Something unexpected happened after the launch of a Falcon 9 rocket to the International Space Station—the first-stage booster did not come back to Earth as intended. Instead, it made an unscheduled landing in the Atlantic Ocean, just off the Florida coast.

At about 7 minutes and 25 seconds after the launch, the first stage began spinning out of control as it descended back toward Kennedy Space Center along the Florida coast. There was a problem with one of the grid fins that are used to stabilize the first stage during its return to Earth through the thickening atmosphere.

"Grid fin hydraulic pump stalled, so Falcon landed just out to sea," SpaceX founder and lead designer Elon Musk tweeted shortly after the rocket landed. "Appears to be undamaged & is transmitting data. Recovery ship dispatched." Later, in response to a question about redundancy of this system, Musk added, "Pump is single string. Some landing systems are not redundant, as landing is considered ground safety critical, but not mission critical. Given this event, we will likely add a backup pump & lines."

Engines stabilized rocket spin just in time, enabling an intact landing in water! Ships en route to rescue Falcon. pic.twitter.com/O3h8eCgGJ7 — Elon Musk (@elonmusk) December 5, 2018

Despite the landing problem, the rocket completed its primary mission with alacrity. After its launch Wednesday afternoon from Cape Canaveral Air Force Station, the first stage delivered the Dragon spacecraft into orbit. The vehicle deployed its solar panels on schedule and appears on course for a rendezvous with the International Space Station later this week. The Dragon is carrying 2.5 tons of cargo.

Since an extended period of trial and error in the mid-2010s, SpaceX has had a remarkable run of success with landing its rockets on land and at sea. The last landing failure of a Falcon 9 rocket the company intended to return to the coast or an ocean-based drone ship occurred on June 15, 2016 during a mission to launch two satellites to geostationary transfer orbit. After launching its heavy payload, the rocket ran out of fuel just before it touched down on the drone ship. Since then, the company has successfully landed 26 Falcon 9 first-stage rockets.

As usual, engineers from SpaceX will assess the problem and apply fixes for future missions. It is not clear how (if at all) this failure might affect the company's launch manifest. The company has one more mission planned for 2018, an important GPS launch for the Air Force. However, SpaceX had not planned to recover that booster anyway.