The US Air Force's second mysterious mini-space shuttle, the X-37B, could be spying on China's space laboratory and the first piece of its space station, Tiangong-1.

Amateur space trackers told the British Interplanetary Society publication Spaceflight that the black-funded spaceplane seemed to be orbiting the Earth in tandem with Tiangong_1, or the Heavenly Palace, leading the magazine to speculate that its unknown mission is to spy on it.

"Space-to-space surveillance is a whole new ball game made possible by a finessed group of sensors and sensor suites, which we think the X-37B may be using to maintain a close watch on China's nascent space station," Spaceflight editor Dr David Baker told the BBC.

America has refused to come clean on exactly what the X-37Bs are meant to be doing up there, but the line on the plane has always been that it's a test prototype of a reusable spacecraft that can carry experiments into and back from space.

According to the Air Force's fact sheet on the project, which was funded from the classified budget:

The X-37B Orbital Test Vehicle is the newest and most advanced re-entry spacecraft. Based on NASA's X-37 design, the unmanned OTV is designed for vertical launch to low Earth orbit altitudes where it can perform long duration space technology experimentation and testing. Upon command from the ground, the OTV autonomously re-enters the atmosphere, descends and lands horizontally on a runway.

As well as being winged for re-entry, the OTV also has a strong heat shield, both perfectly in tune with the (vaguely) stated purpose of the craft.

However since people don't even know how much the things cost, exactly where either one was headed and with what payload when they took off in April 2010 and March 2011 respectively, or for how long they're supposed to be up there, it's assumed the spacecraft have naturally been assigned some sort of clandestine military purpose.

The doomsayers in the Iranian media dubbed the X-37B a "secret space warplane" before the first one went up, adding that it was the first generation of US space Predator drones that would build up the US' space armada.

Slightly less alarmist is the hypothesis suggested by El Reg and others that the mini-space shuttle's purpose has something to do with spy satellites. That could be a spot of sat-napping - grabbing or disabling other countries' satellites while leaving the owners to assume some sort of space accident. Or it could be picking up their own eye-poppingly expensive sats for repair or recycling back at home.

This newest theory, that it might be spying on Tiangong-1, is also a possibility, but it does have a couple of holes in it.

First off, the Chinese spacelab wasn't launched until September 2011, some time after the second X-37B hit Earth's orbit. Of course, the USAF's gizmo is supposedly reprogrammable from the ground so they could conceivably have sent it off in search for the lab once the Chinese got their gear up, but that doesn't explain what it was doing before that.

Secondly, you'd have to wonder why it's worth spying on the Tiangong-1. The lab is unmanned for the moment, so all there'd be to study is the technology of the craft and what experiments it's doing. Still, the US is hugely suspicious of China's space endeavours, so it's more than possible that they'd want to get a look at Tiangong-1 just in case it's doing anything unexpected.

Brian Weedon, a technical adviser to the Secure World Foundation and former orbital analyst with the USAF, told the BBC he thinks that the X-37B is spying on the Middle East and Afghanistan, possibly with new kinds of sensors.

"A typical spy satellite is in a polar orbit, which gives you access to the whole Earth," he said.

"The X-37B is in a much lower inclination which means it can only see a very narrow band of latitudes, and the only thing that's of real interest in that band is the Middle East and Afghanistan.

"Is it spying on Tiangong-1? I really don't think so. I think the fact that their orbits intersect every now and again - that's just a co-incidence. If the US really wanted to observe Tiangong, it has enough assets to do that without using X-37B," he added.

Wilder theories have also reared their heads, such as that both Tiangong-1 and the second X-37B spotted "something else" in space and went to have a look at it - but that seems a little bit like wishful thinking from ET-loving dreamers.

The latest edition of Spaceflight, complete with the X-37B hypothesis, will be published this weekend. ®