Miami - SpaceX on Monday blasted off a secretive US government satellite, known only as NROL-76, marking the first military launch for the California-based aerospace company headed by billionaire tycoon Elon Musk.The payload for the National Reconnaissance Office, which makes and operates spy satellites for the United States, soared into the sky atop a Falcon 9 rocket at 07:15 (11:15 GMT).About 10 minutes after launch, the scorched first stage of the rocket came back to Earth and landed upright at Cape Canaveral, marking the fourth successful solid ground landing for SpaceX."And we have touchdown," a SpaceX commentator said on a live webcast as cheers broke out at mission control."The first stage has landed back at Landing Zone 1. Another good day for us at SpaceX. A beautiful sight to see."Live video of the launch showed the first and second stages of the rocket separating about two and a half minutes into the flight.The larger portion of the rocket, known as the first stage, made a gentle arc and powered its nitrogen thrusters to guide it back to Earth.After a fiery entry burn, the rocket set itself down steadily in the centre of the 91m circular landing zone.Musk is leading an effort in the rocket industry to re-use costly parts rather than jettison them into the ocean after each launch.SpaceX has already made multiple successful landings - some on land and others on floating ocean platforms, known as drone ships.Secretive payload The launch was initially planned for Sunday, but was postponed in the last seconds before liftoff due to a sensor issue with the rocket, SpaceX said.The sensor in question was replaced ahead of Monday's attempt.Little was known about the payload, which a SpaceX commentator described only as a "satellite", due to its classified nature."As a matter of policy and because of classification, NRO does not provide information about our contracts," an NRO spokesperson told AFP.Until now, the US military has spent billions per year exclusively with United Launch Alliance, a joint operation of aerospace giants Boeing and Lockheed Martin, to launch government satellites.SpaceX in 2014 protested the US Air Force's practice of using only ULA, saying it unfairly awarded billions of dollars to a single company for national security launches.SpaceX to was selected to launch NROL-76 "after a competition", said the NRO spokesperson. She said she did not know when the contract was awarded. The contract was first announced last year.SpaceX also has a pair of launch contracts coming up for the Air Force to send GPS satellites into orbit.SpaceX regularly launches unmanned cargo ships to the International Space Station, and is working on a crew capsule that could carry humans into orbit as early as next year.