SpaceX launched its 20th rocket of the year just two days after lofting a record 64 satellites into orbit. On this flight, a brand-new Falcon 9 hoisted a Dragon spacecraft into orbit, bound for the International Space Station. But unlike Monday’s textbook touchdown, today’s landing didn’t quite go as planned.

The Falcon’s first stage, the largest and most expensive portion of the rocket, was expected to navigate itself back to land after launching the Dragon spacecraft. But instead of gently touching down in the middle of SpaceX’s designated landing pad, the booster made an unscheduled plop into the Atlantic Ocean, just off the Florida coast. Video footage shared shortly after the incident shows the booster spinning out of control as it headed towards land.

But what caused the the anomaly? SpaceX officials will need to examine the booster to find out, but Elon Musk tweeted shortly after the incident that he suspected it was an issue with one of the booster’s grid fins, which help stabilize the vehicle as it plummets through the atmosphere. "Grid fin hydraulic pump stalled, so Falcon landed just out to sea," SpaceX founder Elon Musk tweeted soon after the rocket landed. "Appears to be undamaged & is transmitting data. Recovery ship dispatched."

Musk indicated that it’s possible that SpaceX may be able to use the booster after fishing it from the ocean, tweeting that it could be used for an internal SpaceX mission. (That could be a nod to an upcoming launch of one of SpaceX’s Starlink internet satellites.) But others at SpaceX are more circumspect. Hans Koenigsmann, the company’s vice president of flight reliability, says it’s too early to tell if the booster is salvageable. The company’s fleet of recovery vessels was dispatched shortly after launch to tow the booster back to port. According to Marine radio, the fleet secured the booster but is holding onto it until they figure out the best way to bring it home.

Today’s mishap marks the first time that the Falcon has failed to stick a landing on solid ground since SpaceX began recovering boosters. But it’s far from a failure. The incident instead illustrates how the rocket is designed to save itself in case something does go wrong. After the rocket’s first stage separates, it does an aerial flip to ensure it is pointed back towards land. Then the rocket conducts a series of three separate burns to slow itself to gently touch down in the middle of the designated landed zone. But first, it targets a landing point over water until the onboard computer systems ensure that everything is working perfectly.

During a press briefing held shortly after the failed landing, Hans detailed the events of the anomaly, describing how the grid fins appeared to have an issue that caused the rocket to spin out of control. The water landing was ultimately a gentle one, because the rocket’s thrusters had successfully stabilized it during its descent. “As much as we are disappointed that we landed in water, it shows that the system overall knows how to recover itself.”

According to Musk, the fix could be to add a backup system to the current grid fin hardware. "Pump is single string. Some landing systems are not redundant, as landing is considered ground safety critical, but not mission critical,” he added. “Given this event, we will likely add a backup pump & lines."

SpaceX has recovered a total of 33 boosters (including today’s), and while this landing attempt—which involved a shiny new booster—didn’t go as planned, the company will use the flight data to improve future vehicles.

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