense ordered the current mission.

The satellites will be used for experiments involving several U.S. government agencies, as well as some university projects.

SpaceX chief Elon Musk said Tuesday's mission was the company's "toughest rocket launch ever." It required four separate upper-stage engine firings. The satellites also needed to be placed in three different orbits. The process was expected to take about six hours.

SpaceX uses reusable rockets to reduce the cost of space travel for its customers. These include government and military agencies, the U.S. space agency NASA and private companies.

Several minutes after liftoff, the Falcon Heavy's two side boosters returned safely to Earth and landed at Florida's Cape Canaveral Air Force Station. The center booster was supposed to land on a special platform in the Atlantic Ocean, but did not reach its target.

The SpaceX Falcon heavy rocket boosters return to land after a successful launch from the Kennedy Space Center in Cape Canaveral, Fla., early Tuesday, June 25, 2019. (AP Photo/John Raoux)

SpaceX said the landing failure of the center booster was not unexpected because of the difficult nature of the mission. "It was a long shot," SpaceX's Musk said on Twitter.

In addition to the satellites, the Falcon Heavy carried an atomic clock built by NASA, a solar sail project and even human ashes.

The Deep Space Atomic Clock is a technology experiment aimed at giving spacecraft self-flying abilities in the future.

The solar sail project was led by the U.S.-based Planetary Society. The goal of the group's LightSail is to become the first orbiting spacecraft to be powered completely by sunlight. The head of the Planetary Society is scientist and television star Bill Nye, who praised SpaceX for its cooperation. "Hey @elonmusk, thanks for the ride!" he tweeted.

Among the human ashes on the flight were some belonging to astronaut Bill Pogue, who flew on NASA's first space station, Skylab, in the 1970s. Pogue died in 2014.

The Falcon Heavy is the most powerful rocket in use today. Each first-stage booster has nine engines, for a total of 27 firing at the same time during liftoff. SpaceX says the rocket has the ability to lift nearly 64 metric tons into orbit. Only NASA's Saturn V rocket, which took U.S. astronauts to the moon, had the ability to carry more into space.

_______________________________________________________________