FLORIDA • SpaceX has launched its Falcon Heavy rocket from Kennedy Space Centre in Florida, carrying 24 experimental satellites in what Mr Elon Musk's rocket company called one of the most difficult launches it has attempted.

The craft blasted off to cheers from onlookers at 2.30am on Tuesday local time, after a three-hour delay from the original launch time late on Monday.

The boosters separated safely as the craft began its six-hour mission to deploy the satellites.

The two-side booster rockets returned safely to Earth, landing on adjacent Air Force landing pads, but the rocket's centre booster missed its mark, crashing into the Atlantic Ocean.

The mission, dubbed Space Test Programme 2, is the third for the Falcon Heavy rocket, which SpaceX describes as the most powerful launch system in the world.

It was commissioned by the US Department of Defence, the key contractor for commercial space companies like SpaceX.

The company is putting satellites into orbit for agencies including Nasa and the National Oceanic and Atmospheric Administration, defence department laboratories, universities and a non-profit organisation, SpaceX said.

The mission is one of the most challenging in SpaceX history, with four separate upper-stage engine burns and three separate orbits to deploy satellites, the company said on its website.

The payloads on the satellites Falcon Heavy is putting into orbit include an atomic clock Nasa is testing for space navigation, another testing new telescope technologies, and a solar sail project part-funded by the Planetary Society, a non-profit organisation headed by Mr Bill Nye, "The Science Guy" on television presentations.

The LightSail is a crowdfunded project that aims to become the first spacecraft in Earth orbit propelled solely by sunlight, the society, which has championed solar propulsion for decades, said on its website.

REUTERS