SpaceX’s highly reliable Falcon 9 rocket has experienced a rare failure that means the latest batch of the company’s Starlink satellites won’t make it into orbit, the company said Friday, as regulators opened an investigation.

The rocket, a prolific launch vehicle that propels both satellites and astronauts into orbit, blasted off from Vandenberg Space Force Base in California on Thursday night, with the first stage performing well and executing its impressive yet now routine droneship landing.

But the second stage developed a liquid oxygen leak, SpaceX said in a statement, leaving it unable to complete a planned second burn.

Though it deployed the 20 Starlink internet satellites it was carrying, they entered an eccentric orbit with a low point of 135 km (83 miles), roughly half of what it needed to be.

The team worked overnight to send commands to the satellites to try to lift their orbit, but were ultimately unsuccessful.