Kosmos 2491, 2499, 2504 (RS 46, 47) / 14F153

The Kosmos 2491 and 2499 satellites are secretive Russian satellites, which performed apparently orbital rendezvous and inspection manoeuvres.

The exact nature of the payloads remains undisclosed, but observer orbital manoeuvres indicate, that the carried out proximity operations with the Briz-KM upper stage of the launch vehicle. Both payloads were initially not announced during the launch, which led to confusion over the Kosmos numbers of subsequent launches.

The satellites are similar in size and weight to the Yubileyniy satellite, as they share the same payload position during launch with the Strela-3M main payloads on the Rokot-KM launch vehicle. The satellites have been rumored to have ion engines, but the nature of the orbital manoeuvres hints to conventional chemical thrusters.

Both satellites carried also an amateur radio payload. It is called RS-46 on Kosmos-2491 and RS-47 on Kosmos-2499.

Kosmos 2504 launched in March 2015 is believed to be a third satellite in this series. In April, it also began maneuvering.

In January 2020, Kosmos 2491 suffered a break-up event, possibly originating from residual fuel in the propulsion system.