The U.S. Air Force's secretive robotic X-37B space plane mission continues to chalk up time in Earth orbit, nearing 430 days of a spaceflight that while classified appears to be an unqualified success.

The space plane now circuiting Earth is the second spacecraft of its kind built for the Air Force by Boeings Phantom Works. Known as the Orbital Test Vehicle 2, or OTV-2, the space plane's classified mission is being carried out by the Air Force Rapid Capabilities Office.

The robotic X-37B space plane is a reusable spacecraft that resembles a miniature space shuttle. The Air Force launched the OTV-2 mission on March 5, 2011, with an unmanned Atlas 5 rocket lofting the space plane into orbit from the Cape Canaveral Air Force Station in Florida.

Spectacular success

General William Shelton, commander of Air Force Space Command, briefly saluted the high-flying X-37B space plane on April 17 during his remarks at the 28th National Space Symposium in Colorado Springs, Colo.

"Our second X-37 test vehicle has been on orbit for 409 days now" much longer than the 270 day baseline design specifications, Shelton said. "Although I can't talk about mission specifics, suffice it to say this mission has been a spectacular success," he added. [Photos: The X-37B Space Plane's Second Mission]

In a follow-up meeting with reporters, Shelton told SPACE.com: "It's doing wonderful." When asked specifically about when the craft will be brought back down to Earth, Shelton's response was guarded.

"When we're through with it it's going great," Shelton said.

U.S. Air Force Maj. Tracy Bunko, the Pentagon's spokesperson for the X-37B project, told SPACE.com that the space plane's current mission "is still on track and still ongoing."

Bunko said that a third flight of an X-37B spacecraft slated for liftoff this fall will use the same craft that flew the first test flight, the OTV-1 mission, back in 2010. That maiden voyage of the X-37B space plane lasted 225 days. It launched into orbit on April 22, 2010, and then landed on Dec. 3, zooming in on autopilot over the Pacific Ocean and gliding down onto a specially prepared runway at Vandenberg Air Force Base in California.

Read more:

Secret Air Force space plane mission a 'success'