The U.S. Air Force is standing ready for this week's much anticipated return to Earth of a robotic space plane that has spent more than a year in orbit on a secret mission.

Air Force officials say landing day for the unmanned X-37B space plane is imminent, and could occur on Friday (June 15). But weather conditions at its intended landing site at Vandenberg Air Force Base in California, as well as other factors, will determine when the spacecraft's will ultimately land.

"Team Vandenberg is prepared to safely receive the X-37B at a moment's notice," Air Force Lt. Austin Fallin told SPACE.com in an email this week. "Exact landing date and time depend on weather and technical considerations."

The X-37B spacecraft's landing window opened on June 11 and runs through June 18, with Friday being the next opportunity, Fallin said. The exact nature of the space plane's mission is classified, so aside from a brief May 30 statement announcing the upcoming landing in the mid-June timeframe, Air Force officials have remained mum on re-entry details. [Photos: Air Force's 2nd Secret X-37B Mission]

"More information will be released as it becomes available," Fallin said.

- Air Force Lt. Austin Fallin

The X-37B space plane set to land this week is known as the Orbital Test Vehicle 2 (OTV-2) and looks much like a smaller version of NASA's reusable space shuttles. Unlike NASA's shuttles, the OTV-2 is completely unmanned and is powered by a solar panel that allows it to stay in orbit for months at a time.

Air Force officials have said the estimated mission length for its X-37B space planes (there are currently two) is about 270 days, but the OTV-2 mission has far outlasted that timeframe. The OTV-2 mission launched into orbit on March 5, 2011 and has racked up more than 460 days in orbit so far.

The Air Force's X-37B space planes are built by Boeing and originally began as a NASA test program that shifted into the U.S military's control in 2006, first to DARPA and then to the Air Force, due to funding issues. Each X-37B spacecraft is about 29 feet long (8.8 meters) 15 feet wide (4.5 m), and has a payload bay about the size of a pickup truck bed.

The spacecraft are launched into orbit atop unmanned Atlas 5 rockets and are designed to guide themselves back to Earth on autopilot by re-entering the atmosphere and landing on a runway at Vandenberg. The OTV-2 mission and its predecessor were launched from Cape Canaveral Air Force Station in Florida.

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Secret Space Plane Returns?