Force field: The mission to map Earth’s magnetism

At the heart of our planet is a roiling mass of liquid metal. Surrounding the Earth’s solid iron core is a layer of molten iron in constant motion, churning away eternally. At least, we had best hope it’s eternally, because it’s keeping us – and every other living thing on Earth – alive.

The molten core is a self-sustaining dynamo, which is the main source of the planet’s magnetic field. That much, at least, we know; but we have no idea how it works. Finding out more about it is the goal of the Swarm mission, the European Space Agency’s (ESA) attempt to map the texture and fluctuations of the magnetic field; although it may seem paradoxical, the best way to find out what’s happening deep inside the planet is to go into space.

The mission’s three UK-built satellites are currently at EADS Astrium’s Friedrichshafen facility for final assembly and The Engineer was invited to see them before they begin pre-flight checks ahead of their scheduled 2012 launch.

swarm
The three Swarm satellites together in the cleanroom at EADS Astrium’s Friedrichshafen facility, for the last time before their launch in 2012

The magnetic field surrounds the planet in a cocoon of force that forms a barrier against the solar wind – the constant but fluctuating stream of charged particles emitted from the Sun. If the field weren’t there, the solar wind would strip away almost all of the atmosphere, the oceans would boil away into space and the planet would be left unable to support life. Mars, whose magnetic field faded away unknown aeons ago, suffered this fate.

If the magnetic field weren’t there, the solar wind would strip away the atmosphere and the oceans would boil away into space

But the field is complex, and although we’ve known about it since 1600 -an English doctor, William Gilbert, was the first to describe it – we don’t understand how it works. The field is made up from several components. There’s the contribution of the molten core, but how the movement of the liquid iron generates the field is a complete mystery. Another contribution comes from naturally magnetic minerals in the Earth’s crust, such as ores of iron and nickel, and certain magnetic volcanic rocks. The ionosphere – the layer of ionised gases in the atmosphere from about 60km to 500km above the planet’s surface – also generates a magnetic field that contributes to the whole. Lastly, and rather surprisingly, the oceans, made from conductive salt water, generate a magnetic field as their currents move the water across the surface of the planet.

Crust field: magnetic minerals produce sharp fluctuations in the magnetic field, making iron deposits in Russia (top right) and diamond-bearing rocks in the west Sahara (middle) clearly visible

Field layers: the geomagnetic (top), crustal (middle) and oceanic (bottom) magnetic fields

Geofield: a constantly moving layer of molten iron surrounding the solid iron core of the planet generates the geomagnetic field — but scientists are unsure how

It’s becoming increasingly important that we understand more about the magnetic field; primarily, it’s responsible for our ongoing existence, but also because more and more of the technology we rely on depends upon it. The field protects communications, global positioning and Earth observation satellites from the solar wind. It may also be responsible for aspects of the climate.

The gaps in our knowledge became apparent when a German satellite mission called CHAMP, which was launched in 2000, noticed that the magnetic field was declining. In particular, it detected a very weak area of magnetic field over the South Atlantic and part of South America, which allows energetic particles to penetrate deep into the atmosphere; this disrupts GPS satellites and other spacecraft; moreover, astronauts on the International Space Station receive more than 90 per cent of their radiation dose as they pass through this region.

CHAMP burned up in the atmosphere in September, but Swarm is poised to continue its work. Among its objectives, according to Yvon Menard of ESA, is to determine why the magnetic North Pole ’walks’ around 50km per year; why the polarity of the magnetic field periodically inverts – a phenomenon that was first discovered early in the 20th century, and which some believe to be imminent; how the field influences the ’space weather’ of magnetic storms caused by the Sun’s activity; and to understand the still-obscure physics of the magnetic field.

Determining the signals from the various components of the magnetic field has a variety of applications, said Prof Hermann Lühr of the German Research Centre for Geosciences (GFZ) in Potsdam, the science lead for the project. ’We need the geomagnetic field for shielding, so we have to monitor that closely,’ he explained. ’If we look at the magnetic signal from the crust, we can see, for example, the big iron-ore deposits in Russia and Sweden, and the diamond-bearing ores in the west Sahara; this kind of geological information can be very useful for locating valuable minerals and for learning about the formation of the Earth. And if we can detect the magnetic signature of the ocean currents, this can give us a lot of information about the climate, because the ocean transports so much heat.’

CHAMP made the first detection of a magnetic field from the oceans, detecting the signal made by the tides as the Moon pulls huge masses of water back and forth through the geomagnetic field. ’We don’t know yet whether it will be possible to detect ocean currents with Swarm,’ Lühr said. ’It’s very challenging, but the oceans and the climate are linked so strongly and it would be a most useful tool to trace these currents; we currently have no way of doing it.’

Team work: the three satellites will fly together at the start of the mission

Using three satellites rather than the single CHAMP spacecraft will allow Swarm to detect the field’s fluctuations with much higher resolution, Lühr explained. The satellites will fly in polar orbits, about 300-500km up, with the Earth turning underneath them; two satellites will fly alongside each other, while the third will fly in a higher orbit. The two different orbital planes will gradually diverge, so that by the end of the four-year mission, the two planes will be at right angles to each other. This allows magnetic signals to be detected from a variety of different angles and elevations to give fine detail on the texture of the field.

This is a €220m mission; we can’t let any little thing jeopardise it

Albert Zaglauer, EADS Astrium Friedrichshafen

Swarm prime project manager

’Distinguishing between the components of the field is the most challenging part of the mission,’ said Lühr. ’The magnetic fields of the core and the crust are fixed to the Earth, so they are in a rotating frame. The magnetic fields coming from the ionosphere are driven by the Sun, so they sit still with respect to the terrestrial fields.’

Determining which signal comes from the core and which from the crust brings the signals from different angles into play. ’The different sources from the Earth come from different depths, so they have a different spatial spectrum,’ added Lühr. ’If you plot the field you can estimate where the signal comes from, by looking at how the amplitude changes with the scale length; if it’s further away, it changes rapidly, if it’s closer by, it changes slowly.’

The instruments that will allow the satellites to make these measurements required careful development and testing. Each spacecraft carries two magnetometers on the 4.5m boom, which extends from the satellites’ narrow end – a vector magnetometer, which measures the strength and direction of the field, in the middle of the boom, and a scalar magnetometer at the end. ’The satellites themselves are rather noisy, in terms of magnetism, and we want to measure the signal from the Earth, not the noise,’ explained prime project manager Albert Zaglauer. ’So we put the instruments away from the main satellite body, with the most sensitive instrument the furthest away. The scalar magnetometer can measure with accuracies of 1-2 nanotesla, to measure the signals from ocean currents, so we need to make sure it isn’t influenced by the satellite.’

Starcam:the vector magnetometer with its three star cameras on their optical bench is displayed in the foreground

The scalar magnetometer represents the biggest advance of the sensor equipment on Swarm, compared with those on CHAMP, which had a similar shape and layout. ’The vector magnetometer will measure for many years so we need a reference to calibrate it in space,’ Lühr said. ’We now use a very sensitive instrument that is based on helium vapour, rather than the one we used on CHAMP, which was a proton instrument.’

Developed by the French National Centre for Space Studies and the Laboratory of Electronic Information Technology as the French contribution to the project, the scalar magnetometer works on a similar principle to nuclear magnetic resonance spectrometers. It uses a laser to pump energy into atoms of helium-4, and detects the effect of magnetic fields by observing how they ’flip’ the nuclei of the atoms between three different quantum energy levels. ’This is a much better reference and it gives us the possibility to calibrate the three satellites so that we can do simultaneous measurement at different locations – that is the real advantage of Swarm,’ Lühr said.

The vector magnetometer, broadly similar to the instrument that flew on CHAMP, needed careful engineering. The sensor is used in conjunction with three star cameras pointing along the three orthogonal axes that measure the orientation of the magnetometer – vital information to determine the direction of the magnetic field as measured with respect to each of the satellites. The accuracy of this measurement needs to be to within two arc-seconds (an arc-second being one-three-thousand-six-hundredth of a degree) and so the entire assembly of magnetometer and cameras needs to be extremely rigid.

’The sensors are mounted on an optical bench, made of carbon-fibre reinforced polymer and ceramic,’ said Zaglauer. Originally, the bench was made entirely from rigid ceramic, but when this was tested, in an area of southern Spain that enjoys clear skies and is a long way away from any man-made magnetic fields, a problem became apparent. ’We found that the ceramic contained traces – parts per million – of iron, and that was enough to throw the measurements out,’ added Zaglauer. ’So we had to throw the whole thing away and replace the part of the assembly near to the sensor with a stiff CFRP component.’

Removing magnetic influences from the satellite turned out to be a major issue. Ceramics and glues had to be carefully vetted; many contain traces of iron or nickel fillers, any of which could wreck the mission. This care continues as the satellites are prepared for launch. Magnetic metal tools are banned; inside the clean room, all the tools used on Swarm are clearly marked as plastic. Strong magnetic fields are also banned; all visitors have to lock away their mobile phones, keys and credit cards, while audio speakers are strictly prohibited. ’This is a €220m [£196m] mission; we can’t let any little thing like that jeopardise it,’ said Zaglauer.

in depth
spot the difference

A closer look at the Swarm and CHAMP satellites reveals contrasting design features

Each Swarm satellite is similar in form to CHAMP – a 5m-long, narrow, tapering body, with a 4.5m-long boom emerging from the narrow end. However, the similarities are mainly cosmetic.

CHAMP was built from aluminium, but this wasn’t suitable for Swarm. ’We needed a lighter material, but it still needed to be stiff,’ said Andy Jones, project manager at EADS Astrium’s Stevenage facility, where the satellite’s structure and mechanical components were built. ’That’s partly because we’re flying three satellites instead of one and partly because we’re flying a constellation so we need to carry more fuel.’ Each Swarm satellite has 16 attitude control thrusters, providing 2g of thrust (equivalent to putting your hand in front of your mouth and gently blowing) and eight orbital control thrusters each providing 5g of thrust; and they carry 100kg of fuel. ’We have to spend fuel to establish the constellation; we spend half of it in the first quarter of the mission,’ said Zaglauer. ’The three satellites are delivered to the same spot, then two go about 50km lower, and one 40km higher. The satellites weigh the same as CHAMP, but CHAMP carried only 30kg of fuel. We had to save 70kg on other materials, so we switched from aluminium to CFRP.’

The solar panels on Swarm are arranged in an angled ’roof-like’ configuration on the body; partly so the three spacecraft fit inside the launcher fairing ’like slices of a tart’, as Zaglauer said, and partly because the polar orbits mean the angle of the satellites to the Sun changes constantly. The angled shape means they catch the maximum amount of sunlight.

The spacecraft fit inside the launcher fairing like slices of a tart

Albert Zaglauer

The satellites will be launched from Plesetsk in Siberia, fixed into the payload bay with their booms folded back onto the main body. There is only a 50mm clearance between the three spacecraft, but experiments using a shaking table have established that the CFRP structure can resist launch vibrations.

Swarm boom: the spring-loaded mechanism that will be used to deploy the boom of the satellite as it leaves the launcher

The booms deploy using a spring-loaded system. ’We have to make sure it goes into position gently, to avoid jerking the sensors,’ said Andy Jones. ’But the only way to dissipate kinetic energy in space is through friction. So the boom swings past its final position and then moves backwards and forwards, eight times, swinging a little less each time, until it comes to rest.’