The Russian spacecraft closed from hundreds of kilometers away, circled the ICEYE satellite, stayed nearby for an extended period, and then began moving away.
Over the weekend the two were within 500 meters of each other and likely much closer, LeoLabs' Adam Marsh said.
As of Monday, Kosmos 2612 was drifting away at about 17 kilometers per hour, and there were no reports of physical damage.
Its radar can see through clouds and darkness.
LeoLabs called the behavior consistent with sophisticated rendezvous and proximity operations, meaning controlled close approaches between spacecraft, and said it was not a routine fly-by.
Astrophysicist Jonathan McDowell and Marsh both said this is the first known low-Earth orbit rendezvous between two active satellites without the owner's approval.
A Russian official said in 2022 that civilian satellites supporting Ukraine's military are a "legitimate target."
ICEYE US, which owns the satellite, said it is tracking the situation and that its constellation is designed for continuity and resilience. The Finnish parent company has launched 80 satellites and has four more scheduled to fly this week on a SpaceX rideshare mission. Its customers also include the US military and several European defense ministries. Retired Lt. Gen. John Shaw, a former deputy commander of US Space Command, called the move a provocation and said the best reading so far is a message that Russia will act aggressively near platforms it sees as threats. US Space Command chief Gen. Stephen Whiting said in April that Russia is operationalizing anti-satellite weapons. Analysts have also tracked Kosmos 2612 since its April launch, when at least four of six satellites from the same Soyuz mission appeared to line up with ICEYE-X36's orbit.
