In the early morning of 16 June, 2012, a top secret spaceplane made a picture perfect landing at the Vandenberg Air Force Base in California. To those unfamiliar with the vehicle, it might have looked roughly similar to the US space shuttle, the manned spacecraft that shuttled astronauts into space for three decades.

But this spaceplane, called the X-37B Orbital Test Vehicle, is very different. While it looks like a plane, is launched on a rocket, has a cargo bay and uses some of the same technology as the shuttle, such as thermal shielding to protect it during reentry, it is smaller and unmanned. It is designed to stay in orbit for months on end and can automatically land back on Earth. Perhaps more crucially, the Boeing-designed plane is operated by the US Air Force and its mission is a closely held secret, prompting a slew of speculation about its true purpose.

Since the first X-37B was launched in 2010, amateur satellite spotters have carefully followed the robotic spacecraft’s orbit, while those unconnected with the program have speculated that the plane could be anything from an anti-satellite weapon to so-called “on demand reconnaissance,” shorthand for a spy satellite that can be placed over any country in the world. Compounding the mystery was the launch of a second vehicle in 2011, which stayed in orbit for 469 days, long exceeding the Air Force’s stated maximum requirement of 270 days for the spaceplane.

Now, a third launch is slated for 11 December, according to an Air Force spokesperson, once again ramping up the rumour mill. So, what do we actually know about the plane?

Tactical response

Early reports focused on the X-37B’s seeming resemblance, at least in size and weight, to the X-20 Dynasoar (short for Dynamic Soarer), a 1950s-era hypersonic vehicle that was envisioned for a variety of military missions, including bombing and sabotaging enemy satellites. However, experts familiar with the X-37B programme emphasized that its technology is actually closer to the recently retired space shuttle (a fact reinforced by Boeings’ proposal for a crewed version of the vehicle known as the X-37C). The Air Force blandly described the role of the X-37B in a factsheet given to media as a "reliable, reusable, unmanned space test platform”.

The Air Force also says the mini-shuttle has two objectives: testing “reusable spacecraft technologies” and conducting “experiments which can be returned to, and examined, on Earth”. Again, this is similar to the stated aims of the space shuttle. But many forget that earlier craft also had a secret military role. Although ostensibly a civilian program, it conducted a series of missions from 1982-1992 on behalf of the National Reconnaissance Office, carrying a series of classified spy satellites.

Similarly, most outside experts now agree that it’s likely the robotic space plane is being used for some sort of secret reconnaissance. “I think the guess that makes most sense is quick-response tactical imaging, meaning hours to a couple of days from request to delivery,” says Allen Thomson, a former CIA analyst.

Thomson says it is also possible that it could have a more mundane but useful task, such as “maintaining up-to-date general purpose mapping imagery.” However, if that is the case, Thomson says that it could be a waste of money. “I think that the commercial satellites could and should do that cheaper and better than X-37B,” he says. It is a view backed by parts of the scientific community.

Indeed, the X-37B launches comes in the middle of a larger debate about the role of government-operated spy satellites, which have proven enormously costly but can provide some of the most advanced imagery, versus commercial satellite imagery. The US intelligence community recently slashed its budget for commercial imagery, indicating that it was going back to greater reliance on its own classified satellites.

The view that the X-37B is a reconnaissance platform is strengthened by observations from amateur satellite watchers, who track the vehicles’ orbits, and noticed that it has similar orbits to spy satellites and scientific remote sensing craft. In addition, they noticed the craft changing its orbital path several times during its test flights.

This is to be expected, says Joan Johnson-Freese, professor of national security studies at the US Naval War College in Newport. “The upcoming launch will continue just to see what the vehicle can do,” she says. “One of the things they are testing is maneuverability. The problem with satellites in orbit is they are very predictably in certain orbits at certain times, and thus vulnerable to anti-satellite weapons (ASAT).”

Johnson-Freese says the military has long been interested in the ability of a spacecraft that has “the ability to evade, to maneuver, to not be in a predicable place at a predicable time.”

She expects the Air Force will be “pressing the envelope for manoeuvrability and duration” during the X-37B’s next flight. “That will give them the idea of potential missions where avoidance of ASATs comes into play.”

‘No weapons’

Although its orbits may be difficult to predict in advance, its tracks show where the X-37B has been and its likely purpose, says Brian Weeden, a technical advisor at the Secure World Foundation, a Washington-based foundation that focuses on space issues.

For example, he says, the X-37B flew at inclination of 42.79 degrees, which tells you how far north and south in latitude the spacecraft can see. “The tradeoff is that something at a 90 degree polar orbit covers the whole world, but its frequency is less; it may arrive only every couple of days,” he says. “If something is 40 or 45 degrees, it would be covering a smaller portion [of the earth], but more often. “

At a 42.79 degree inclination, the X-37B would be useful for looking at a geographic region such as the Middle East, says Weeden, pouring cold water on one theory that that it was used to spy on China’s spacelab, Tiangong-1. And given the current political context, he says, the Middle East “makes sense”.

Weeden also suggests the X-37B’s orbit may indicate that the military is trying out a new sensor system, such as radar imaging or hyperspectral sensors, which collect information across different wavelengths. He suggests this could be the case, because unlike satellites collecting light in the visible wavelength, the X-37B’s orbits are not synchronized with the sun, a trick used to maintain a predictable angle between the sun, satellite and ground.

But, like with many of the theories surrounding the X-37B, he warns, “it is just speculation.”

For its part, the Air Force itself is silent about the plane’s use, only speaking to allay fears that it was a weapon. "I don't know how this could be called weaponisation of space. It's just an updated version of the space shuttle type of activities in space," said Gary Payton, the Air Force's deputy undersecretary for space programmes, in 2010. "We, the Air Force, have a suite of military missions in space and this new vehicle could potentially help us do those missions better."

Whatever its purpose; the X-37B is perhaps one of the few bright spots among the Pentagon’s hypersonic test vehicle programs. The Air Force’s X-51A Waverider, a scramjet powered hypersonic missile, suffered a fatal mishap earlier this year on a test flight, and never reached hypersonic speeds. Separately, another hypersonic prototype, known as the Falcon Hypersonic Test Vehicle-2, suffered mishaps in both flight tests, plunging into the ocean.

Those other efforts, which are focused on creating missiles, are still in early testing phase, while the X-37B is clearly further along.

“I think the one interesting question is whether this is just test and evaluation, or is it being used to be support real world operations,” says Weeden. But like most questions about the plane, it is one that is currently impossible to answer.

If you would like to comment on this story, head over to our Facebook page or message us on Twitter.