A mysterious Russian spacecraft has maneuvered into a new orbit around Earth right behind a secret U.S. spy satellite.

The unusual move by Russian Cosmos 2542 on Jan. 20 allows it to closely watch the American KH-11, a $4 billion orbital telescope staring down at Earth. And there's not much that U.S. space operators can do about it.

For the Americans, getting tailed by the Russians in peacetime is annoying. During wartime, it could be a prelude to an attack.

Cosmos 2542 is what space operators call an "inspection satellite." Fitted with sensors and thrusters, the mini-fridge-size satellite can maneuver close to other spacecraft and scan them. Some inspection-sats could double as weapons, tampering with or even destroying enemy spacecraft.