1995 WINNER

Mission to Study Virtually Nothing

By Nick Flowers

LOOKING up at Mars tonight you might be forgiven for thinking that a transparent vacuum stretched between Earth and the red planet. Actually Solar System space is bathed in the extended atmosphere of the Sun a fact that is betrayed to us only during auroral displays. This atmosphere is tenuous and insubstantial. Streaming out of the sun is the solar wind - a wind with less than 10 particles per cubic centimetre (cup your hands together... you are now holding 300 million million million particles of air). Compared with our atmosphere it would appear as thin as air is to rock and standing in it would not disturb one hair on your head although it would be moving past at 400 km/s. This ghost of a wind is so hot that its constituent electrons and ions are stripped apart. As charged particles they are subjected to the magnetic forces of the Earth's field trapping some in the magnetosphere - the comet shaped region where the magnetic field of the Earth dominates particle motion.  Less than 500 kg of particles can be trapped in the magnetosphere but due to their energy they can be as deadly to spacecraft as the myriad of fragments of broken rocket known as space debris. Scientists hope to learn more
about the solar wind and the hazards in near-earth orbit in January
when the European Space Agency launches the 4 spacecraft of the
Cluster mission. The craft will fly in closely-choreographed orbits
to probe the 3-dimensional nature of Sun's interaction with the
Earth's shielding magnetic fields.

Cluster will be launched on the maiden flight of the new European
Ariane V, one of the most powerful rockets yet constructed and will be injected into a polar orbit. This orbit will annually rotate around from the magnetotail to the magnetosphere's boundary layer and out into the volatile solar wind permitting observations of the complex physics of many regions. The spacecraft are expected to last 2 years taking the satellites through a punishing regime of particle radiation every 75 hours, pushing electronics subsystems to design limits that required military chips developed to survive nuclear attack.

The need for such rad-hard technology was driven by one of the
objectives of the mission: the measurement of the Van Allen Radiation Belts - a feature which ultimately may well be the  cause of the flotilla's demise. The Cluster spacecraft can expect the radiation from these regions to cause a number of mission threatening problems. Due to advances in smaller microchip designs the probability of a penetrating electron causing substantial damage to a circuit has increased. Electrons hitting transistors can leave a trail of damage significant enough to flip individual bits of memory, corrupting data or randomising programs. Although bit-flip effects are debilitating, more serious damage can occur which may permanently disable entire
memory chips.

Effects are not isolated to such exploration missions - the Meteosat series of weather satellites that provide regular global images have experienced glitches.  These interfered with infrared scanning devices, denying forecasters valuable weather system information. A far more serious loss was experienced last year when Canada lost two of its Anik Communications satellites due to a particle storm. This caused communications blackouts throughout the country. Scientists hope that data from the 4 Cluster spacecraft will contribute to the aodelling of complex particle systems near geostationary orbit where communications, weather, and defence satellites work. Explaining the processes that can cripple spacecraft such as the Aniks will provide more accurate descriptions of the magnetosphere. This will permit
future satellite designers to reduce costs by using off-the-shelf
components where possible whilst not taking risks with electronics.

Dr Andrew Fazakerley of the Cluster science team at the Mullard
Space Science Laboratory (MSSL), University College London hopes to learn more about the Earth's interaction with its space-environment:"Cluster is to solar-terrestrial physics as a new telescope capable of viewing a new part of the spectrum is to astronomy - 4 spacecraft flying in close formation will give us new insight into the processes controlling the ebbs and flows of dangerous particles".

MSSL is one of several UK research institutions providing
instrumentation for integration with the European spacecraft. MSSL's contribution will be a set of sensors designed to detect high and low energy electrons.

"We have broken new ground by creating a set of identical sensors. This satisfies our need to measure small differences between the electron populations on each satellite" added Dr. Fazakerley. Loss of spacecraft in orbit last year cost $310m with the Aniks alone. With many new communications satellites being planned, the intrepid spacecraft of the Cluster mission will provide valuable insights into the nature of the magnetosphere and the solar wind, enabling future satellites to avoid the pitfalls of near-earth space.