Enlarge / A Russian Rokot launcher is seen on April 25, 2018, at the Plesetsk Cosmodrome in northern Russia. (credit: Stephane Corvaja/ESA via Getty Images)

On Christmas Day, 2013, the relatively small Russian Rokot rocket launched from the Plesetsk site in the northern part of the country. The mission carried three small military communications satellites, but observers noted that the mission appeared to eject a fourth object into orbit.

A few months later Russia confirmed that this object was a satellite, and it came to be known as Cosmos 2491. To the surprise of many sky watchers, this satellite then began to perform novel orbital maneuvers, such as raising and lowering its orbit, that demonstrated rendezvous and proximity operations.

Then it happened again. In May 2014 another Rokot booster carried three communications satellites into orbit as well as a fourth object, which was designated Cosmos 2499. Finally, this happened a third time in April 2015, with a third mystery satellite known as Cosmos 2504.