The U.S. Air Force launched its fifth flight of the X-37B Orbital Test Vehicle on Thursday, but the long-running project is still mysterious.
But what exactly the X-37B does in orbit is classified - aside from launch and landing dates and minor operational details, the Air Force provides little on the vehicle's objective.
The Air Force is known to have at least two of the Boeing-built drones that resemble a small space shuttle.
It was SpaceX's 16th successful return of a first-stage booster. For comparison, the space shuttles were 122 feet (37 m) long, with 78-foot (24 m) wingspans.
Artist's concept of the X-37B spaceplane in orbit.
USAF officials said they want to use a variety of rockets for the X-37B programme to launch quickly if warranted.
This is the first time SpaceX has provided a lift for the experimental mini-shuttle. The Air Force usually says it's running experiments to test new satellite technology. X-37B's fourth mission set an endurance record, racking up 718 days in orbit before landing in early May at Kennedy Space Center, Florida.
Up next on SpaceX's docket is the third launch of Iridium's NEXT series of polar orbit satellites, now aiming for a launch from the Vandenberg Air Force Base at 6:06 a.m. PST, October 4. We also don't know how long the X-37B will stay in orbit, although its last mission was almost two years in length (270 days to be exact). Over all four previous missions, the space craft have spent a total of 2,085 days in space.
The SES 11/EchoStar 105 spacecraft has been delivered to Cape Canaveral from its Airbus Defense and Space factory in Toulouse, France.
It will now need to be moved from its base as Hurricane Irma approaches Florida in time for Saturday morning.