Cluster

Cluster is a constellation of four spacecraft flying in formation around Earth. They are making the most detailed investigation yet of how the Sun and Earth interact beyond sunlight alone. The four Cluster spacecraft work together collecting three-dimensional information about how the perpetual stream of subatomic particles given out by the Sun, the solar wind, interacts with Earth’s natural cloak of magnetism, the magnetosphere.

These interactions can damage satellites in orbit and power stations on Earth. The original operations of the Cluster mission were scheduled to run from February 2001 to January 2003 but, in the absence of any significant hardware problems, a series of extensions have meant that Cluster has now spent a decade passing in and out of our planet's magnetic field, returning invaluable data.

Spacecraft

Each Cluster spacecraft is shaped like a giant disc, 1.3 m high and 2.9 m wide, with a cylinder in the centre. Six spherical fuel tanks are attached to the outside of this central cylinder. The fuel they carried accounted for more than half the launch mass of each spacecraft, although most of it was consumed soon after launch in the manoeuvres needed to reach their operational orbits. In addition, each spacecraft has eight thrusters for smaller orbit changes, which are required to place the spacecraft into the various configurations used for their science.

Also around the central cylinder is the main equipment platform. Electrical power comes from six curved solar panels attached around the outside of the platform. Five batteries are used to supply power during eclipses when the spacecraft enter Earth's shadow. Recently, to conserve the limited lifespan of the batteries, Cluster has been put to sleep during eclipses.

Rod-shaped booms opened out on each spacecraft once they reached orbit to provide two antennas for communications and two for sensors. In addition, four wire booms extended more than 40 m to measure the changing electrical fields around the spacecraft.

The individual spacecraft are called Samba, Tango, Rumba and Salsa.

Journey

First Cluster duo launch 16 April 2000

Cluster was launched by two Russian Soyuz rockets. The first pair of Cluster satellites lifted off on 16 July 2000 from the Baikonur Cosmodrome in Kazakhstan. The second pair was launched from the same location a month later.

Each pair was placed into an initial elliptical orbit that swept them from 200 to 18 000 km in altitude. Then they used their own onboard propulsion systems to reach the final operational orbit, another elliptical path around Earth from 19 000 to 119 000 km in altitude.

Over the years, Cluster’s maximum altitude gradually increased to above 130 000 km. To keep a good communications link with ground stations, the four spacecraft were lowered in November 2009. The current orbit takes them from a few hundred kilometres at closest approach to Earth to around 125 000 km at furthest.