Boeing X37B, US Top Secret Spy Plane lands after 674 days in Orbit

This June 16, 2012 file image from video made available by the Vandenberg Air Force Base shows an infrared view of the Boeing X-37B unmanned spacecraft landing at Vandenberg Air Force Base. 

The purpose of the U.S. military's space plane is classified, only fueling speculation about why it has been orbiting Earth for nearly two years on this, its third mission. 

The plane is expected to land this week at a Southern California Air Force base. 

Credit: AP Photo /Vandenberg Air Force Base

A top-secret space plane landed Friday at an Air Force base on the Southern California coast.

The plane spent nearly two years circling Earth on a classified mission. Known as the Boeing X-37B, it resembles a mini space shuttle.

It safely touched down at 9:24 a.m. Friday, officials at Vandenberg Air Force Base said.

Just what the plane was doing during its 674 days in orbit has been the subject of sometimes spectacular speculation.

Several experts have theorized it carried a payload of spy gear in its cargo bay.

Other theories sound straight out of a James Bond film, including that the spacecraft would be able to capture the satellites of other nations or shadow China's space lab.

In a written release announcing the return of the craft, the Air Force only said it had been conducting "on-orbit experiments."

The Boeing X-37B program has been an orphan of sorts, bouncing since its inception in 1999 between several federal agencies, NASA among them. It now resides under the Air Force's Rapid Capabilities Office.

The plane that landed Friday is one of two built by Boeing. This is the program's third mission, and began in December 2012.

The plane stands 9 1/2 feet tall and is just over 29 feet long, with a wingspan under 15 feet. It weighs 11,000 pounds and has solar panels that unfurl to charge its batteries once in orbit.

The Air Force said it plans to launch the fourth Boeing X-37B mission from Cape Canaveral, Florida, next year.

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The X-37B, which weighs five tonnes and is 29 feet (8.9 meters) long, can return material to Earth in the way of the retired shuttle Orbiter program but is designed to stay in orbit for much longer at 270 days.

The last X-37B returned in June after orbiting for 469 days in a test of endurance.

The United Launch Alliance, a joint venture between Boeing and Lockheed Martin, approved the X-37B at Cape Canaveral in Florida after finding no danger following an anomaly during a separate launch two months ago.

The company said in a statement that a Global Positioning System (GPS) satellite was put into orbit as expected on October 4 but that a fuel leak took place inside the thrust chamber, triggering an investigation.

Patrick Air Force Base gave notice of a hazard from a launch between 10:45 am to 5:15 pm (1545 to 2215 GMT) on Tuesday.

Authorities have said little more about the X-37B. An Air Force fact sheet described it as "experimental test program to demonstrate technologies for a reliable, reusable, unmanned space test platform for the US Air Force."

The secretive nature of the equipment on the X-37B has led to speculation in the media over its true nature, with some experts saying it could eventually be designed to tamper with satellites from rival nations.

China in 2007 became the first nation after the United States and the former Soviet Union to shoot down one of its own satellites, in a test seen in Washington as a sign of the rising power's ambitions in space.

The X-37B project was launched by the space agency NASA in 1999 before being adopted by the Defense Advanced Research Projects Agency, which designs new technologies for the US military.