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.

The inspection-sat launched from the Plesetsk Cosmodrome north of Moscow back in November. Riding atop a Soyuz rocket, Cosmos 2542 settled into orbit between 250 miles and 550 miles over Earth’s surface.

“The purpose of the experiment is to continue work on assessing the technical condition of domestic satellites,” the Russian defense ministry stated.

But it was apparent early on that an American satellite that trackers call USA 245 was the real target. Cosmos 2542’s original orbit allowed it to pass within a few hundred miles of the KH-11 every 11 or 12 days, noted Michael Thompson, an American graduate student who moonlights with a small space company and, in his spare time, tracks satellites.

It’s surprisingly easy to do. Amateur sat-trackers all over the world use telescopes and government data to keep track of many of the world’s roughly 2,200 active satellites, more than half of which are in low orbit between 100 and 1,200 miles above Earth.

Between November and January, Cosmos 2542 mostly stayed a respectful distance from the KH-11 as the U.S. National Reconnaissance Office satellite went about its business snapping high-resolution photographs of America’s rivals.

Then in mid-January, Cosmos 2542 passed close to the spy satellite—and made its move. Instead of drifting away like it usually did, Cosmos 2542 performed a series of maneuvers between Jan. 20 and Jan. 23 and essentially matched orbits with USA 245.

Sat-tracker Nico Janssen noticed the maneuver and fed data to Thompson, who performed his own analysis, then started tweeting.

“Cosmos 2542 is loitering around USA 245 in consistent view,” Thompson tweeted Thursday. “As I’m typing this, that offset distance shifts between 150 and 300 kilometers [93 and 186 miles] depending on the location in the orbit.”

At that range, Cosmos 2542 can probably take pretty detailed photos of KH-11. “The relative orbit is actually pretty cleverly designed,” Thompson tweeted. “Cosmos 2542 can observe one side of the KH-11 when both satellites first come into sunlight, and by the time they enter eclipse, it has migrated to the other side.”

It’s not clear how much Russia can learn from photographing the KH-11. “Personally, I think the intelligence value of observations of optical spy satellites like this one are probably marginal,” Thompson tweeted.

The NRO reportedly operates four KH-11s. They traditionally maintain orbits that dip as low as 160 miles and climb as high as 620 miles, allowing the satellites to modulate between viewing huge swaths of Earth at low resolution and much smaller sections of the planet at high resolution.

By coordinating the orbits of the KH-11s, the NRO can maintain simultaneous wide and narrow surveillance. This is obvious to the world’s satellite-trackers. The NRO never comments on the KH-11s’ operations, but the shipping-container-size sats aren’t hard to see from the ground.

And since it’s public knowledge that the KH-11s use the same kind of lens that forms the basis of NASA’s Hubble space telescope, anyone with expertise in optics can estimate the KH-11s’ capabilities.

Spying on the spy satellite might not be the point. Russia has deployed several mysterious inspection satellites since 2014. China, Japan, Sweden, and the United States have launched their own inspection craft. The U.S. Air Force’s mysterious X-37B robotic mini-shuttle might be the most famous spacecraft with inspection capabilities.

All of these craft have had potential as weapons.

“You can probably equip them with lasers, maybe put some explosives on them,” Anatoly Zak, an independent expert on Russian spacecraft, told The Daily Beast in 2015. “If [one] comes very close to some military satellite, it probably can do some harm.”

In maneuvering Cosmos 2542 to closely tail an American spy satellite, Russia could be practicing for war.

KH-11s aren’t known to possess any defensive systems. Not that they would be useful during peacetime. Thompson said there’s not much the NRO’s satellite-operators can do about the Russian interloper “besides grumble at the U.N.”