SpaceX successfully launched a secret U.S. government payload called Zuma on Sunday and landed its rocket back on Earth, in the company's first mission of 2018.

The Zuma spacecraft was attached to one of SpaceX's Falcon 9 rockets. It was launched into orbit from the Cape Canaveral Air Force Station in Florida. The Falcon 9 successfully landed back to base.

Landing and reusing rockets is the main aim of SpaceX scientists, who argue that it reduces the cost of launches and allows it to perform more missions.

SpaceX did not reveal the purpose of Zuma because it is classified, but the mission marked Elon Musk's company's first in 2018.

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Last year, SpaceX carried out 18 successful launches, the highest in any calendar year. The firm is aiming to carry out one launch every two weeks.

SpaceX is now looking towards its next challenge, launching the Falcon Heavy, its largest rocket to date. Musk announced on social media last week that the Falcon Heavy would be launched at the end of the month, after several delays.

For the test flight, the rocket will not carry a customer's payload. Instead, Musk will launch an original Tesla Roadster playing David Bowie's "Space Oddity" into "a billion year elliptic Mars orbit."