The United States Air Force is called the air force, not the space force, but it has been testing a space plane recently that has brought much speculation. The mysterious X-37B space plane will launch into orbit around Earth from Florida’s Cape Canaveral Air Force Station. The unmanned spacecraft is scheduled to launch on Wednesday at 10:45 a.m. ET atop a United Launch Alliance Atlas 5 rocket. This will be the reusable space plane’s fourth mission. The craft is known as OTV-4, which stands for Orbital Test Vehicle-4.

The majority of X-37B’s payloads and specific activities are classified, so no one outside the Air Force is really sure what the spacecraft is supposed to be doing once it leaves Earth. The secrecy surrounding the spacecraft has led to some speculation that the machine might be some sort of space weapon. Air Force officials have repeatedly rejected that notion and say that the vehicle simply tests a number of new space technologies.

For example, the space plane boasts a type of ion engine called a Hall thruster, Air Force officials said, as reported by NBC News. NASA is also flying an experiment on OTV-4. The agency’s Materials Exposure and Technology Innovation in Space investigation will see how exposure to the space environment affects nearly 100 different types of materials. The results should aid in the design of future spacecraft, NASA says.