Russian satellite pursues secret US military satellite.

A Russian satellite, Kosmos-2558, appears to be stalking a classified US military satellite, USA-326, in a cat-and-mouse chase through space. The behavior of Kosmos-2558, and the lack of a formal explanation from Russia, has led space observers to believe that the probe is stalking USA-326. It's at least the third satellite Russia has launched that appears to be an "inspector" — a spacecraft aiming to gather up-close data on another satellite. The US military is tracking the Russian spacecraft, and experts suggest that Russia is experimenting with new space stalking technology.