The Air Force hasn’t revealed much about its X-37 pilotless space plane, fueling speculation that it’s being developed for military purposes.It may seem like a harmless, miniature version of the space shuttle, but some industry analysts are wondering if the secretive robotic spacecraft set to launch Thursday from Cape Canaveral has a more sinister side.

"Are we looking at a new space vehicle or an orbital bomber that's capable of attacking from space?" said John Pike, director of Globalsecurity.org, a website for military policy research. "At this point, it's hard to say."

The U.S. Air Force, which has been developing the X-37 pilotless space plane, isn't saying much. Even the launch has been kept out of the public eye, and only Pentagon-sanctioned photographers can take pictures of the vehicle.

The program isn't entirely classified. The Air Force revealed that it's about 29 feet long, or about the size of a small school bus, with stubby wings that stretch out about 15 feet. It is one-fifth the size of the space shuttle and can draw on the sun for electricity using unfolding solar panels.

It is the latest version of a spacecraft that initially began more than a decade ago as a NASA program to test new technologies for the space shuttle. When President George W. Bush decided to retire the space shuttle, the Pentagon took over the program and shrouded its development in secrecy.

The Pentagon won't say how much it has spent on the space plane or what its ultimate purpose may be. It has been equally cryptic about when the spacecraft would return to Earth. The space plane is designed to be launched atop an Atlas V rocket and then land on its own at Vandenberg Air Force Base.

"In all honesty, we don't know when it's coming back," said Gary E. Payton, deputy undersecretary for Air Force space programs, in a conference call this week. He did say that the vehicle can stay in space for up to nine months.

Air Force officials also deflected questions about using the X-37 for military missions, saying that the spacecraft is simply a way to test new technologies, such as satellite sensors and components.

But analysts question whether the Pentagon would be willing to spend possibly hundreds of millions of dollars for an orbiting laboratory at a time when the government is tightening spending.

"It wouldn't be in the defense budget if it didn't have defense capabilities," said Pike of Globalsecurity.com. "The idea of a small-winged vehicle that can go into orbit and perform military missions has been around for half a century."