In this artist's impression, two units of the Cluster II flotilla are seen piggy-backed atop the Soyuz and Fregat boosters as the rocket's payload fairing separates. Photo: ESA

The four Cluster II satellites will be lifted into orbit, in pairs, on board Soyuz rockets provided by the Russian-French Starsem company. For this mission, the old Soviet era workhorse has been upgraded with the addition of a newly developed Fregat restartable upper stage.

The Soyuz will place the upper stage and its Cluster II payload into a preliminary orbit inclined at 64.8° to the equator. Then, 8 minutes 48 seconds after lift off, the Fregat payload assist module and its two spacecraft will separate from the booster. The Fregat main engine will fire almost immediately to achieve a circular orbit of approximately 200 km altitude. About one hour later, the engine will fire again to inject the spacecraft into a 200 km x 18,000 km elliptical orbit.

The two satellites will then be released, one after the other. Using their own onboard propulsion systems, they will gradually change their orbital inclination from 64.8° to 90°, eventually reaching the final operational orbit with an apogee of 119,000 km and a perigee of 19,000 km. They will be joined a few weeks later by the second pair of satellites. If all goes well, the science programme should begin in earnest around October.

< Anatomy of a Cluster II spacecraft

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