An artist's impression of the Cluster quartet - Rumba, Salsa, Samba and Tango. Photo: Dornier Satelliten Systeme GmbH

Some 100 scientists and engineers gathered last week at the European Space Technology Research Centre (ESTEC) in the Netherlands for the 36th Cluster Science Working Team (SWT) meeting. They were greeted with the news that the four Cluster spacecraft - Rumba, Salsa, Samba and Tango - are now dancing in a new formation around the Earth.

At the start of the two-day gathering, Cluster deputy project manager Alberto Gianolio told the SWT that the recent orbital manoeuvres had been extremely successful. With the quartet now flying some 2000 km apart in a tetrahedron formation, the spacecraft are ready to begin their first observations of the Earth's huge magnetic tail or magnetotail.

Further details were provided by Cluster spacecraft operations manager Sandro Matussi, who explained that the complex sequence of engine firings had been completed with great precision. The manoeuvres had been carried out so meticulously that only two of the four spacecraft orbits had to be altered on 3 June, the final day of the four-week phase to set up the new orbital configuration.

"Rumba and Salsa had been moved so precisely during previous manoeuvres that there was no need to fire their thrusters," said Matussi. "Even Samba and Tango required only very small drift manoeuvres."

"Our original plan was designed to cover possible errors in the manoeuvres," he explained. "However, the excellent performance of the spacecraft and the very thorough mathematical calculations of our flight dynamics colleagues at the European Space Operations Centre in Darmstadt, Germany, meant that we only had to make 23 of the 28 scheduled engine firings."

Even as the Cluster quartet were beginning their latest celestial dance, the 11 Cluster principal investigators met to discuss their preferences for the next mission phase that will start in February 2002. They eventually agreed to close the gap between each spacecraft to just 100 km - an intimacy unrivalled by any flotilla of scientific spacecraft during more than four decades of space exploration.

"By bringing the spacecraft so close together, we will be able to study the key regions of the magnetic bubble that surrounds the Earth - the magnetosphere - in unprecedented detail," said Cluster project scientist Philippe Escoubet. "This will probably be our only opportunity to obtain small-scale, three-dimensional measurements of the polar cusps, the bow shock and the magnetopause."

"The observations will revolutionise our understanding of the physical processes taking place where the electrically charged particles in the supersonic solar wind collide with the Earth's magnetic field," he said.

This schematic diagram illustrates Cluster's orbit at the beginning of June 2001, after orbital manoeuvres. (The view is of the magnetosphere from above the North pole.) The interspacecraft distance is about 2000 km and the tetrahedron is located at apogee. The Cluster orbit is indicated in red. Photo: ESA

Meanwhile, the global community of more than 200 Cluster scientists has begun to analyse the avalanche of results from the first six months of Cluster science operations. The SWT was given a foretaste of some of the papers to be published later in the year in a special edition of the journal Annales Geophysicae. They included remarkable new studies of radio waves generated above the Earth, never-before-seen fluctuations in the magnetic field, and particle motions in the inner magnetosphere.

"Since four identical spacecraft have never before examined near-Earth space, the data from Cluster are continually breaking new ground," said Dr. Escoubet. "The analysis is very complex but also extremely exciting."

"We have completed the first chapter of the Cluster adventure, but the rest of the story promises to be even more compelling," he added.