In April 1997, ESA committed to re-fly the Cluster mission. The agency used spare parts left from the original project to assemble a replacement spacecraft known as Phoenix and contracted the Daimler-Benz Aerospace-led consortium to build three similar satellites for the Cluster 2 mission.

On August 4, 1998, ESA and the Paris-based Starsem company signed an agreement to launch the Cluster 2 spacecraft in two pairs aboard Soyuz/Fregat rockets during the period May to August 2000. The Cluster mission's science goals require two pairs of spacecraft to take off within 42 days.

The original launch dates for the Cluster 2 mission were set for June 15 and July 13. Recently however, ESA decided to delay the mission to review a potential problem with the spacecraft's attitude control thrusters. The engineers discovered small fuel leaks in the thrusters built by DaimlerChrysler and commonly used in the European spacecraft, including Cluster.

ESA is currently waiting for the results of the vacuum-chamber tests, which would validate the modifications in the thrusters for the Cluster 2 mission. The formal results of the tests, which would give green light to the mission, are expected on Friday.

The July 12 launch date is likely to change, since it coincides with the planned launch of the Proton booster carrying the Zvezda service module, a crucial element of the International Space Station. Although the two missions use different launch facilities in Baikonur, they do share tracking stations.

Patrick Bonguete, deputy managing director of operations at Starsem, said the July launch date might shift by one or two days. "For the time being we are discussing [the issue] with the Russian Space Agency, so maybe we will delay or advance our launch date," Bonguete said.

Two Soyuz launchers for the mission have been delivered to Baikonur on March 5 and April 5. The Fregat upper stages, which will propel the pairs of Cluster spacecraft into highly elliptical orbits, will be shipped to the Kazakh launch site in the second half of May and mid-June.

By July 4, the first pair of Cluster spacecraft will be covered with a protective shroud, and the next day the Cluster/Fregat stack will be integrated with their launch vehicle.

The Soyuz launcher will inject the Cluster/Fregat stack into suborbital trajectory. The first ignition of the Fregat engines will place the spacecraft into a 125-mile (200-kilometer) circular orbit with inclination 64.9 degrees toward the equator.

When the stack reaches the correct position relative to the sun, the Fregat engines will fire again, sending the pair into the highly elliptical orbit with the highest point around 11,185 miles (18,000 kilometers) above the earth's surface. The Fregat and Clusters will separate, one by one, shortly after the second maneuver.

The Clusters will use their own engines to increase the inclination of the orbit up to around 90 degrees and the altitude to 11,800 by 73,945 miles (19,000 by 119,000 kilometers).

The Cluster 2 launch will be the first operational mission for the Soyuz/Fregat booster. The launcher completed its first test launch on February 9. On March 20, Soyuz/Fregat flew exactly the launch profile planned for the Cluster 2 mission.