The U.S. Air Force launched an unmanned spacecraft Tuesday for a repeat mystery mission, two years after making the first flight of its kind.

The 29-foot X-37B spaceplane circled the planet for seven months in 2010 and is now embarking on a new secret mission being referred to a OTV-3, or Orbital Test Vehicle, flight No. 3.

While little is known about the secret mission, one scientific observer, Jonathan McDowell of the Harvard-Smithsonian Center for Astrophysics, speculates to the Associated Press that he spaceplane is carrying sensors designed for spying and likely is serving as a testbed for future satellites. As cited by the AP, McDowell dismisses rumors of "exotic ideas" for the X-37B as a new space weapon or shadowing a Chinese satellite.

The scientist grants that he is not 100 percent sure what the spaceplane is carrying, but says on-board sensors could be capable of imaging or intercepting transmissions from hot spots like, for example, terrorist training sites in Afghanistan. "All the sorts of things that spy satellites generally do," he said.

Experts would most likely agree that the most important achievement of the launch is that mystery mission flight is the first reflight.

"That is pretty cool," McDowell told the AP, "reusing your spacecraft after a runway landing. That's something that has only really been done with the shuttle."

NASA's space shuttles, which stretch 22 feet long, with 78-foot wingspans and weights of more than 170,000 pounds, are now forever inactive.

They were launched, from 1981 to 2011, with two strap-on booster rockets and an external fuel tank feeding three main engines.

According to the AP, The X-37B wingspan is 15 feet, and the 11,000-pound, Boeing-built vessel requires the United Launch Alliance's hefty Atlas V for hoisting. It is solar-powered once in orbit, the AP adds.