Cluster II spacecraft loses its magnetism
14 June 1999
A steady stream of scientists and engineers coming and going. A strangewooden building hidden behind a high wire fence deep in the Bavarianforests. Unusual humming sounds coming from inside the structure. Asuitable case for an X-Files investigation by Mulder and Scully? Far from it.This unusual setting is actually the location for a final series of ground tests on the first Cluster II satellite as it nears completion prior to launch next summer. Flight model (FM) 6 spent last week undergoing a thorough examination of its magnetic properties. The aim is to demonstrate that, once it is placed into orbit, the spacecraft will not play havoc with sensitive experiments to measure magnetic fields in near-Earth space by turning into a giant magnet.
Although a wooden shed is not the place one would normally expect to find a spacecraft, this unique facility - the only one of its type in Europe - is the ideal place to study a satellite's magnetic characteristics.
The key to the success of the operation is the absence of any outside magnetic sources - hence the location in the middle of a forest, 5 km from the main IABG test facilities and far from overhead power lines and other electromagnetic interference. Even staff working inside the room have to be magnetically 'clean', so all metal objects such as wrist watches, keys or coins, must be left outside.
The test facility itself has wooden walls and ceiling, and a raised wooden floor. For the duration of the tests, FM6 sits on a rotary table in the middle of a 15 metre wide room. The spacecraft is surrounded on all sides by a box-shaped framework of aluminium beams which support a large, copper coil system. Only one person, who manually rotates the spacecraft, is allowed inside this metal frame.
The first phase of the test programme is to cancel out the Earth's magnetic field and reduce the magnetism in the room to a negligible level. This is done by sending a strong electrical current through the large coils. Technicians and engineers can then assess the magnetic state of the spacecraft at the end of its assembly and test cycle.
FM6 is then subjected to a demagnetisation procedure known as a 'deperm'. This is achieved by passing an alternating electrical current through small portable coils located within a metre of the spacecraft.
If all goes well, the magnetic field at the end of the boom sensor should be below 0.25 nanotesla (200 000 times lower than the local strength of Earth's magnetic field in Munich). This is so weak that it cannot be measured directly on the ground, so engineers have to predict its level, using mathematical models based on measurements from test facility sensors around the spacecraft.
It is important to reach this level of extreme demagnetisation because, once the spacecraft is in orbit, a magnetometer on the end of a 5 metre long boom will be deployed to measure the local magnetic field and its tiny variations. A magnetised spacecraft would disrupt these readings.
Unfortunately, although the Cluster II spacecraft have been made, as far as possible, from materials which do not tend to become magnetic, problems inevitably arise with certain metallic components such as valves and relays.
The answer is to compensate by gluing small magnets (about 3 cm long and 5mm thick) to the body of the spacecraft. These cancel out any weak magnetic fields generated by spacecraft components. This 'damping' process is especially important near the tip of the 5 metre boom which will be used by the Flux Gate Magnetometer experiment.
Another troublesome source of disturbance could be any 'soft' magnetism induced in some of the spacecraft components. The Cluster II team need to be certain that their spacecraft will not become magnetised again when it is removed from the test facility. For this phase of the tests, engineers use the smaller coils to simulate worst case magnetic conditions by raising the level of magnetism around the spacecraft to six times the strength of Earth's magnetic field.
With these final checks out of the way, the spacecraft is cleaned up again by being subjected to another 'deperm'. FM6 is then moved back to the main IABG facility where it is fitted with a magnetic monitor. This triggers an alarm if the spacecraft is exposed to any threatening magnetic fields. For the next 12 months it will be stored in a container, protected from dust and other contamination by a continuous flow of inert gas.
One down, three more to go.
