Ten years flying in formation: The legendary Cluster quartet

On 1 September 2000, just a few weeks after launch, the four individual satellites of the Cluster mission began coordinated orbits, marking the formal start of formation flying.

Since then, the four satellites -- dubbed Samba, Tango, Rumba and Salsa -- have gone on to collect some of the most detailed data ever on the physical properties of space between Earth and the Sun, and on the interactions between the charged particles of the solar wind and Earth's atmosphere. In all, over 2.6 terabytes of data -- enough to fill 3300 CDROMS -- have been delivered from space.

The Cluster mission was lofted into orbit in two dual launches on Soyuz boosters from Baikonur Cosmodrome, Kazakhstan, on 16 July and 9 August 2000.

Cluster provides a decade of results

Cluster's results have enabled scientists to construct a three-dimensional model of the magnetosphere and to understand better the processes taking place inside it. Since 2000, over 1200 scientific papers have been published in hundreds of journals using Cluster data, making an enormous contribution to our understanding of the Solar System.

Throughout the decade, one critical requirement has been the need for continuous flight control, which has been managed by the Cluster Mission Operations team at ESA's European Space Operations Centre (ESOC).

Ten years of mission control expertise

Supported by flight dynamics specialists, mission planning engineers and ground station controllers, Cluster operations engineers have been responsible for retrieving the scientific data produced by the four spacecraft, regularly sending up commands to operate the spacecraft and their scientific instruments and manoeuvring the spacecraft in their precise orbits.

Twice per year on average, the formation has undergone a major realignment, with inter-spacecraft distances being adjusted by tens of thousands of kilometres through a series of thruster burns. This provides the precise changes in positioning required by the science investigators who manage the 11 instruments mounted on each satellite.

Orbiting just 17 kilometres apart

At one point, in June 2007, Samba and Tango were orbiting just 17 km apart -- the closest ever that two ESA spacecraft have achieved in routine operations. To date, the satellites have been commanded through 771 manoeuvres -- an enormous number in spacecraft navigation terms.

"For the past decade, mission operations have succeeded through a combination of skill, initiative, teamwork and intense effort. Everyone on the team can be proud of a significant scientific and technical achievement," says Juergen Volpp, Cluster Spacecraft Operations Manager.

Like any 'old-timers', the Cluster satellites are showing wear after years of continuous exposure to the harsh radiation environment around Earth. However, with the exceptions of the batteries, all major systems are still functioning well. Engineers have devised ingenious work-arounds to account for battery issues and the normal reduction in electrical power as the solar panels degrade.

Cluster's return on investment

The Cluster mission, originally planned for 27 months, is now operating until the end of 2010 and is due to be extended until December 2012, with an option to go on until 2014.

"We look forward to several more years of operational effectiveness for the mission, which will provide even more scientific results. Cluster is providing some of the best returns on investment ever seen in an ESA mission," says Volpp.