In 2010, a new U.S. Air Force test vehicle, the X-37B, took flight from Cape Canaveral in Florida. The unmanned plane, which looks a bit like a compact version of the space shuttle, later landed at Vandenberg Air Force Base in California, completing its autonomous flight. "There is no one on the ground with a joystick flying it," Lieutenant Colonel Troy Giese, the vehicle's systems program director, said in a statement before launch.

An autonomous cross-country flight would be impressive enough, but the X-37B covered far more ground than that on its maiden flight. The plane, which launched in April on an Atlas 5 rocket and entered Earth orbit, circled the planet probably thousands of times (details on the flight are sketchy) before landing in December—more than seven months later.

Now the Air Force is preparing to launch a second X-37B space plane, which the military says can stay aloft for as many as 270 days—about 15 times the duration of the longest shuttle mission. The launch window opens at 3:39 P.M. Eastern Standard Time on March 4. (United Launch Alliance, which operates the Atlas 5 rocket, is streaming video from the launch.)

The Air Force has been intentionally vague about the purpose of the robotic spacecraft, saying that the program is testing "reusable technologies for America's future in space and operating experiments which can be returned to and examined on Earth." But the specs of the vehicle itself are public. The X-37B weighs five metric tons and is nearly nine meters long with a 4.5-meter wingspan. (The space shuttle, in comparison, is 37 meters long with a 24-meter wingspan.) The space plane has a modest experiment bay that can open to space, which has led to speculation that it may be used to deploy satellites. The X-37 project began as a NASA endeavor before the military took it over in 2004.

Photo of X-37B: Courtesy USAF