Three top-secret satellites launched by Russia between 2013 and 2015 have recently been reactivated, and appear to be carrying out missions to rendezvous with other man-made objects in orbit, performing comparatively dramatic maneuvers to make their intercepts. Despite assurances from the Russian government that these vehicles are benign, there is speculation that these satellites might be anti-satellite weapons, designed to maneuver close to another satellite and destroy it with an onboard weapon.

Because there are a vast number of variables affecting the orbit of a spacecraft that can’t be anticipated prior to launch, many long-term satellites are equipped with basic maneuvering thrusters that can help correct their orbits when factors such as atmospheric drag or uneven heating cause them to drift slightly off course. These thrusters, however, are only used for comparatively minute adjustments, and are not typically capable of the major course changes that Kosmos 2491, 2499 and 2504 have been making.

The first of these satellites, Kosmos 2491, was launched on December 25, 2013, along with three other ordinary communications satellites (Kosmos 2488, 2489 and 2490), possibly to hide 2491’s launch from foreign observers. While the three communications satellites were publicly announced by Russia, Kosmos 2491 itself went unmentioned until foreign powers reported tracking its movements. The other two maneuverable satellites, Kosmos 2499 and 2504, were launched with the same configuration, each accompanying three regular communications satellites during their launches on May 23, 2014, and March 31, 2015, respectively.

Originally designated as space debris, Kosmos 2499 was tracked making maneuvers between May 29 and May 31, 2014. On June 24, the satellite adjusted the height of its own orbit — effectively changing its speed to allow for a July rendezvous with the derelict rocket booster that originally brought it into orbit. Kosmos 2499 eventually came to within a kilometer (0.6 miles) of its target.

Kosmos 2504 performed a similar maneuver, coming to within a mile of its own spent booster in April 2015. On April 16, the booster was tracked as climbing to a slightly higher orbit, possibly pushed higher by physical contact with Kosmos 2504 itself. After changing orbit once again, the satellite went silent for nearly two years, until April 2017, when it changed course again to intercept a wrecked component of the Chinese Fengyun-1C weather satellite, destroyed in 2007 during a test of China’s anti-satellite missile system.

Despite the shroud of secrecy that these satellites are hidden under, we can safely say that they are at the very least a testbed for an experimental form of ion engine, much like one of the experiments performed aboard the USAF’s top-secret X-37B spaceplane: In the same month that Kosmos 2499 was launched, an article on the Moscow Institute of Physics and Technology’s website congratulated the Keldysh Research Center for the successful launch of a JSC Information Satellite Systems-developed spacecraft that utilizes their reaction control thrusters, "based on the new generation of Hall type plasma engines". While the article "Plasma engines of the new generation have successfully started a regular work in space [sic]" doesn’t specifically name the spacecraft involved, the date of the article’s publication coinciding with the launch of a satellite that would later demonstrate exceptional maneuverability is too close a coincidence to ignore.

Maneuverable satellites are not a new development, with reconnaissance satellites having made use of rocket motors to adjust their orbits to take them over their intended targets for decades. In December 2014, Oleg Ostapenko, then-head of Russia’s space agency Roscosmos, said that these satellites were launched for peaceful purposes, and not built to be anti-satellite weapons. However, the secrecy surrounding Kosmos 2491, 2499 and 2504, along with the apparent ability to come within orbital spitting distance of other objects, isn’t helping ease fears that this might be a new anti-satellite weapon. Weighing only 50 kilograms (110 lbs), it’s not likely that they’re carrying any weapons, and there are more practical ways to knock out an enemy’s satellites.

"In most cases, it’s far easier to jam a satellite’s communications or hit it with a missile than try and do some sort of destructive co-orbital rendezvous," explains the Secure World Foundation’s Brian Weeden. Regardless, the sheer maneuverability of such satellites can still be important, because "the capability to do rendezvous and proximity operations … has a whole bunch of applications — civil, commercial and military."

For readers that are curious, the positions of Kosmos 2491, Kosmos 2499 and Kosmos 2504, and their companion communications satellites can be seen at stuffin.space. Note that the orbits are not in real time, but are calculated using daily updates from SpaceTrack.org. — so they may — or may no longer be — in the orbits indicated.