Launch of Atlas V OTV3 from Cape Canaveral AFS, FL. December 11,
Launch of Atlas V OTV3 from Cape Canaveral AFS, FL. December 11, 2012 Patrick H. Corkery—© 2012 United Launch Alliance. All rights reserved.

The Air Force Just Launched a Secret Space Plane

The U.S. Air Force sent its unmanned X-37B spacecraft into orbit for the fourth time on Wednesday

The X-37B is one of two spacecraft, built for the Air Force by Boeing, first launched in 2010. Its mission? That's a secret.

The two planes have been on three journeys to date: one from April to December of 2010, then from March 2011 to June 2012, and finally a third flight from December 2012 all the way until this past October.

The Air Force has remained mum despite long-running speculation about the purpose of these two spacecraft, the New York Times reports, and in 2010 the Pentagon denied that the planes have anything to do with weapons. Wednesday's launch of one of the spacecraft (it is also unknown which of the two launched) happened at Cape Canaveral, Fla.

The X-37B spaceplane was carried into the sky by an American Atlas V rocket. Watch a video of the launch below:

This article originally appeared on Fortune.com

See Air Force One's Transformation Over 70 Years

A view of Air Force One on the runway.
After the original Air Force One, a C-87A Liberator Express nicknamed Guess Where II, was deemed unsafe for presidential use, this Douglas C-54 Skymaster, nicknamed Sacred Cow was introduced for President Franklin D. Roosevelt in 1945. It was equipped with a radio telephone, sleeping area, and elevator for President Roosevelt's wheelchair.Thomas D. McAvoy—The LIFE Picture Collection/Getty Images
A view of Air Force One on the runway.
Faded color photograph of the Independence in flight.Date: ca. 1947 The Independence
President Dwight Eisenhower’s private plane
British Royalty, Royal Tour of the United States, pic: October 1957, Washington, USA, HM, Queen Elizabeth and the Duke of Edinburgh pictured alongside US, President Dwight Eisenhower at the airport welcome
Air Force One
AIR FORCE ONE REAGAN
Barack Obama,
Airplane Takeoffs And Landings At Chicago's O'Hare International Airport.
This Boeing 747-8 Intercontinental jetliner, the first VIP-configured aircraft, rolls out for an undisclosed customer for takeoff from Paine Field in Everett, Washington
After the original Air Force One, a C-87A Liberator Express nicknamed Guess Where II, was deemed unsafe for presidential
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Thomas D. McAvoy—The LIFE Picture Collection/Getty Images
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