The U.S. military likes to be a little sneaky with its robotic space planes. Unlike typical spacecraft, these vehicles can shift their orbits, frustrating the global network of skywatchers who keep track of just about every man-made object rotating the planet.

But the sleuths have their tricks, too. They've tracked down the X-37B on its second secret mission. And the information the skywatchers are finding says quite a bit about the classified operations of this mysterious spacecraft.

It took the amateur sleuths nearly a month to hunt down the first X-37B after it launched on its inaugural mission. That's an eternity in sky-spotting time.

The second time around was easier. The U.S. space plane was discovered just four days after it blasted into orbit, earlier this month. Cape Town, South Africa's Greg Roberts – "a pioneer in using telescopic video cameras to track spacecraft, chalking up exceptional results over the years," according to Space.com – spotted this second secret spacecraft, just like he found the first.

The X-37B has generated intense interest, long before it ever left the ground. Boeing originally developed the 29-foot unmanned craft – a kind of miniature Space Shuttle – for NASA. Then, the military took over in 2004, and the space plane went black. Its payloads were classified, its missions hush-hush.

Depending on who you talk to, the space plane could be a prototype commando transport, an orbiting bomber or (most likely) a spy-above-the-skies. It could launch, repair or reposition U.S. satellites in low orbit. It could sneak up and disable or steal enemy satellites. Its pickup-bed-sized payload bay is particularly enticing to observers.

And now, there are observers – plural. Ontario's Kevin Fetterof filmed the X-37B "gliding past the binary star Eta Serpentis," according to Space.com, which has – video of the flyby. Skywatchers Brad Young, Bill Young, and Alberto Rango have all reported sightings, as well.

You can even see the space plane for yourself: The X-37B is traveling in a slightly elliptical orbit more than 200 miles up, swooping from 43 degrees north latitude to 43 degrees south. (Check the real-time map at n2yo.com.)

That orbit gives some indications about what the space plane is actually doing up there. The typical spy satellite has a polar orbit, which means "it can cover the whole earth, and it can fly over the same spot at the same sun angle each time it comes overhead," explains Brian Weeden, a former Air Force Space Command officer, now with the Secure World Foundation.

The X-37B, on the other hand, is orbiting around the fat middle of the planet, traveling over the Middle East, Africa, and fair chunk of China. "It means they are giving up global coverage and predictable shadow lengths, but getting more frequent passes," Weeden says. The orbit lends credence to the idea that the space plane is an orbiting spy.

So does the X-37B's altitude. It's flying pretty low – one of the rare orbiters traveling beneath the International Space Station. "The lower you are," Weeden notes, "the higher resolution you can get in any imagery."

And the easier you are to spot from the ground.

Photo: U.S. Air Force. Illustration: Scott Madry/International Space University