A Russian satellite has positioned itself uncomfortably close to an American spy satellite in orbit around Earth, leading space trackers to speculate that the foreign vehicle is doing some spying of its own.

The Russian spacecraft is meant to inspect other satellites, and experts in the space community believe it may now be keeping a watchful eye on the secretive US vehicle. But the motivation behind this in-space stalking is still unknown.

All January, amateur satellite trackers have been keeping tabs on the weird behavior of this Russian probe, known as Kosmos 2542. Launched in November of last year, Kosmos 2542 has been orbiting in the same plane as a satellite operated by the National Reconnaissance Office called USA 245, which has been in space since 2013. Now, Kosmos 2542 is nearby USA 245 all the time

The NRO is a military agency that specializes in surveillance and operates a large swath of classified satellites that are thought to spy on places all over the world — so it’s entirely possible USA 245 is doing something the US might want to keep secret. The fact that the two satellites are in the same plane isn’t enough to raise alarm though, as the satellites only passed close by each other every 10 days or so.

“[It] is suspicious, but doesn’t prove anything, as there are a lot of different satellites in that plane,” Michael Thompson, a graduate teaching assistant at Purdue University specializing in satellites and astrodynamics, writes in an email to The Verge.

Kosmos 2542 attracted particular attention last week when it performed a series of maneuvers, using its onboard thrusters to get closer to USA 245. Now, Kosmos 2542 is nearby USA 245 all the time. Thompson writes that the Russian satellite has been in constant view of its US target for nearly two weeks now. The two satellites range between 150 to 300 kilometers apart at any given time, which essentially makes them neighbors in the vast area of low Earth orbit. Kosmos 2542 is slowly drifting away, but it will be within a direct line of sight of USA 245 for weeks or even months, according to Thompson.

Thompson revealed all this info in a Twitter thread, suggesting that Kosmos 2542 is doing some inspection of one of America’s assets. And it’s not a completely unexpected conclusion to make. Before Russia launched the satellite, the Russian Ministry of Defense claimed that the spacecraft was, indeed, designed to inspect other satellites in space, according to the site Russian Space Web, which follows Russia’s space industry. Most assumed that it would be inspecting other Russian spacecraft, not classified spy satellites operated by the US.

Of course, we ultimately don’t know the real reason Kosmos 2542 made those maneuvers. But most experts say there’s really only one good explanation: one satellite is stalking the other.“Right now it’s the most likely explanation we have.”

“The conclusion that it’s shadowing the NRO satellite is speculation but one that’s informed by the orbital data,” Brian Weeden, director of program planning for the Secure World Foundation, writes in an email to The Verge. “Right now it’s the most likely explanation we have for why the Russian satellite has been maneuvering the way it has and why it’s in that orbit.”

It’s not clear what happens next. Chances are probably good no harm is going to come to the US satellite, since Russia claims its probe is just meant for inspection. However, concerns have been raised about what satellites could do to one another in space if they got close enough. The Defense Department has sounded the alarm about satellites ramming into other satellites, spraying them with chemicals, or shooting them with lasers in order to destroy them. That kind of in-space warfare hasn’t quite happened yet, but it’s certainly on the radar of the US government.

Plus, there isn’t a set protocol about what to do when another country’s satellite gets too friendly. “One of the big concerns is that we don’t have any agreed rules or norms about how these close approaches should be done,” says Weeden. “That means an increased risk someone might get the wrong perception about what’s going on, perhaps even mistaking it for an attack.”“we don’t have any agreed rules or norms about how these close approaches should be done.”

It’s not as if this kind of behavior is completely novel, though. Weeden noted that both Russia and China have done close inspections of their own satellites in the past. And in 2015, a Russian satellite known as Luch put itself next to two US communications satellites operated by the company Intelsat, and stayed there for five months before moving. On the flip side, the US is also guilty of this practice, says Weeden. The American military operates a series of satellites through its Geosynchronous Space Situational Awareness Program (GSSAP), which are tasked with approaching and checking out satellites operated by other countries.

For now, Thompson says he’ll be continuing to monitor the whereabouts of Kosmos 2542. The details of the satellite’s path are available on space-track.org, a website that publishes tracking data collected by the US Air Force on as many satellites and pieces of debris in orbit as possible. He notes that he’s definitely not the only one watching, either. “Since the orbits for these Russian satellites are public info, anyone who wants to can watch for it, and I know that plenty of people in our community are,” says Thompson.