A global team of vigilant satellite watchers has caught the U.S. Air Force's robotic X-37B space plane on camera as the craft circles Earth on its fourth mysterious mission.

The unmanned X-37B launched May 20 atop a United Launch Alliance Atlas V rocket from Florida's Cape Canaveral Air Force Station. Details about the current mission — known as Orbital Test Vehicle-4, or OTV-4 — are classified, but the satellite trackers' new observations are shedding some light on the spacecraft's activities.

"It's in a lower orbit than normal … had us confused for a while, as I thought it would be the standard operating orbit," tracker Greg Roberts, who lives in South Africa, told Space.com. "The inclination is also lower than before." [X-37B Space Plane's 4th Mystery Mission in Photos]

Roberts captured pictures of the space plane in two-second–long exposures through what he called "cursed power and telephone lines."

Mission number four

The X-37B is built by Boeing Network & Space Systems, a unit that also designs and delivers satellites used for communications, navigation, intelligence and weather monitoring. The robotic vehicle looks like a miniature version of NASA's now-retired space shuttle.

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