Outer space

Secretive X-37B military space plane marks 700 days in orbit

The U.S. Air Force's robotic X-37B space plane has now been circling Earth for 700 days on its latest hush-hush mission.

That mission — known as Orbital Test Vehicle 4, or OTV-4 — began on May 20, 2015, with a liftoff atop a United Launch Alliance Atlas V rocket from Florida's Cape Canaveral Air Force Station.

As the name implies, the current mission is the fourth for the X-37B program. These orbital jaunts have been getting progressively longer: OTV-1 launched in April 2010 and spent 224 days in space, OTV-2 lifted off in March 2011 and orbited Earth for 468 days, and OTV-3 began in December 2012 and operated for nearly 675 days. [The Most Dangerous Space Weapons Ever]