ESA Science & Technology - #5: Electric and magnetic fields under control for Solar Orbiter

After shipment from prime contractor Airbus Defence and Space in Stevenage, UK, last year, the Solar Orbiter spacecraft has been undergoing a series of tests at the premises of IABG in Ottobrunn, Germany, testing its thermal and mechanical properties, the deployment of several elements, and most recently its electromagnetic compatibility (EMC) and magnetic behaviour.

Solar Orbiter in the anechoic chamber. Credit: Airbus Defence and Space

The EMC testing took place in an anechoic chamber at IABG, where the spacecraft was isolated from external electromagnetic interference. The chamber walls are covered with thousands of pointy pyramids that fully absorb reflections of electromagnetic waves, muting also any sound echoes and creating an eerie silence. Performed between 8 and 22 May, these tests were needed to verify that the spacecraft's electrical equipment will be fully electromagnetically compatible throughout all phases of the mission.

Electromagnetic compatibility is a key aspect of any space mission, because electrical and electronic equipment can be a source of electromagnetic emissions and/or sensitive to such emissions, so the compatibility between the various onboard emission sources and their susceptibilities needs to be verified before launch.

One part of the EMC tests was designed to check that the electromagnetic emissions from various parts of the spacecraft – particularly the various radio antennas onboard – did not interfere with other spacecraft subsystems. During this phase of the tests, the spacecraft's high gain, medium gain and low gain antennas that will provide telemetry, tracking and communication (TT&C) for the mission were turned on one at a time, checking that all other systems were still operating properly.

The engineers also had to verify the compatibility of the spacecraft's systems with sensitive external equipment. The Solar Orbiter EMC tests verified that no emissions from the spacecraft would interfere with the launch vehicle's radio receivers and transponders or with nearby radio antennas during the launch preparations and lift-off from the Cape Canaveral launch site in Florida, USA.

Solar Orbiter in the magnetic field simulation facility. Credit: ESA–S. Corvaja

In addition, some specific compatibility tests were performed to determine whether the Radio and Plasma Waves (RPW) instrument would be affected by the spacecraft's electromagnetic emissions. This instrument is sensitive to electric and magnetic signals, so it was important to characterise the fields produced by the spacecraft in order to be discriminated later on from the actual measurements of the fields in space.

Once in space, Solar Orbiter will deploy three 7-m long monopole antennas that are part of the RPW instrument. However, these are too long to be deployed in the EMC chamber, so the tests were carried out using shorter placeholder antennas plugged into the instrument, together with an external antenna linked to a receiver.

Another phase of the EMC tests checked the compatibility of another one of the RPW elements – the search coil magnetometer (SCM) – with the low frequency alternating current (AC) magnetic fields emitted by the spacecraft. The flight version of Solar Orbiter will carry the SCM in the middle of the instrument boom, but deployment of the actual instrument boom during the EMC test was not possible, so an electrically representative boom mock-up was prepared and used with the SCM qualification model connected to the spacecraft.

Analysis of the test data showed that the spacecraft meets the EMC requirements with respect to interactions with the TT&C subsystems onboard and with external equipment during launch. Analysis of the EMC test for the RPW instrument yielded results that were consistent with prior expectation from unit level testing and analysis, but further characterisation will be necessary during in-flight and in-orbit commissioning.

Solar Orbiter in the magnetic field simulation facility. Credit: ESA–S. Corvaja

While most spacecraft undergo only EMC testing, missions that involve measuring magnetic fields in space with exquisite accuracy – such as Solar Orbiter, which will measure the magnetic field of the solar wind with its Magnetometer (MAG) and with the SCM component of the RPW instrument, or magnetospheric plasma missions like ESA's Cluster and Swarm – require an additional set of tests to fully characterise their magnetic properties.

The IABG magnetic field simulation facility. Credit: ESA–S. Corvaja

These tests were conducted in a unique facility, the magnetic field simulation facility (MFSA), located just outside the IABG premises in a nearby forest to avoid interference with human-generated magnetic fields. In addition to that, the facility consists completely of non-magnetic materials like wood, and contains twelve 15-m coils – nearly as large as the building – to create a homogeneous magnetic environment that compensates Earth's own magnetic field, simulating outer space conditions.

Solar Orbiter in the magnetic field simulation facility. Credit: ESA–S. Corvaja

Performed between 18 and 25 June, the testing first verified the magnetic behaviour of the unpowered spacecraft, to make sure the magnetic field of Solar Orbiter is low enough so that the MAG instrument can operate at its most sensitive range. Later, the tests proceeded with electric current powering the spacecraft – first looking at direct current (DC) and finally alternating current (AC).

The last scenario is especially important because it measured possible variations in the spacecraft's magnetic field during specific operations, for example with extra currents generated by the motors or by mobile elements such as the changeable optical filters used by the cameras on board to take images of the Sun.

The analysis of the magnetic tests indicates that the mission requirements were met within the limits of the testing facility. After launch, in the even quieter environment of space, further measurements during the commissioning phase will complement the results of these tests to fully characterise the magnetic properties of the spacecraft.

The spacecraft electromagnetic compatibility and magnetic cleanliness are especially important aspects for the science that Solar Orbiter will perform, not only for the RPW (EMC aspects) and MAG (magnetic properties) instruments. They are also key elements for the two particle detector instruments on board – the Solar Wind Plasma Analyser (SWA) and the Energetic Particles Detector (EPD) – in order to reconstruct the path of incoming particles that might be deflected by any residual spacecraft electric or magnetic field.

Throughout the mission development, a specific EMC working group, involving representatives of industry, ESA and all instrument teams, has looked into these aspects, setting stringent requirements for the overall spacecraft design. With this phase of the test campaign now completed, the teams are pleased to see that these choices eventually paid off, as the satellite checks off another important milestone on the way to launch.

About Solar Orbiter

Solar Orbiter's mission is to perform unprecedented close-up observations of the Sun. Its unique orbit will allow scientists to study the Sun and its corona in much more detail than previously possible, and to observe specific features for longer periods than can ever be reached by any spacecraft circling the Earth. In addition, Solar Orbiter will measure the solar wind close to the Sun, and provide high-resolution images of the uncharted polar regions of the Sun.

It will carry 10 state-of-the-art instruments. Remote sensing payloads will perform high-resolution imaging of the Sun's atmosphere – the corona – as well as the solar disk. Other instruments will measure the solar wind and the solar magnetic fields in the vicinity of the orbiter. This will give us unprecedented insight into how our parent star works, and how we can better predict periods of stormy space weather, which are related to coronal mass ejections (CMEs) that the Sun throws our way from time to time.

Scheduled for launch in February 2020, Solar Orbiter will take just under two years to reach its initial operational orbit, taking advantage of gravity-assist flybys of Earth and Venus, and will subsequently enter a highly elliptical orbit around the Sun.

Solar Orbiter is an ESA-led mission with strong NASA participation.