Cluster II

The current mission Cluster II is investigating the small-scale structure of the Earth's plasma environment, such as those involved in the interaction between the solar wind and the magnetospheric plasma, in global magnetotail dynamics, in cross-tail currents, and in the formation and dynamics of the neutral line and of plasmoids.

The four spacecraft have been collecting the most detailed data yet on small-scale changes in near-Earth space and the interaction between the charged particles of the solar wind and Earth's atmosphere. This enables scientists to build a three-dimensional model of the magnetosphere and to better understand the processes taking place inside it. The Cluster mission control centre is located at ESOC, Darmstadt, Germany.

The Flight Control Team

Juergen Volpp

The Flight Control team (FCT) operates Cluster from a Dedicated Control Room located at ESOC, Darmstadt.

Spacecraft Operations Manager, Juergen Volpp, oversees a team of engineers from flight dynamics, ground facilities, navigation and mission data systems.

Mission operations

Fregat upper stage deploying Cluster

Launch and early orbit phase

The first pair of Cluster satellites lifted off on 16 July 2000, the second pair one month later on 9 August 2000. This gap allowed fewer people to be used for mission control in the European Space Operations Centre (ESOC) in Darmstadt (Germany).

At each launch, two Cluster satellites were placed in an elliptical orbit whose height varied from 200 to 18 000 km above Earth. The two satellites of each launch were then released, one after the other and used their own on-board propulsion systems to reach the final operational orbit (19 000 to 119 000 km from the planet).

Once the booster reached the correct altitude, after liftoff, the Fregat payload assist module and its two Cluster spacecraft were released. The Fregat main engine fired almost immediately to achieve a circular orbit of approximately 200 kilometres high. About an hour later, the Fregat engine fired again to inject the spacecraft into an elliptical orbit.

The two satellites were released, one after the other. Each Cluster spacecraft main engine performed six major manoeuvres, using the large amount of on-board fuel (about half of each satellite's launch mass).

VMC image sequence showing Cluster's Tango and Rumba separation

Day-to-day operations

One of the most challenging aspects of Cluster is to control four identical spacecraft. The day-to-day operation of the spacecraft is carried out by the European Space Operations Centre (ESOC) in Darmstadt, Germany.

During the two-year mission, signals to and from the spacecraft are sent through ESOC. The centre controls the spacecraft and their payloads, and carries out all activities related to mission planning and scheduling.

Routine ESOC operations are interrupted every six months to conduct orbit manoeuvres. Controllers adjust the separation distances between the four spacecraft in order to allow the study of different scientific phenomena.

The ground station - Maspalomas

Maspalomas S-band and X-band ground station

Spacecraft telemetry, command and tracking is carried out primarily by the ESA ground station in Maspalomas, Spain with support of ESTRACK's Perth (Australia), Villafranca (Spain), and New Norcia station.

In the critical early stages between launch and final orbit insertion, additional ground stations in Kourou (French Guiana), Perth (Australia) and Kiruna (Sweden) were used.

Ground segment & mission control system

Cluster ground segment

The Cluster ground segment is used to monitor and control the four satellites, and to receive, archive and distribute the science data. It is composed of the Mission Operations Centre (MOC), ESA's Ground Stations and Communications Network (ESTRACK), the Cluster Science Data System (CSDS), and the Joint Science Operations Centre (SOC).

The Cluster Mission Control Centre at ESOC is responsible for the:

• Transfer of the four satellites into their operational orbits and their correct orbital positions
• Combined operation of the four spacecraft and their payloads
• Collection and distribution to the scientific community of all the raw science data, as well as monitoring of spacecraft health, orbits, etc.

The vast amount of scientific information sent back by the four spacecraft is handled by CSDS. It supports scientific data processing and distribution activities. SOC coordinates the science operations. Its main task is to merge all commands coming from the 11 instrument teams into an overall command schedule, which is then sent through ESOC to the 44 spacecraft instruments.

The platform and payload

Cutaway diagram of one of the Cluster spacecraft

The platform

The Cluster spacecraft resemble giant 'Lego' sets, assembled by hand from thousands of individual parts. Each one is shaped like a giant disk, 1.3 m high and 2.9 m across. In the centre is a cylinder with an aluminium honeycomb structure covered with a skin of carbon-fibre reinforced plastic. The equipment panel inside this cylinder supports the main engine, two high-pressure fuel tanks and other parts of the propulsion system.

Six spherical fuel tanks made from titanium are attached to the outside of this central cylinder. The fuel they carry accounts for more than half the launch weight of each spacecraft. Most of this fuel was consumed soon after launch, during the complex manoeuvres required to reach their operational orbits. Each spacecraft also carries eight thrusters for smaller changes of orbit.

Cluster observes 'magnetic reconnection'

Each spacecraft carries an identical set of 11 instruments to investigate charged particles, electrical, and magnetic fields. These were built by European and American instrument teams led by Principal Investigators.

The Cluster scientific community includes the ESA Project Scientist, 11 principal investigators, and more than 250 co-investigators from ESA Member States, the United States, Canada, China, the Czech Republic, Hungary, India, Israel, Japan, and Russia.