SpaceX successfully launched its 16th cargo resupply mission, CRS-16, to the International Space Station (ISS) today. The Dragon spacecraft will arrive at the ISS early Saturday morning Eastern Standard Time. However, SpaceX did not achieve its secondary objective of landing the first stage of the Falcon 9 rocket back at Cape Canaveral Air Force Station as it has done in the past. Instead, it made an almost perfect landing on the ocean about two miles offshore.

SpaceX founder and CEO Elon Musk issued several tweets about the anomaly, including video of the stage slowly settling on the water and then tipping over. The company is retrieving the booster and may be able to reuse it anyway.

Tracking shot of Falcon water landing pic.twitter.com/6Hv2aZhLjM — Elon Musk (@elonmusk) December 5, 2018

He also tweeted imagery from a camera onboard the booster as it made its way down. The booster began spinning, but righted itself before it reached the water.

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

His immediate reaction was that a hydraulic pump on one of the grid fins failed. Two of the four grid fins are visible on the image in the tweet.

Grid fin hydraulic pump stalled, so Falcon landed just out to sea. Appears to be undamaged & is transmitting data. Recovery ship dispatched. — Elon Musk (@elonmusk) December 5, 2018

SpaceX’s success in landing Falcon 9 first stages and reusing them has become so routine that today’s failure is newsworthy even though no other commercial space launch company does this at all. SpaceX has successfully landed 32 boosters, 11 of which were on land. The others were on autonomous drone ships at sea.

On Monday, SpaceX launched a Falcon 9 from Vandenberg Air Force Base, the SSO-A Smallsat Express mission, using a first stage that was making its third flight. It successfully landed on a drone ship. SpaceX returns the boosters to one of its two drone ships or to a landing pad depending on the trajectory of the launch and other considerations. In the case of SSO-A, a Delta IV Heavy rocket was on a nearby pad getting ready to launch a high priority national security satellite so the Air Force apparently did not want SpaceX attempting to land the Falcon 9 so close to it.

Today’s landing was intended to be on land. SpaceX Vice President for Build and Flight Reliability Hans Koenigsmann told reporters at a post-launch press conference that as the booster descends, its target point is out at sea. As it gets closer to Earth, systems aboard the rocket move the impact point toward the Landing Zone as the landing burn is underway. Thus, if anything goes awry, as it did today, it lands in the water without endangering people or infrastructure.

“I would say in terms of public safety, the vehicle kept well away from anything that could pose even the slightest risk to population or property. So public safety was well protected here, and as much as we are disappointed in this missed landing, or landing in the water instead of land, it shows the system overall knows how to recover from certain malfunctions.”

The booster continued to function after it landed in the water, going through its automated safeing process and transmitting data. SpaceX was getting ready to tow it back to land at the time of the press conference.

Koenigsmann said he does not expect the landing failure to impact future launches. The next launch is of an Air Force GPS III satellite and SpaceX is not planning to recover the booster on that mission. After that is an Iridium launch in January and he anticipates that whatever caused the problem today will be identified and remedied by then.

Despite Musk’s tweets, Koenigsmann cautioned that they need time to determine exactly what happened. “Honestly I am a little bit puzzled myself” as to how the rocket stopped itself from spinning. It could be that deployment of the landing legs stabilized the vehicle, but he said it needs to be investigated in detail. They also do not know exactly when the anomaly began — before or during the entry burn.

While SpaceX is disappointed at the unsuccessful landing, its customer, NASA, is delighted with the successful Dragon launch. NASA ISS Deputy Program Manager Joel Montelbano called it an “incredible launch.”

Dragon is taking 5,600 pounds of supplies, equipment and scientific experiments, including mice, to the ISS. The launch was delayed one day because NASA discovered that the food bars for the mice were contaminated with mold. They had to get new food for the “moustronauts” from NASA’s Ames Research Center in California and there was not enough time to do that and meet the original December 4 launch date.

Dragon will arrive at the ISS about 6:00 am ET on Saturday morning where it will be grappled using the robotic Canadarm2 and then installed onto one of the ISS docking ports. It is joining five other “visiting vehicles” currently docked there: two Soyuz crew spacecraft, two Russian Progress cargo spacecraft, and a Northrop Grumman Cygnus cargo spacecraft.