The unmanned X-37B spacecraft was launched May 20 2015 atop a United Launch Alliance Atlas 5 rocket from Florida’s Cape Canaveral Air Force Station. The liftoff will begin the reusable space plane’s fourth mission, which is known as OTV-4 (short for Orbital Test Vehicle-4). Most of the X-37B’s payloads and specific activities are classified, so it’s not clear what the space plane will be doing once it leaves Earth. This secrecy has led to some speculation that the vehicle might be some sort of space weapon. Air Force officials have repeatedly rejected that notion, saying that the X-37B flights simply test a variety of new space technologies.
For example, the space plane is carrying a type of ion engine called a Hall thruster on OTV-4, Air Force officials said. This Hall thruster is an advanced version of the one that powered the first three Advanced Extremely High Frequency military communications satellites, the officials added. 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.
The X-37B looks like a miniature version of NASA’s now-retired space shuttle. The robotic, solar-powered space plane is about 29 feet long by 9.5 feet tall (8.8 by 2.9 meters), with a wingspan of 15 feet (4.6 meters) and a payload bay the size of a pickup-truck bed. Like the space shuttle, the X-37B launches vertically and lands horizontally, on a runway.,,,
Excerpts from Mike Wall, Air Force Gets X-37B Space Plane Ready for Its Next Mystery, SPACE.COM, May 18, 2015