The U.S. Air Force is preparing to launch a secret payload into space Wednesday after completing a Launch Readiness Review on its X-37B Orbital Test Vehicle. The unmanned craft is scheduled to take off from Cape Canaveral, Florida, aboard an Atlas V rocket built by United Launch Alliance, though questions about its purpose remain.

The robotic X-37B is a solar-powered space plane that looks remarkably similar to a smaller version of the traditional NASA space shuttle. The length and purpose of the spacecraft's mission remain classified, though the Hall thruster that will propel the X-37B into orbit has previously been used to send Advanced Extremely High Frequency military satellites into space.

This is the fourth time the X-37B has been deployed on a secret mission. Previous reports revealed the craft's payload will include 100 small samples of polymers, coatings and composites, leading to a suggestion from the New Scientist that NASA could be testing to see how long this kind of material will last in space.

The most recent X-37B mission ended after 674 days, in October 2014.

“Space is so vitally important to everything we do,” Maj. Gen. Tom Masiello, an Air Force Research commander, said in a recent military statement. “Secure comms [intelligence and surveillance reconnaissance], missile warning, weather prediction, precision navigation and timing all rely on it, and the domain is increasingly contested. A more efficient on-orbit thruster capability is huge. Less fuel burn lowers the cost to get up there, plus it enhances spacecraft operational flexibility, survivability and longevity.”

The Atlas V rocket, scheduled for takeoff at 10:45 a.m. EDT Friday, will also ferry the first LightSail mission into space. The LightSail, a project originated by astrophysicist Carl Sagan and built by Bill Nye's Planetary Society, is a small “solar sail” craft that aims to lower aviation costs by running almost entirely on solar power.