(Update 8 a.m. Thursday — Air Force says the space plane won't be coming back for the next couple of days due to weather issues at Vandenberg. )

After an unprecedented 15 months in orbit, the military's unmanned X-37B experimental spaceplane is due back to Earth, possibly as early as today.

The robotic spaceship, which resembles a miniature space shuttle, blasted off aboard an Atlas 5 rocket on March 5, 2011, from Cape Canaveral Air Force Station in Florida. The mission was billed as a follow-up to a sistership's 224-day debut flight in 2010.

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The X-37B, also known as the Orbital Space Vehicle, is due to make an automated landing at either California's Vandenberg Air Force Base or Edwards Air Force Base between June 6 and June 18.

The Air Force says it has been using the X-37B to test new technologies, but will not elaborate. Another aspect of the program is to develop systems and processes to quickly and inexpensively refurbish the vehicles for reflight.

The X-37B that flew in 2010 is being prepared for a return to orbit this fall, said Air Force spokeswoman Tracy Bunko.

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The spaceplanes are about 29 feet long and 15 feet wide. Each has a payload bay about the size of a pickup truck bed and solar panel wings  to provide power.

 Image: The first X-37B landed at Vandenberg Air Force Base in California after a 224-day spaceflight. A sistership is due home as early as today. Credit: U.S. Air Force.