For over two years, the Air Force's mysterious X-37B space plane has been in Earth's orbit, as part of a top secret military mission.

That enigmatic mission comes to an end today, potentially, as the plane returns to Earth, landing at Vandenberg Air Force Base in California. Although the military won't confirm the landing date and time, reports state that the officials are shutting down air space around Vandenberg from 8 a.m. to 5 p.m. PST today.

However, the exact time of landing depends on weather and visibility conditions. According to the latest weather reports, the day will be partly cloudy, which may affect landing.

"Team Vandenberg stands ready to implement safe landing operations for the X-37B Orbital Test Vehicle, the third time for this unique mission" says Col. Keith Baits, 30th Space Wing commander.

As with many military missions, the Air Force is staying tight-lipped about the robotic X-37B's mission in space, although the Air Force states that it is testing reusable spacecraft technology.

"Technologies being tested in the program include advanced guidance, navigation and control, thermal protection systems, avionics, high temperature structures and seals, conformal reusable insulation, lightweight electromechanical flight systems, and autonomous orbital flight, re-entry and landing," says an Air Force statement.

The original mission for the solar-powered X-37B was only supposed to last nine months, so its lengthier time in space, 672 days, leads to speculation of more dubious activities, such as spying and even possibly testing out a new space weapon.

This is the third complete mission for this particular plane, designed by Boeing for movement through low-Earth orbit, about 110 and 500 miles above the planet. The plane looks a lot like a smaller version of NASA's shuttles and is about 29 feet long with a wingspan of around 15 feet. Like those shuttles, it uses a rocket for launching into orbit. It is too small for manned missions, but does have its own cargo bay.

The Air Force believes that this is not the last space flight for the X-37B and that its secret mission is just getting started. However, the military won't unveil further details about future launches or mission objectives.

Although Vandenberg Air Force Base served as a previous landing space for the X-37B, Boeing is reworking an old NASA shuttle hangar at Kennedy Space Center in Florida for future landings. This makes sense as the space plane always launches from nearby Cape Canaveral Air Force Station.