SpaceX launched a Falcon 9 rocket from Vandenberg Space Force Base in California on May 11 carrying a classified payload of U.S. National Reconnaissance Office (NRO) spy satellites.

The mission, called NROL-172, is part of an ongoing effort to expand a new “proliferated architecture” network designed to increase the number and capability of U.S. reconnaissance satellites.

The satellites, built with SpaceX and Northrop Grumman, are intended to deliver more signals and imagery from multiple orbits, though their exact functions remain secret.

The rocket’s first stage successfully landed on a SpaceX drone ship in the Pacific Ocean shortly after liftoff, Space.com has reported.

This was SpaceX’s 55th Falcon 9 launch of the year, most of which have supported either U.S. government missions or the company’s Starlink internet constellation.