SpaceX’s Falcon 9 rocket experienced a rare anomaly on Thursday night (July 11).

The incident took place during the launch of SpaceX’s Starlink internet satellites from Vandenberg Space Force Base in California. Initially, the Falcon 9 performed well; its two stages separated on schedule, and the first stage successfully landed on a drone ship about eight minutes after liftoff as planned.

However, the rocket’s upper stage, which was carrying 20 Starlink spacecraft to low Earth orbit, encountered a problem, according to SpaceX founder and CEO Elon Musk.

“Upper stage restart to raise perigee resulted in an engine RUD for reasons currently unknown. Team is reviewing data tonight to understand root cause. Starlink satellites were deployed, but the perigee may be too low for them to raise orbit. Will know more in a few hours,” Musk said via X about two hours after liftoff.

“RUD” is short for “rapid unscheduled disassembly” — SpaceX-speak for an explosion or disintegration. (Perigee, in case you were wondering, is the point in an object’s orbit when it’s closest to Earth.)

An hour after Musk’s post, SpaceX provided more details via the company’s X account.

“During tonight’s Falcon 9 launch of Starlink, the second-stage engine did not complete its second burn. As a result, the Starlink satellites were deployed into a lower-than-intended orbit. SpaceX has made contact with 5 of the satellites so far and is attempting to have them raise orbit using their ion thrusters,” the company wrote.

In a reply to that post, Musk mentioned that this effort “will probably not work, but it’s worth a shot.”

The Falcon 9’s upper stage, powered by a single Merlin engine, appeared unusual on this flight. SpaceX’s launch webcast showed a buildup of fluffy white ice near the engine as it fired in space — an uncommon sight that might indicate a propellant leak. However, this is just speculation; neither SpaceX nor Musk has commented on the ice at this point.

The Falcon 9 is the most frequently flown rocket today, with 69 launches completed in 2024 alone. Known for its reliability, the Falcon 9 has experienced only one full in-flight failure, which occurred during the launch of a robotic Dragon cargo capsule to the International Space Station (ISS) in June 2015.

The Falcon 9 is also human-rated and has launched 13 crewed missions, nine of which sent astronauts to the ISS for NASA. It is currently unclear how Thursday’s incident will impact the rocket and its future missions; SpaceX will need to determine the cause of the issue and find a solution.

Thursday’s launch deployed 20 Starlink satellites, 13 of which are capable of providing service directly to cell phones. Most Falcon 9 missions now support the Starlink megaconstellation, which has over 6,100 operational satellites. Of the 69 Falcon 9 launches in 2024, 49 have been dedicated to Starlink missions.