This may not seem a particularly noteworthy event, but since the four Cluster II satellites resemble gigantic flying fuel tanks, filling them with 650 kg of toxic propellant is rather more prolonged and hazardous than queuing up at the local petrol station.

The first step in the process was to pressurise the spacecraft's helium tanks. In flight, this inert gas will be required to force the oxidiser and fuel into the combustion chamber of the main engine and the eight thrusters on each satellite.

Pressurisation was followed on 6 June by the start of the crucial filling operations. The entire procedure was undertaken in the Hazardous Processing Facility (HPF), part of the Starsem Payload Facilities at Baikonur Cosmodrome in Kazakhstan.

This delicate operation was monitored and remotely controlled via intercom and TV screens by specialist engineers in a separate control room. Only two technicians wearing special protective 'scape' suits were allowed inside the HPF clean room, while the fire brigade remained on standby outside.

After the fuelling got under way, everyone hoped and prayed that no difficulties would occur. Once the operations begin, it is very difficult to call a halt and almost impossible to go back and start again, since the spacecraft are then considered as hazardous items and any unforeseen activity would require a major review by the safety authorities.

Fuelling is a two-stage operation. On 6 June, FM 6 was loaded with 405 kg of nitrogen tetroxide (NTO), followed by FM 7 the next day. After the empty oxidiser drums were removed and exchanged for those containing the fuel, two more days were required to fill the tanks of both spacecraft with 245 kg of monomethylhydrazine (MMH). In both cases, the flow and total quantity of propellants being transferred were carefully controlled by weighing the spacecraft.

By 10 June, the fully fuelled satellites were ready to be transported to another clean room for the start of their combined operations with the Fregat upper stage.

The next pair of Cluster II satellites will take their turn in the space garage shortly, with fuelling of the last spacecraft (FM 5) expected to be completed on 22 June.

The Reaction Control System of each 1.2 tonne Cluster II spacecraft consists of six titanium propellant tanks -- three for the nitrogen tetroxide oxidiser and three to hold the MMH fuel -- plus two pressure tanks, eight 10 N thrusters, one 400 N main engine and 80 metres of piping.

The fuel and the oxidiser that they carry accounts for more than half the launch weight of each spacecraft (650 kg out of a total launch mass of 1200 kg).

Most of this liquid propellant will be consumed by the main engine in the first orbits after launch during the complex manoeuvres required to reach operational orbit. A substantial amount will also be required by the thrusters that will alter the orbital separation of the four spacecraft as they fly in formation above the Earth's polar regions.