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X-37B re-entry
NASA / Boeing
An artist's illustration of the unmanned X-37B space plane during re-entry.
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updated 6/16/2012 12:59:45 PM ET 2012-06-16T16:59:45

The U.S. Air Force's robotic X-37B space plane finally returned to Earth Saturday (June 16), wrapping up a mysterious mission that lasted more than year in orbit.

The unmanned X-37B spacecraft, also known as Orbital Test Vehicle-2 (OTV-2), glided back to Earth on autopilot, touching down at California's Vandenberg Air Force Base at 5:48 a.m. PDT (8:48 a.m. EDT, 1248 GMT). The landing brought to an end the X-37B program's second-ever spaceflight, a mission that lasted more than 15 months with objectives that remain shrouded in secrecy.

U.S. Air Force / Michael Stonecypher
An X-37B robotic space plane sits on the Vandenberg Air Force base runway during post-landing operations on Dec. 3, 2010. Personnel in self-contained protective atmospheric suits conduct initial checks on the robot space vehicle after its landing. This same craft is due to launch again in fall 2012.

Air Force officials announced the X-37B space plane's successful landing in a brief statement posted on the Vandenberg website and emailed to reporters.

"Team Vandenberg has put in over a year's worth of hard work in preparation for this landing and today we were able to see the fruits of our labor," said Col. Nina Armagno, 30th Space Wing commander at Vandenberg. "I am so proud of our team for coming together to execute this landing operation safely and successfully." [Photos: Air Force's 2nd Secret X-37B Mission]

The X-37B stayed in orbit for 469 days this time, more than doubling the 225 days its sister ship, OTV-1, spent in space last year on the program's maiden flight. Officials at Vandenberg said the spacecraft conducted "on-orbit experiments" during its mission. The landing window for the X-37B actually opened on June 11, and was expected to close on Monday (June 18).

An extended, mysterious mission
OTV-2 launched aboard an Atlas 5 rocket from Florida's Cape Canaveral Air Force Station on March 5, 2011. The space plane was designed to stay aloft for 270 days, but the Air Force kept it flying well beyond that milestone in a mission that officials recently called a " spectacular success."

"With the retirement of the space shuttle fleet, the X-37B OTV program brings a singular capability to space technology development," said X-37B program manager Lt. Col. Tom McIntyre in today's statement. "The return capability allows the Air Force to test new technologies without the same risk commitment faced by other programs. We're proud of the entire team's successful efforts to bring this mission to an outstanding conclusion."

Exactly what the spacecraft, which is built by Boeing's Phantom Works division, was doing up there for so long is a secret. The details of the X-37B's mission, which is overseen by the Air Force's Rapid Capabilities Office, are classified, as is its payload.

AIAA / Grantz / Boeing
A size chart shows how the Boeing-built X-37B robot space plane compares to NASA's space shuttle, a larger version of the spacecraft called the X-37C and an Atlas 5 rocket.

This secrecy has led to some speculation, especially online and abroad, that the X-37B could be a space weapon of some sort — perhaps a sophisticated satellite-killer. Some experts also suspect that the vehicle may be an orbital spy platform.

The Air Force, however, has worked to tamp down such speculation, stressing repeatedly that the X-37B isn't doing anything nefarious hundreds of miles above the Earth's surface.

"This is a test vehicle to prove the materials and capabilities, to put experiments in space and bring them back and check out the technologies," Richard McKinney, the Air Force's deputy undersecretary for space programs, said shortly after OTV-1 landed in December 2010.

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"My words to others who might read anything else into that is, 'Just listen to what we're telling you,'" McKinney added. "This is, pure and simple, a test vehicle so we can prove technologies and capabilities."

The X-37B looks a bit like NASA's recently retired space shuttle, but it's far smaller. The X-37B is about 29 feet (8.8 meters) long and 15 feet (4.5 m) wide, with a payload bay about the size of a pickup truck bed. Two X-37B vehicles could fit inside the payload bay of a space shuttle.

The spacecraft's orbital longevity is enabled by its solar array, which generates power after deploying from its payload bay.

Cargo and crew carrier?
While the X-37B currently flies only hush-hush missions for the Defense Department, its spaceflight role may be expanded in the future.

In its current state, the vehicle could fly cargo missions to the International Space Station, docking to the orbiting outpost's common berthing port, Boeing officials have said.

Boeing is also looking into building a larger variant of the spacecraft called the X-37C, which could ferry up to six astronauts to the space station. The X-37C would be 65 to 80 percent bigger than the X-37B.

NASA / MSFC
NASA's Marshall Space Flight Center image shows on-orbit functions for the reusable X-37 space plane, now under the wing of the U.S. Air Force.

Originally, NASA used the X-37B as an experimental test bed until funding for the project ran out in 2004. The vehicle then passed to the Defense Advanced Research Projects Agency, and it was ultimately turned over to the Air Force in 2006.

Vandenberg officials said the next X-37B mission will launch sometime later this year, most likely during autumn. That mission will use the first X-37B to fly, OTV-1, lofting it to orbit for a second flight.

Follow SPACE.com senior writer Mike Wall on Twitter at@michaeldwall or SPACE.com@Spacedotcom. We're also onFacebook andGoogle+.

© 2013 Space.com. All rights reserved. More from Space.com.

Photos: Month in Space: January 2014

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  1. Southern stargazing

    Stars, galaxies and nebulas dot the skies over the European Southern Observatory's La Silla Paranal Observatory in Chile, in a picture released on Jan. 7. This image also shows three of the four movable units that feed light into the Very Large Telescope Interferometer, the world's most advanced optical instrument. Combining to form one larger telescope, they are greater than the sum of their parts: They reveal details that would otherwise be visible only through a telescope as large as the distance between them. (Y. Beletsky / ESO) Back to slideshow navigation
  2. A balloon's view

    Cameras captured the Grandville High School RoboDawgs' balloon floating through Earth's upper atmosphere during its ascent on Dec. 28, 2013. The Grandville RoboDawgs’ first winter balloon launch reached an estimated altitude of 130,000 feet, or about 25 miles, according to coaches Mike Evele and Doug Hepfer. It skyrocketed past the team’s previous 100,000-feet record set in June. The RoboDawgs started with just one robotics team in 1998, but they've grown to support more than 30 teams at public schools in Grandville, Mich. (Kyle Moroney / AP) Back to slideshow navigation
  3. Spacemen at work

    Russian cosmonauts Oleg Kotov, right, and Sergey Ryazanskiy perform maintenance on the International Space Station on Jan. 27. During the six-hour, eight-minute spacewalk, Kotov and Ryazanskiy completed the installation of a pair of high-fidelity cameras that experienced connectivity issues during a Dec. 27 spacewalk. The cosmonauts also retrieved scientific gear outside the station's Russian segment. (NASA) Back to slideshow navigation
  4. Special delivery

    The International Space Station's Canadian-built robotic arm moves toward Orbital Sciences Corp.'s Cygnus autonomous cargo craft as it approaches the station for a Jan. 12 delivery. The mountains below are the southwestern Alps. (NASA) Back to slideshow navigation
  5. Accidental art

    A piece of art? A time-lapse photo? A flickering light show? At first glance, this image looks nothing like the images we're used to seeing from the Hubble Space Telescope. But it's a genuine Hubble frame that was released on Jan. 27. Hubble's team suspects that the telescope's Fine Guidance System locked onto a bad guide star, potentially a double star or binary. This caused an error in the tracking system, resulting in a remarkable picture of brightly colored stellar streaks. The prominent red streaks are from stars in the globular cluster NGC 288. (NASA / ESA) Back to slideshow navigation
  6. Supersonic test flight

    A camera looking back over Virgin Galactic's SpaceShipTwo's fuselage shows the rocket burn with a Mojave Desert vista in the background during a test flight of the rocket plane on Jan. 10. Cameras were mounted on the exterior of SpaceShipTwo as well as its carrier airplane, WhiteKnightTwo, to monitor the rocket engine's performance. The test was aimed at setting the stage for honest-to-goodness flights into outer space later this year, and eventual commercial space tours.

    More about SpaceShipTwo on PhotoBlog (Virgin Galactic) Back to slideshow navigation
  7. Red lagoon

    The VLT Survey Telescope at the European Southern Observatory's Paranal Observatory in Chile captured this richly detailed new image of the Lagoon Nebula, released on Jan. 22. This giant cloud of gas and dust is creating intensely bright young stars, and is home to young stellar clusters. This image is a tiny part of just one of 11 public surveys of the sky now in progress using ESO telescopes. (ESO/VPHAS team) Back to slideshow navigation
  8. Fire on the mountain

    This image provided by NASA shows a satellite view of smoke from the Colby Fire, taken by the Multi-angle Imaging SpectroRadiometer aboard NASA's Terra spacecraft as it passed over Southern California on Jan. 16. The fire burned more than 1,863 acres and forced the evacuation of 3,700 people. (NASA via AP) Back to slideshow navigation
  9. Where stars are born

    An image captured by NASA's Spitzer Space Telescope shows the Orion Nebula, an immense stellar nursery some 1,500 light-years away. This false-color infrared view, released on Jan. 15, spans about 40 light-years across the region. The brightest portion of the nebula is centered on Orion's young, massive, hot stars, known as the Trapezium Cluster. But Spitzer also can detect stars still in the process of formation, seen here in red hues. (NASA / JPL-Caltech) Back to slideshow navigation
  10. Cygnus takes flight

    Orbital Sciences Corp.'s Antares rocket rises from NASA's Wallops Flight Facility on Wallops Island, Va, on Jan. 9. The rocket sent Orbital's Cygnus cargo capsule on its first official resupply mission to the International Space Station. (Chris Perry / NASA) Back to slideshow navigation
  11. A long, long time ago...

    This long-exposure picture from the Hubble Space Telescope, released Jan. 8, is the deepest image ever made of any cluster of galaxies. The cluster known as Abell 2744 appears in the foreground. It contains several hundred galaxies as they looked 3.5 billion years ago. Abell 2744 acts as a gravitational lens to warp space, brightening and magnifying images of nearly 3,000 distant background galaxies. The more distant galaxies appear as they did more than 12 billion years ago, not long after the Big Bang. (NASA / NASA via AFP - Getty Images) Back to slideshow navigation
  12. Frosty halo

    Sun dogs are bright spots that appear in the sky around the sun when light is refracted through ice crystals in the atmosphere. These sun dogs appeared on Jan. 5 amid brutally cold temperatures along Highway 83, north of Bismarck, N.D. The temperature was about 22 degrees below zero Fahrenheit, with a 50-below-zero wind chill.

    Slideshow: The Year in Space (Brian Peterson / The Bismarck Tribune via AP) Back to slideshow navigation
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