Image: X-37B robotic space plane
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.
By Senior Writer
updated 6/8/2012 8:33:37 PM ET 2012-06-09T00:33:37

After spending more than a year orbiting our planet on a hush-hush mystery mission, the U.S. Air Force's X-37B space plane is due to return to Earth any day now.

Air Force officials haven't announced exactly when the robotic X-37B space plane will land. But on May 30, they said that the spacecraft's homecoming should occur in the "early- to mid-June timeframe" — a window that will close in the next week or so.

The Air Force launched the X-37B on March 5, 2011, sending the reusable space plane design on its second-ever space mission. The X-37B currently zipping around Earth is known as Orbital Test Vehicle-2, or OTV-2.

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California's Vandenberg Air Force Base is viewed as the most likely primary landing site for OTV-2. Edwards Air Force Base, also in California, serves as a backup. [Photos of the 2nd Secret X-37B Mission]

The X-37B looks much like NASA's now-retired space shuttles, only much smaller. The space plane is about 29 feet long by 15 feet wide, with a payload bay the size of a pickup truck bed. A solar array packed in the payload bay powers the spacecraft. For comparison, two entire X-37Bs could fit inside the payload bay of a space shuttle.

OTV-2's flight represents a big jump for the X-37B space plane. The vehicle has been aloft for 462 days as of June 8, more than doubling the on-orbit time of the first space-flown X-37B, known as OTV-1.

OTV-1 launched in April 2010 and landed that December, staying on orbit for 225 days — well under the spacecraft's supposed 270-day limit. OTV-2 is nearly 200 days over that limit on its current flight, and the calendar keeps ticking over.

Just what OTV-2 has been doing for so long up there remains a mystery, because the space plane's mission and payloads are classified. Air Force officials have said that the vehicle's chief task is testing out new technologies for future satellites, and some experts think that's likely to be the case.

For example, the space plane may be observing the Middle East and Afghanistan with some brand-new spy gear, according to Brian Weeden, a technical adviser with the Secure World Foundation and a former orbital analyst with the Air Force.

OTV-2's orbit — which takes it repeatedly over the stretch of Earth from 43 degrees north latitude to 43 degrees south latitude — supports this interpretation, Weeden told SPACE.com earlier this year. The space plane's orbital parameters are classified, but amateur astronomers have been tracking OTV-2's movements since it blasted off.

The Air Force also probably wants to push the X-37B's limits on this second flight, to see just what the space plane can do, said Weeden, who published a report in 2010 that investigated the X-37B and its likely missions.

Follow SPACE.com senior writer Mike Wall on Twitter@michaeldwall or SPACE.com @Spacedotcom. We're also on Facebook and Google+.

© 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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