With a few minor exceptions, the Cluster II spacecraft are identical to their predecessors. Although all of them are outwardly similar in appearance, the present-day Phoenix (FM 5) is built around the first spacecraft structure ever manufactured during the original Cluster programme back in 1992. Phoenix also differs slightly from its companions by having the original Cluster analogue transponder and signal amplifier. However, to all intents and purposes, Phoenix can be considered to be a new spacecraft.

Graphic: Astronomy Now/ESA

The other three Cluster II spacecraft are identical, although even they differ slightly from the original satellites. Significant modifications made to the overall design include the addition of a solid state data recorder with a larger memory; two new computers, a new high power digital transponder, and experiment booms which have been slightly shortened to fit inside the protective fairing on the Soyuz rocket. Various components which are no longer manufactured have also been replaced.

The same applies to the scientific payload. Under the original revival plan, Phoenix was to have carried experiments leftover from the first Cluster mission. However, following the decision that, since it was necessary to make three new units for each experiment, it would be just as easy and cost-effective to make four, most of its science instruments have now been completely rebuilt.

The spacecraft have been assembled by hand from thousands of individual parts. Built into each 550 kg structure are six propellant tanks, two pressure tanks, eight thrusters, one main engine, 80 metres of pipework, about 5 km of wiring, 380 connectors and more than 14,000 electrical contacts.

Each drum-shaped Cluster II spacecraft is 1.3 metres high and 2.9 metres in diameter and weighs in at approximately 1.2 tonnes, of which more than half is accounted for by propellant. This enormous supply of fuel is necessary to enable the spacecraft to reach their operational orbits and then to adjust their separation during the two-year mission.

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