 months, Air Force officials have said. [Infographic: The X-37B Space Plane]
What exactly the vehicle does while circling the Earth for so long is a mystery, since the craft's payloads and missions are classified. Partly as a result of the secrecy, some concern has been raised — particularly by Russia and China — that the X-37B is a space weapon of some sort.
But the Air Force has repeatedly denied that charge, claiming that the X-37B's chief task is testing out new hardware for future satellites — instruments like sensors and guidance, control and navigation systems. And that's likely to be the case, experts say.
"It gives the Air Force the ability to test-fly some of this hardware," said Brian Weeden, a former Air Force orbital analyst who works as a technical adviser for the nonprofit Secure World Foundation.
Weeden suspects the X-37B is testing gear for the National Reconnaissance Office, the intelligence agency that builds and operates the U.S.'s spy satellites. That would explain all the secrecy, he said.
Second mission for the X-37B
The Air Force's other X-37B, known as OTV-1, launched last April and returned in December after spending 224 days in space. While its mission was also classified, technology-testing was OTV-1's primary job, too, Air Force officials have said.
And things presumably went well, experts say, or the Air Force wouldn't be launching the craft's twin a few short months later.
While the X-37B is likely trying out new hardware, the vehicle itself is experimental — hence the "X" designation — so these flights should also help the Air Force assess the space plane as well as its payload.
"Part of its mission is to test out reusable technologies and to see how quickly they can turn around these vehicles and launch them again," Weeden said.
Boeing's Space and Intelligence Systems division builds the X-37B for the Air Force. Originally, NASA used the space plane as an experimental test bed until funding for the project ran out in 2004.
The vehicle then passed to the Defense Advanced Research Projects Agency and was ultimately turned over to the Air Force in 2006.
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