The U.S. Air Force is set to launch its secretive "space plane" on Wednesday, the fourth flight of the mysterious unmanned X-37B Orbital Test Vehicle.
Unlike previous missions, however, the Air Force has offered details of at least part of its mission: an experimental Hall thruster. This electric propulsion device ionizes a noble gas such as xenon. The device would allow a spacecraft to carry larger payloads and perform more orbital maneuvers than one powered by traditional rocket engines.
"Space is so vitally important to everything we do," Maj. Gen. Tom Masiello, commander of the Air Force Research Laboratory, said in a news release. "Secure comms, ISR, 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 X-37B is capable of extended missions in space; one such journey lasted 675 days. The Air Force has said little about what the spacecraft has been doing, and the details about the Hall thruster represent some of the most detailed explanations it has offered about an X-37B flight.
The craft resembles the space shuttle, but is roughly a quarter of the size of its retired cousin. Like the shuttle, it's both reusable and capable of landing like an airplane, as shown in this Air Force image from a previous mission:
Unlike the shuttle, the X-37B is unmanned and launched from a rocket. In this case, it will launch on the United Launch Alliance’s Atlas V rocket departing from Cape Canaveral Air Force Station in Florida.
The X-37B will be contained inside the payload fairing atop the rocket, or the white portion visible in this image released by the National Reconnaissance Office:
— NRO (@NatReconOfc) May 19, 2015
Along with the space plane, the rocket will carry 10 small satellites, called "CubeSats," with experiments from the the U.S. Naval Academy, Cal Poly, the Planetary Society, the Air Force Research Laboratory and Aerospace Industries, the National Reconnaissance Office (NRO) said on Twitter.
The agency released this image of the CubeSats:
— NRO (@NatReconOfc) May 18, 2015
The NRO also posted a short summary of the experiments online.
The CubeSats are in the aft bulkhead carrier, which is in the bottom portion of the payload fairing. A more detailed breakdown of Atlas V and what's inside it is visible in this brochure from United Launch Alliance.
One of the experiments is the Planetary Society's solar sail, a project based on a design by Carl Sagan. Called LightSail and funded by donations, the mylar sheet will use radiation from the sun to maneuver.
"You can go to very distant destinations in the solar system without any fuel," Planetary Society CEO Bill Nye "The Science Guy" said, according to USA Today. "You get a continuous very small push, indefinitely. You can tack, just like a sailboat, this really elegant, wonderful thing."
The small prototype is expected to fly in low-earth orbit for about 30 days ahead of a larger experiment scheduled for next year. That mission is being funded in part by a Kickstarter campaign.