The four Cluster spacecraft. Flying in formation around the Earth, they relay detailed information about how solar wind affects our planet in 3D (credit: ESA)

The four Cluster spacecraft. Flying in formation around the Earth, they relay detailed information about how solar wind affects our planet in 3D (credit: ESA)

Plasma shock waves

Launched by two Russian rockets in the summer 2000, the four Cluster spacecraft then used their own thrusters to get to an elliptical orbit with a closest distance to the Earth of nearly 20 000 km, and getting as far away as nearly 120 000 km: nearly a third of the distance to the moon. Cluster?s task is to sample the Earth?s magnetosphere.

Cluster senses many different physical processes, including the shockwave generated when the solar wind slams into the Earth. Our planet and its magnetic field are an obstacle to the supersonic plasma as it flows away from the Sun. Therefore a ‘bow shock’ forms to slow down and deflect the plasma around the Earth.

Shocks form wherever an obstacle sits in a supersonic plasma flow. The Hubble space telescope has captured images of a bow shock, about 0.25 light years across, formed ahead of a star ploughing through the Orion Nebula. Astrophysical shocks are very energetic (they’re the source of some of the highest-energy particles we know about) but to understand how those particles get such huge amounts of energy, we need to know more in general about how shocks work in plasmas. The Earth's bow shock is a good case to study, because it's close enough that we can get plenty of information back about the physics happening there.

Cluster is not the first spacecraft to visit the Earth’s magnetosphere. So what makes it so different? The answer is that the four spacecraft fly in close formation, giving us four tracks through whatever physical process is happening around the spacecraft. It's therefore possible to sample a volume of space and make measurements in three-dimensions. Imperial College, London developed 3-axis magnetometers, University College London’s Mullard Space Science Laboratory designed and built electron detectors and Sheffield University developed the wave processors for each satellite.

Find our more about the Cluster mission at the European Space Agency's website!

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