Air Force's X-37B mini-shuttle shrouded in secrecy

• The X-37B is scheduled to launch on its third mission Tuesday from Cape Canaveral

CAPE CANAVERAL, Fla. — The military's mysterious mini-shuttle is set to launch this week on a classified mission that has captured the imaginations of everyone from amateur satellite trackers to anti-nuclear protestors and potential military adversaries Russia and China.

Built by Boeing's secretive Phantom Works in Huntington Beach, Calif., the Air Force X-37B spacecraft is rumored to be everything from a space bomber to a satellite-killer or a test-bed for advanced spy satellite sensors.

The Air Force is revealing little.

"Inquiring minds want to know, right?" said John Pike, director of GlobalSecurity.org, a leading source of defense, space and intelligence information.

"But posing this question presumes that (the mini-shuttle) does serve some specific purpose. And I think that might be imposing greater rationality on the whole thing than is warranted."

Birthed by NASA in 1999, the project shifted to the Defense Advanced Research Projects Agency (DARPA) in 2004, and then to the U.S. Air Force in 2006. Tuesday's planned launch of the winged vehicle will be the X-37B's third mission.

Pike thinks the program exists, well, because it exists.

"To the extent that it does have a purpose, I think its purpose is to keep the Chinese guessing as to what the purpose is."

The Union of Concerned Scientists holds a similar view. The nonprofit group says that bureaucratic inertia "may help keep the space-plane concept alive."

"In a time of tightening budgets, the administration and Congress should take a close look at the X-37B program and figure out why they're spending money on a program that has no persuasive rationale," said Laura Grego, a senior scientist with the organization.

Here's what we do know about the mini-shuttle set to launch atop an Atlas V rocket at Cape Canaveral Air Force Station in Florida:

— About one-fourth of the size of a NASA shuttle orbiter, the X-37B is a reusable, robotic vehicle.

— The unmanned spacecraft has no crew cabin, no life support systems, and neither the Air Force nor NASA has indicated a desire to upgrade it for human spaceflight.

— From stem to stern, the spaceship is 29 feet in length, about the size of a small school bus.

— The solar-powered spaceship is designed to remain in orbit up to 270 days. The second X-37B mission flew for 469 days.

In comparison, shuttles were powered by fuel cells that limited its orbital flights. The longest shuttle mission lasted 17 days, 15 hours and 53 minutes.

— The project's total cost is unknown because the budget has been classified since the X-37B project was transferred to DARPA in 2004.

What is known: NASA, Boeing and the Air Force spent $208 million ($125 million from NASA, $67 million from Boeing with the Air Force chipping in $16 million) on initial development between 1998 and the end of 2002. In November 2002, Boeing was awarded a $301 million contract to continue development.

— The X-37B's most unique capability: It can re-enter Earth's atmosphere and land autonomously — with no pilot.

Boeing built two X-37B spacecraft for orbital flight. The first launched from Cape Canaveral in April 2010, the second blasted off in March 2011. Both of those missions concluded with landings at Vandenberg Air Force Base in California. The Air Force is also looking at consolidating landing, refurbishment and launch operations on the Space Coast.

So, what exactly does the X-37B do?

And what doesn't it do?

China and other adversaries "see it as yet another example of the U.S. developing space weapons, contrary to the U.S. public position — that it doesn't have any space weapons," said Brian Weeden, technical adviser to Secure World Foundation, which promotes peaceful uses of outer space.

"There is no proof to support their claims, but that doesn't really matter. Just as some in the U.S. have misrepresented or exaggerated Russian and Chinese programs to advance certain political positions, Russia and China are going to do the same."

Dave Webb, chairman of the Global Network Against Weapons & Nuclear Power in Space, says the X-37B "is part of the Pentagon's effort to develop the capability to strike anywhere in the world with a conventional warhead in less than an hour."

But other analysts say the X-37B is no space bomber.

"Absolutely not," said Weeden, a former Air Force official with experience in space and ballistic missile operations. "The laws of physics are a pretty harsh mistress and make such systems impractical and not very useful."

Some question whether the X-37B itself might be a delivery system for a nuclear bomb — whether the spaceship is intended to re-enter Earth's atmosphere on autopilot and dive-bomb an enemy target.

That's highly unlikely. Just like NASA's shuttle orbiters, the X-37B is an unpowered glider during reentry.

"It would be a very expensive, sitting duck for any air-defense system in the world," Weeden said.

Some surmise the X-37B is a satellite-tracker or a satellite-killer. Or both.

But the U.S. military knows the X-37B is "a very high-interest object" for amateur satellite observers and military officials in Russia and China.

So, then, what is the X-37's mission?

Gary Payton, deputy undersecretary of the Air Force for Space Programs and a former shuttle astronaut, says the spacecraft is primarily a technology demonstrator and a platform for space experiments.

"The primary objectives of the X-37 are (developing) a new batch of reusable technologies for America's future, plus learning and demonstrating the concept of operations for reusable experimental payloads," Payton told reporters prior to the X-37B's inaugural launch.

"Take a payload up, spend up to 270 days on orbit. They'll run experiments to see if the new technology works, then bring it all back home and inspect it to see what was really going on in space," Payton said.

The first X-37B mission was a 224-day shakedown cruise — a flight primarily aimed at "proving that the vehicle itself can get up in space, do a job, get back down," Payton said.

Then "the most important demonstration" would be determining whether the vehicle could be prepped for another flight rapidly, and at low cost, he said.

Objectives of the second flight — the first flight of the Air Force's second X-37B vehicle — probably were similar.

Weeden said the spacecraft could serve as test-beds for advanced sensors for spy satellites.

"I think it is very likely, and this may be the primary mission of the X-37B," Weeden said. "As far as the types of sensors, it could be virtually anything, but my guess would be radar, hyperspectral or infrared."

Pike remains skeptical.

"Over time, most of the money got spent just to keep the program going," Pike said.

"It acquired a life of its own. And now to the extent that it might be said to have any larger purpose, it would be to bewilder the Chinese."