Moving time has arrived for the four Cluster spacecraft. During the next few weeks, the satellites will carry out a series of maneuvers that will increase their separation distances. Eventually, each spacecraft will have drifted to a distance of 1,245 miles (2,000 kilometers) from its partners, compared to the current 375 miles (600 kilometers).

The first engine firings took place on the evening of 10 May, starting with spacecraft 3 (Samba) and progressing through spacecraft 4 (Tango), 2 (Salsa) and 1 (Rumba). Further adjustments of each spacecraft will take place until the heavenly dance reaches its finale on June 3.

"We will be using only 5 to 10 percent of the fuel reserves on each spacecraft during these maneuvers," said Cluster project scientist Philippe Escoubet. "This leaves plenty to spare for future readjustments of the four spacecraft."

"The maneuvers now being undertaken will enable us to rearrange the spacecraft so that the tetrahedron formation will be shifted to the Earth's magnetic tail region," he added. "In the huge tail, we will be studying much larger structures, such as magnetic waves and particle acceleration, so the spacecraft have to be moved farther apart."

The tail observations start in June and last for six months, he said. "This will be the first time that this region has been studied simultaneously by four identical spacecraft. By August, the spacecraft [cluster] apogee -- its farthest distance from the Earth -- will be in the center of the magnetic tail at a distance of 120 000 kilometers (74,568 miles) from Earth. This will be a very exciting time for us."

Teamwork is vital if the orbit changes are to be successfully carried out. The current maneuvers would not be possible without advance planning by the flight dynamics team at the European Space Operations Center (ESOC) in Darmstadt, Germany.

The commands to fire the thrusters are uploaded to the computers on each spacecraft one day before a particular maneuver is scheduled to take place. The modified orbits are later checked as the quartet fly within range of the European Space Agency ground stations of Villafranca 1 and 2 in Spain and Perth in Australia.

"Flight dynamics have done a tremendous job in planning the 28 maneuvers, generating the commands and then verifying that everything goes alright," said Sandro Matussi, Cluster spacecraft operations manager at ESOC.

"Engineers are monitoring the situation day and night when the maneuvers are taking place, ready to intervene if necessary. Only eight of the thruster firings take place when the satellites are not 'visible' from Spain or Perth."

During this period of unusual activity, the temporary buildup of gases around the spacecraft means that Cluster payload operations have to be reduced. Some instruments, for example, FGM and WEC, can remain in operation, but others such as ASPOC, CIS, EDI, PEACE and RAPID, which operate at high voltages, have to be switched off or put in a standby mode. This prevents the possibility of electrical arcing that could damage the satellites and their instruments. These instruments are switched on again between eight and 24 hours after each maneuver is completed and the cloud of gases around the spacecraft has dissipated.

This activity has inevitably created extra work for the Joint Science Operations Center (JSOC) at the Rutherford Appleton Laboratory in the United Kingdom, since each of the 42 instruments on Cluster has its own operating schedule.

"Once again, this job has been perfectly carried out with the planning tools available at JSOC," said Philippe Escoubet. "Everyone has put in a great effort to make these orbital maneuvers a success."