This Air Force mission is the fifth Boeing X-37B Orbital Test Vehicle launch.
SpaceX put the USAF's X-37B space plane-developed by Boeing-into space from Cape Canaveral Air Force Station, ahead of what is expected to be rough weather as Hurricane Irma approaches Florida. It's also created to land like the Shuttle, using a landing strip like you'd use for an airliner.
This was the first time SpaceX has provided a lift for the experimental mini-shuttle.
Evacuations in advance of Hurricane Irma's Florida landfall have already begun, but SpaceX managed to get its Falcon 9 rocket launched, and its first-stage booster landed, at the end of a smooth countdown.
By noon, the Air Force had declared the launch a success. "A strong relationship with our mission partners, such as the Air Force Rapid Capabilities Office, is vital toward maintaining the Eastern Range as the World's Premiere Gateway to Space".
What happened to the X37-B after the Falcon 9 rocket pushed the classified payload beyond Earth's atmosphere is a little less certain.
Watch today's SpaceX rocket launch live
The X-37B, also called the Orbital Test Vehicle, weighs about 11,000 pounds (5 metric tons) and has typically orbited Earth at altitudes between 200 and 250 miles (320 to 400 kilometers).
The U.S. Air Force's X-37B space plane fleet has begun its fifth secret flight with the launch of the OTV-5 mission on September 7, 2017. The X-37B is going to a higher inclination orbit than usual but the X-37B's main mission is classified and is being fiercely guarded.
The fifth mystery mission of the U.S. Air Force's X-37B space plane is now underway.
Air Force Secretary Heather Wilson disclosed during an early June congressional hearing that SpaceX, not ULA, would be launching X-37B this time around. The uncrewed vehicles look like NASA's now-retired space shuttle orbiters, but are much smaller; each X-37B is 29 feet (8.8 meters) long and 9.6 feet (2.9 m) tall, with a payload bay the size of a pickup truck bed.
The thermal spreader experiment will test three oscillating heat pipes.