The U.S. military's uncrewed X-37B space plane is nearing one year in orbit on its latest secret mission.

The robotic space plane launched atop a United Launch Alliance Atlas V rocket on May 20, 2015, kicking off the X-37B program's fourth flight. This mission, dubbed OTV-4 (short for Orbital Test Vehicle-4), remains a clandestine affair.

"I can confirm the fourth OTV mission is approaching one year on orbit," Air Force spokeswoman Capt. Annmarie Annicelli said in response to Space.com's inquiry about the X-37B's activities. [The X-37B's Fourth Mystery Mission in Photos]

Annicelli had nothing further to add.

Mysterious minishuttle

The X-37B looks like a miniature adaptation of NASA's now-retired space shuttle orbiter. The military space plane is 29 feet (8.8 meters) long and 9.6 feet (2.9 m) tall, and has a wingspan of nearly 15 feet (4.6 m). For comparison, the space shuttle was 122 feet (37 m) long, with a wingspan of 78 feet (24 m).

The X-37B spacecraft has a payload bay about the size of a pickup-truck bed. It has a launch weight of 11,000 lbs. (4,990 kilograms) and is powered on orbit by gallium arsenide solar cells with lithium-ion batteries.

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