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).
Though we don't know much about the X-37B's mission specifics, we do know a little bit about where it's going in orbit thanks to SpaceX.
About 2.5 minutes into the flight, the Falcon 9's two stages separated. The X-37B, built by Boeing, is an uncrewed vehicle, but resembles the Space Shuttle on a smaller scale.
The X-37B will glide back to Earth for a runway landing to conclude its mission, likely targeting a homecoming at the Shuttle Landing Facility at the Kennedy Space Center, where the OTV-4 mission ended. For comparison, the space shuttles were 122 feet (37 m) long, with 78-foot (24 m) wingspans.
SpaceXhas been valued at $21bn (£16bn), making it one of the most valuable privately owned companies in the world. While the secretive nature of the Orbital Test Vehicle missions mean that live coverage ended after the first stage landed, this resulted in fantastic live coverage of the landing, as is now routine. The previous missions relied on United Launch Alliance's Atlas V rockets.
According to the U.S. Air Force, this mission was also launched into, and will be landed from, "a higher inclination orbit than prior missions to further expand the X-37B's orbital envelope".
You can watch a livestream of the launch and landing attempt below. The goal was to get the launch up before the arrival of Hurricane Irma, and they succeeded.
But this week's rocket launch is not what an ordinary SpaceX launch looks like. "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".
As usual, SpaceX aims to land its leftover booster back at Cape Canaveral for eventual reuse.