PARIS — Three European Space Agency (ESA) satellites designed to study the Earth’s magnetic field will be launched together on a Russian Rockot vehicle operated by the German-Russian Eurockot Launch Services company under a contract signed April 9, Eurockot and ESA announced. The organizations also signed a separate contract for the launch of a yet-undetermined ESA Earth observation or science satellite.

Bremen, Germany-based Eurockot will design an adaptor to separate the three Swarm spacecraft from the Rockot vehicle’s upper stage in a launch now planned in mid-2012. One satellite will operate in an orbit of about 530 kilometers in altitude at the start of its service. The other two will operate at around 490 kilometers.

The Swarm satellites, under construction by Astrium Satellites of Germany under a contract valued at 86 million euros ($116 million), had been set for a mid-2011 launch before the launch contract was signed.

Danilo Muzi, an ESA Earth observation project manager, said April 9 that it appears Astrium would be able to deliver the satellites in the second half of 2011, but the construction and testing of the Swarm payload dispenser by Eurockot will push the launch into 2012.

Peter Freeborn, Eurockot’s marketing manager, said Eurockot has openings before 2012 but the dispenser, and the fact that the ESA contract was not signed earlier, has resulted in a delayed launch date. Under the contracts signed April 9, Eurockot committed to giving ESA two launch slots between December 2011 and November 2012.

Rockot, a converted SS-19 ballistic missile, is launched from the Plesetsk Cosmodrome in northern Russia.

Peter B. de Selding was the Paris bureau chief for SpaceNews.