For almost two years, an unmanned space plane bearing a remarkable resemblance to NASA’s space shuttle has circled the Earth, performing a top-secret mission. It’s called the X-37B Orbital Test Vehicle — but that’s pretty much all we know for certain.

Officially, the only role the Pentagon acknowledges is that the space plane is used to conduct experiments on new technologies. Theories about its mission have ranged from an orbiting space bomber to an anti-satellite weapon.

The truth, however, is likely much more obvious: According to intelligence experts and satellite watchers who have closely monitored its orbit, the X-37B is being used to carry secret satellites and classified sensors into space — a little-known role once played by NASA’s now-retired space shuttles.

For a decade between the 1980s and early 1990s, NASA’s space shuttles were used for classified military missions, which involved ferrying military payloads into space. But the shuttles’ military role rested on an uneasy alliance between NASA and the Pentagon. Even before the 1986 Challenger disaster, which killed all seven crewmembers, the Pentagon had grown frustrated with NASA’s delays.

Now, with the X-37B, the Pentagon no longer has to rely on NASA — or humans.

The X-37B resembles a shuttle, or at least a shrunken-down version of it. Like the space shuttles, the X-37B is boosted into orbit by an external rocket, but lands like an aircraft on a conventional runway. But the X-37B is just shy of 10 feet tall and slightly less than 30 feet long.

Its cargo bay, often compared to the size of a pickup truck bed, is just big enough to carry a small satellite. Once in orbit, the X-37B deploys a foldable solar array, which is believed to power the sensors in its cargo bay.

“It’s just an updated version of the space shuttle type of activities in space,” insisted one senior Air Force official in 2010, the year of the first launch, when rampant speculation about the secret project prompted some to question whether it was possibly a space bomber.

For several years, the X-37B was developed in plain sight, with the military saying it was just a test vehicle. But in 2009, the Air Force suddenly said it was classified, and it went from being just another technology project to an object of obsession for amateur satellite spotters and aviation enthusiasts.

On Dec. 11, 2012, the X-37B was launched for a third time, and that vehicle has now spent over 600 days in space.

And despite the secrecy surrounding its mission, the space plane’s travels are closely watched. The Air Force announces its launches, and satellite watchers monitor its flight and orbit. What is not revealed is what’s inside the cargo bay and what it’s being used for.

While the X-37B requires a rocket to boost it into orbit, its success may be helping to revive dreams of a true reusable space plane that can take off and land like an aircraft. A real space plane has long been a dream of the Pentagon, but it has also long been a sinkhole for money. Most of those efforts have fallen by the wayside, stymied by the technology needed to boost a space plane into orbit, not to mention the prohibitive costs.

In the 1950s, for example, the Air Force pursued the X-20 Dynasoar, short for Dynamic Soarer, a hypersonic vehicle that was, in fact, designed to be a space bomber. It was eventually canceled.

In the 1980s, the Pentagon funded the National Aerospace Plane, which Ronald Reagan hailed as a new “Orient Express” that would make travel from Washington to Tokyo more like a brief train trip. Pentagon officials privately cringed at the hype, knowing the technology was likely years away. A decade later, and with more than $1 billion spent, the National Aerospace Plane was also canceled.

Now, the Pentagon’s Defense Advanced Research Projects Agency (DARPA) is taking another shot at this elusive goal with a project called the Experimental Space Plane, or XS-1 for short. DARPA is already funding several companies to work on the space plane, which is supposed to fly “10 times in 10 days” at speeds of over Mach 10. Whether this project will be any more successful than its predecessors has yet to be seen.

As for the X-37B, it’s unclear what may be next. While the secrecy surrounding the X-37B has attracted more attention to its mission, many of the more farfetched theories have fallen by the wayside.

A bomber, it turns out, would be an incredibly inefficient use of a space plane, which doesn’t carry much fuel, and so would be hard to position for an attack. Even more exotic weapons, like a space-based laser, are well outside the realms of modern technology. (The Pentagon has spent billions trying to develop lasers for use in space, with no luck.)

But presuming, as most experts do, that it’s used to carry spy satellites, what has it accomplished? It’s most likely the X-37B has been used to capture imagery of the world’s political hot spots: North Korea and Iran have both topped the list of possible targets.

The X-37B itself could be operating as a maneuverable satellite — one that can change its orbit with relative ease, and return to Earth for repairs or upgrades. A space drone.

After operating in space for nearly two years, it’s hard to argue with the X-37B’s success as a space plane. By flying without people, the military’s space plane avoids the costs — not to mention the dangers — involved with putting humans in space.

What is harder to assess, however, is the X-37’s overall value to the military. Space planes are supposed to provide economical access to space, but to date, the Pentagon has declined to release any funding information about its robotic space plane, citing its classified mission.

The real question is whether the X-37B and its payload are providing any new imagery that is useful to the military. The National Reconnaissance Office, which is responsible for the Pentagon’s secret spy satellites — and has presumably built whatever is being carried on the X-37B — has been criticized in recent years for favoring high-priced satellites over cheaper commercial imagery.

In other words, the robotic space plane, which is unclassified, is undoubtedly a technological success, but it’s unclear whether its secret payload is really doing anything particularly unique.

Only the Pentagon can answer that question, and so far, it hasn’t.

Sharon Weinberger is working on a book about the Defense Advanced Research Projects Agency.