The U.S. Air Force's mysterious space plane, the Boeing X-37B, will be returning to space next month for another mission. However, the Air Force has divulged more information about what the spacecraft will be doing in the heavens.

The X-37B, also known as the Orbital Test Vehicle (OTV), is a reusable unmanned spacecraft the Air Force operates to test new technologies, including advanced propulsion systems, according to the U.S. Air Force.

Space Flight Now reports X-37B will return to space for its fourth mission on May 20 aboard an Atlas V rocket launched from Cape Canaveral, Florida. Unlike previous missions, the Air Force has disclosed what the X-37B will test: the Hall electric thrusters.

The Hall electric propulsion thrusters are 4.5-kilowatt units that use electricity and xenon to produce the energy needed to power a craft through space, ionizing and accelerating xenon gas. Advanced Extremely High Frequency military communications satellites use the thrusters to maneuver through space and Aerojet Rocketdyne produces them.

"This experiment will enable in-space characterization of Hall thruster design modifications that are intended to improve performance relative to the state-of-the-art units onboard AEHF," the Air Force stated.  

Unlike solid chemical rockets, the electric engines lack the thrust required for high-speed movement, with a mere output of 0.06 pound thrust. However, xenon does not weigh as much as hydrazine, which fuels most rockets, making craft with Hall thrusters smaller and less expensive.

Three AEHF satellites currently orbit Earth and three more are to be launched in the future. Valued at $1 billion each, the satellites are designed to survive nuclear scenarios and continue to provide communication to U.S. forces and government.

Control of space has taken paramount importance as other nations continue to enter orbit. CBS's "60 Minutes" highlighted on April 26 the Air Force's Space Command, which oversees all space missions and satellites.

Military satellites support everything from guided missiles and smart bombs to the Google Maps application on a smart phone. The Pentagon's space budget titters around $10 billion though it could reach as high as $25 billion and the focus on space will continue as nations like China enter the field.

"It's a competition that I wish wasn't occurring, but it is. And if we're threatened in space, we have the right of self-defense, and we'll make sure we can execute that right," Gen. John Hyten, the head of Air Force Space Command, told "60 Minutes."

The Air Force Research Laboratory developed the test plans for X-37B's mission, which the Rapid Capabilities Office will administer, explains Space Flight Now.

"The experiment will include collection of telemetry from the Hall thruster operating in the space environment as well as measurement of the thrust imparted on the vehicle," the Air Force said in a statement.

"The resulting data will be used to validate and improve Hall thruster and environmental modeling capabilities, which enhance the ability to extrapolate ground test results to actual on-orbit performance."