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The intriguingly long voyage of the unmanned X-37B has come to a conclusion at last. But the mystery of the mission lingers on.

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The US Air Force space plane, one of just two X-37B vehicles in the Pentagon's inventory, landed Friday morning under the auspices of the 30th Space Wing at Vandenberg Air Force Base in California after 674 days in space -- that is, 22 months.

And that's about all that the space plane's handlers would say about the mission, aside from the terse statement that it "conducted on-orbit experiments."

"The mission is our longest to date and we're pleased with the incremental progress we've seen in our testing of the reusable space plane," the Air Force said in a statement.

The Air Force also said it plans to start the next X-37B mission sometime in 2015, launching from Cape Canaveral in Florida.

Almost as remarkable as the length and the hush-hush nature of the mission is one of the signature skills of the Boeing-built X-37B -- the unmanned spacecraft flies autonomously on its return trip to Earth.

The Air Force has said precious little -- ever -- about its X-37B missions, leading to wide-ranging speculation about what the diminutive space vehicle has been up to up there, or is building toward. Theories hit on everything from terrestrial surveillance to satellite launches (or, conversely, satellite killing) to weapons platform aimed at ground targets.

Or it could be more mundane: these could simply be shakedown cruises to see how a space plane, one with no human aboard, fares on extended junkets into orbit and back. It can't be easy to work all the kinks out of the algorithms that enable a space plane to maneuver in orbit, and in re-entry and landing, without a human at the controls.

The boilerplate description on the Air Force's fact sheet is not exactly illuminating: "The primary objectives of the X-37B are twofold: reusable spacecraft technologies for America's future in space and operating experiments which can be returned to, and examined, on Earth."

Measuring 29 feet long, with a wingspan of less than 15 feet and a cargo bay equivalent to that of a pickup truck, the 11,000-pound X-37B looks like a junior version of NASA's space shuttles. That's not really a coincidence, since both trace their roots to the space agency's research into lifting-body flying machines. In fact, the X-37B program was a NASA project until 2004, before it was shifted first to the Defense Advanced Research Projects Agency and then on to the Air Force.

This was the third spaceflight of an X-37B, also known as the Orbital Test Vehicle, since missions began five years ago. The first, which ended in December 2010, lasted 224 days, and the second, which ended in June 2012, endured for 469 days, or a year and four months.

This latest mission began December 11, 2012, when an Atlas V rocket carrying the space plane lifted off from the Cape Canaveral Air Force Station in Florida.

There are two OTV craft in the X-37B fleet. The one that flew on the just-ended mission is OTV-1, which also carried out the first of the flights to date. Confusingly the numbers in the the OT-x designations seem to be used loosely both for the spacecraft (-1 and -2) and for the missions (-1, -2, and now -3).

NASA last week said it has entered into an agreement with the Air Force's X-37B program for use of the Kennedy Space Center's Orbiter Processing Facility (OPF) Bays 1 and 2 -- former space shuttle hangars -- to process the X-37B for launch. Boeing is performing construction upgrades in those facilities that are targeted to be complete in December.

But the flights of the X-37B -- however many eventually take place -- could just be the groundwork for the next generation of space plane.

In July, DARPA took a first step back into the game, announcing the start of design work toward the XS-1, which also would be a reusable unmanned vehicle for "aircraft-like access to space." The agency didn't mind mentioning potential payloads: "XS-1 seeks to deploy small satellites faster and more affordably, and develop technology for next-generation hypersonic vehicles."

It's probably not too much of a surprise that Boeing is in the mix, winning a $4 million preliminary design contract for its concept of the XS-1. (Teamed with Boeing is Blue Origin, the spacecraft-minded company owned by Amazon CEO Jeff Bezos.) The other two Phase 1 contracts went to Northrop Grumman (working with Virgin Galactic) and to Masten Space Systems (working with XCOR Aerospace).

Late on Friday, Boeing declared its own cheery but unrevealing sentiments ("the X-37B program performs risk reduction, experimentation and concept of operations development for reusable space vehicle technologies") about the return of the X-37B.

"We congratulate the Air Force Rapid Capabilities Office and the 30th Space Wing at Vandenberg Air Force Base on this third successful OTV mission," said Ken Torok, Boeing director of Experimental Systems. "With a program total of 1,367 days on orbit over three missions, these agile and powerful small space vehicles have completed more days on orbit than all 135 Space Shuttle missions combined, which total 1,334 days."

Update October 18 at 7:38 a.m. PT: Added photos, along with comment from Boeing.

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