An X-37B space plane launched into space on a secret mission for the U.S. Air Force on May 20.

Tagging along on the launch were 10 CubeSat satellites, including the LightSail craft designed by The Planetary Society. This is the fourth mission for the automated X-37B space vehicle.

An Atlas 5 rocket lifted off from the Cape Canaveral Air Force Station at 11:05 a.m. EDT. Defense Department officials are releasing little information about the nature of the flight, as even the length of the mission is classified.

The LightSail craft will test technologies needed to construct a fully-functional spacecraft that will use the pressure of light from the sun to sail around the inner solar system. Like sailboats on Earth, these vehicles will be able to sail with the wind or tack into it to travel toward our parent star.

The X-37B is 29 feet long and has a wingspan that stretches 15 feet across. The Air Force currently operates two of the craft, which resemble the now-retired space shuttles. Like that earlier human-occupied spacecraft, the X-37 lifts off from Earth in a vertical configuration and returns to Earth landing on a runway like a regular airplane. However, these new vehicles are much smaller, as both of the craft could fit within the cargo bay of one of the now-retired space shuttles.

Air Force officials call the X-37B a "reliable, reuseable, unmanned space test platform" but speculation continues that it is designed to search out and destroy satellites from other nations. Military sources say the fourth flight of the orbital test vehicle, known as OTV-4, is being carried out to test equipment that could be used for future aircraft and space-faring vehicles.

"OTV missions allow us to examine a payload system or technology in the environment in which it will perform its mission. The unique aspects of the OTV allow us to mature these new technologies and inspect them following the de-orbit sequence," Captain Chris Hoyler, spokesman for the U.S. Air Force, said.

The first test flight of the X-37B took off in April 2010 and spent 225 days in orbit. In March 2011, the OTV-2 mission sent an X-37B into space for 469 days. The third flight, OTV-3, stayed in space for an amazing 675 days following its launch in December 2012.

On this flight, almost 100 samples of various materials will be exposed to the harsh conditions of space during OTV-4 to test how they withstand the rigors of an orbital environment. A new version of an ion engine will also be subjected to tests aboard the flight.

"A more efficient on-orbit thruster capability is huge. Less fuel burn lowers the cost to get up there, plus it enhances spacecraft operational flexibility, survivability and longevity," Air Force Major General Tom Masiello said.

The Planetary Society is planning to launch a second LightSail craft into space in 2016. This will be a fully-functional solar sail vehicle, a type of spacecraft first proposed decades ago and popularized by the late astronomer Carl Sagan.

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