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A combined electric and magnetic field sensor

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In the context of ESA’s Comet Interceptor mission aiming at a comet flyby, BIRA-IASB has designed and built a combined electric and magnetic field sensor. Combining these two types of instruments required special precautions to avoid that both sensors interfere with each other. As is typical for space applications, the combined instrument had to be lightweight. This offered our engineers a challenging mechanical design problem … but the job was completed successfully!
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The Comet Interceptor mission

The European Space Agency’s Comet Interceptor mission aims at performing a comet flyby. The mission is relatively small, yet packed with a lot of instrumentation.

In order to avoid the payload to become complex and heavy, the instrument teams for the FGM magnetometer (U. Auster, Technical University Braunschweig, Germany) and the COMPLIMENT electric field probes (P. Henri, LPC2E, Orléans, France; N. Edberg, IRF, Uppsala, Sweden; J. Peterson, IRF, Kiruna, Sweden; J. De Keyser, BIRA-IASB, Brussels, Belgium) proposed to combine both instruments into a single sensor.

The COMPLIMENT probes and the combined sensor

The COMPLIMENT sensor acts as an electric field sensor. It consists of two Aluminum spheres with 8 cm diameter, each one mounted on a boom that positions the spheres about 1 m away from the spacecraft to avoid interference produced by the spacecraft itself. Being exposed to free space, these spheres behave like an electric contact with the space environment as the charged particles that fill space may collide with the sphere.

By playing with their electric potential and measuring the current, the most important characteristics of the plasma environment can be measured. One of the two spheres has been converted into a combined sensor.

A design challenge

Our mechanical engineers were faced with the following problem: how can we embed the magnetometer inside the spherical probe, while enveloping the magnetometer in an electrically conducting “bag” that shields the electric probe from electric perturbations that necessarily arise from operating the magnetometer. All of this had to be lightweight, yet strong enough to sustain the vibration loads that are experienced by the sensor at the tip of its boom during launch.

The BIRA-IASB engineers iterated their design with the different teams involved and came up with a nice scheme. The “bag” is made of a thin foil and is kept in place with a soft polyurethane framework, nicely midway between the magnetometer and the spherical probe – the ideal configuration from the electric point of view.

After ordering all the components in industry, the spheres and the foil were prepared in the BIRA-IASB workshop and were shipped to Germany for integration with the magnetometer and the boom.

Success

“The proof of the pudding is in the eating.” The entire team was thrilled with the outcome of the vibration tests that mimic the mechanical loads during launch: the probe survived the testing, and the thin foil seemed to have stayed nicely in place.

The feasibility of the combined sensor concept has been demonstrated. Further electric and magnetic characterisation of the combined sensor is now underway.

FGM COMPLIMENT Vibration Tests
The combined FGM-COMPLIMENT sensor prototype mounted onto the tip of the deployable boom during the vibration tests. Click to see the movie (Credit: TU Braunschweig).

Perspectives

The next step is to build the instrument prototypes and the flight instrument following the schedule imposed by ESA.

 

Want to know more?

Snodgrass, C., and Jones, G.H. (2019). The European Space Agency’s Comet Interceptor lies in wait. Nature Communications, 10, A5418. https://doi.org/10.1038/s41467-019-13470-1 Open Access Logo

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A prototype of the combined FGM-COMPLIMENT sensor during integration, showing how the magnetometer is enclosed by the thin anti-interference foil. (Credit: BIRA-IASB)