The mystery space plane X-37B of the United States

The US military's space plane X-37B that can keep track of terrorists has been circling Earth for nearly two years in the program's third mission and is expected to land soon at Vandenberg Air Force Base, about 130 miles northwest of Los Angeles nzherald.co.nz.
The project is at present with the Air Force's Rapid Capabilities Office.
It seems there are two of these robotic planes, and the one that is due to land soon had been launched on the back of an Atlas V rocket from Cape Canaveral, Florida, on Dec. 11, 2012. As far as size is concerned, it 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 is installed with solar panels that unfurl to charge its batteries once it is in orbit. However, there are no details on the purpose of this top secret mission and what it has been doing in space.
One theory is that it has a relatively small cargo bay which was used to ferry sensors that can capture detailed images or communications on Earth - for example, to track Iran's nuclear program or suspected terrorists.
Another theory is that it has been designed to give the Chinese something to worry about what it is for and, to arrive at a solution, it would have to spend (and waste) time and resources trying to counteract a range of possibilities. Other theories are that X-37B could grab another nation's satellites, or come close to inspect it from close quarters or even shadow China's space lab.