WASHINGTON - The US Air Force on Tuesday launched a space plane at Cape Canaveral in Florida, which carries a mystery payload for an undisclosed mission.

A United Launch Alliance Atlas 5 rocket carrying the US military's X-37B experimental space plane lifts off from launch complex 41 in Cape Canaveral, Florida Dec 11, 2012. [Photo/Agencies]

The space plane, called X-37B, is an unmanned mini space shuttle. It was sent into space aboard an Atlas V rocket. US media reports speculate it carries sensors designed for spying and likely serves as a testbed.

This is the mini shuttle's third flight. The program is being run by the Air Force's Rapid Capabilities Office, and there are two space planes in the fleet, both built by Boeing.

Although media reports are abuzz with speculations about the true nature of the program, the Air Force said it is to " demonstrate a reliable, reusable, unmanned space test platform for the United States Air Force."