A SpaceX Falcon 9 rocket blasted off from California on Sunday to put a climate-monitoring satellite into orbit, NASA said.
Technology entrepreneur Elon Musk's SpaceX then planned to attempt to land the rocket on a barge in the Pacific Ocean, which would mark its second milestone a month after it nailed a spaceflight first with a successful ground landing in Florida.
The 22-story tall rocket lifted off through thick fog from Vandenberg Air Force Base on the central California coast at 10:42am PST (1842 GMT).
After sending the US- and European-owned Jason-3 satellite on its way to orbit, the rocket's first stage separated and turned around in an attempt to touch down on a platform floating in the Pacific Ocean, said officials with privately-held SpaceX, also known as Space Exploration Technologies.
After years of testing, including two failed ocean landing attempts, SpaceX last month nailed a touchdown on land in Florida, a key step in founder Elon Musk's quest to develop a cheap, reusable rocket.
SpaceX does not yet have federal clearance to land rockets at Vandenberg, prompting Sunday's ocean try, company vice president Hans Koenigsmann told reporters on Friday.
Being able to land at sea also gives the company flexibility to recover rockets used on more demanding missions, such as launching heavy satellites, when boosters do not have enough fuel left to reach land.
The 1,200-pound (550 kg) Jason-3 satellite is the fourth in a series of ocean-monitoring satellites, which are now taking center stage in monitoring Earth's climate.