First off, the Chinese spacelab wasn't launched until September 2011, some time after the second X-37B hit Earth's orbit. 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."
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