The X-37B spy plane is ready to take off on a new mission, starting May 20. The nature of the highly secretive flight remains a mystery.

The U.S. Air Force operates the X-37B, which resembles a miniature version of the retired Space Shuttle. Designed by the Phantom Works division of Boeing, the 29-foot spacecraft, with a 15-foot wingspan, features a cargo bay which is 7 feet long and 4 feet wide.

This fourth test flight of the reuseable robotic vehicle will launch aboard an Atlas 5 rocket, manufactured by the United Launch Alliance. Liftoff is scheduled to take place somewhere between 10:45 a.m. and 2:45 p.m., EDT. Mission engineers will announce the exact time of the launch on the day of the mission. Two days before the flight, weather predictions called for a 60 percent chance of favorable conditions for liftoff.

The first three test flights of the plane utilized a pair of the spacecraft, which together logged 1,368 days in space. Following their flights, the vehicles touched down at Vandenberg Air Force Base in California, directed by autonomous systems aboard the craft. Those three flights spent more time in space than the combined totals of 135 flights over 30 years carried out during the space shuttle program.

"We are excited about our fourth X-37B mission. With the demonstrated success of the first three missions, we're able to shift our focus from initial checkouts of the vehicle to testing of experimental payloads,"  Randy Walden, director of the Air Force Rapid Capabilities Office, said.

Air Force officials have stated they will test several systems aboard the X-37B during its fourth test flight, including a potentially revolutionary form of electrical propulsion. This engine is said to be an advance on the Hall Current Thrusters currently in use aboard some communications satellites managed by the Department of Defense. These devices use xenon gas to create a small thrust, capable of keeping satellites in orbit around the Earth.

The Materials Exposure and Technology Innovation in Space (Metis) experiment on the X-37B will expose almost 100 material samples, each just over an inch in diameter, to the harsh environment of space. This research will examine how well various substances hold up to the cold and vacuum experienced by spacecraft orbiting our planet.

"By exposing materials to space and returning the samples to Earth, we gain valuable data about how the materials hold up in the environment in which they will have to operate," Miria Finckenor, principal investigator for Metis at the Marshall Space Flight Center, told the press.

The launch also will carry the LightSail project managed by The Planetary Society. This first test flight will see how well new technologies work in a small spacecraft that eventually will operate like a sailboat in space, driven by pressure from light emanating from the sun. The first operational flight of the LightSail spacecraft design will likely take place in 2016.

Between 2001 and 2013, more than 4,000 samples of different materials were exposed to conditions in space as part of experiments performed by astronauts aboard the International Space Station.

One X-37 was used for the first and third test flights, while another was utilized for the second test. The Air Force would not reveal which vehicle would be used for the fourth flight.

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