MARE to the Moon – our M-42 radiation meter with a smart solution for saving power
The MARE experiment includes a set of 16 radiation measurement devices called M-42, which the Biophysics Working Group of the Radiation Biology Department at the DLR Institute of Aerospace Medicine developed, tested and now finally built for the NASA Artemis I mission. M-42 is an active radiation meter. This means that it needs to obtain power from somewhere for the radiation detector (a small silicon diode) and the associated measurement electronics, and for storing the resulting measurement data. This power can be supplied either directly via a USB cable or batteries that simply plug into the M-42 via two connectors.
In principle, this sounds very simple. However, anyone who has ever hoped that their mobile phone would last until the next charging opportunity knows how dependent we are on batteries and their capacity. This poses a big challenge for this mission in particular. Our M-42 measuring instruments and the mannequins Helga and Zohar are part of NASA's Artemis I mission, but we do not get data or power interfaces to the Orion spacecraft.##markend##Our entire setup must function completely independently. This means that we will be reliant on batteries for the entire duration of the mission. To make matters worse, several weeks can pass between the installation of the equipment and the mission launch. This is the waiting time on the launch pad. In the worst case, the time can even be extended to two or three months. The actual flight duration will then be another 42 days.
Can you think of a mobile phone that lasts several months on a single battery charge? – It's rather unlikely. So, our most important question was – how can we build a measuring device that runs on batteries and remain functional for such a long time? Because the power consumption must be as low as possible the developers of the measurement electronics were tasked with the first challenge.
The M-42 barely consumes any power and can definitely measure over the maximum mission duration of 42 days with the two batteries included. But the possibly months-long waiting time cannot yet be bridged with these batteries from installation to launch. So, the next challenge was – how can we solve this problem and go on to build a system that works even better! (Of course, these two questions were solved at the same time in the development of the electronics, I just separated them now to increase the excitement).
We're rather proud of how this problem was solved, because not only were we able to successfully address this complex task, but we also think our method is very elegant. We built an acceleration sensor into our M-42 measuring device, because as soon as a rocket begins to lift off, there is an additional, upward acceleration that can be measured by that sensor.
With this, our M-42 now works according to the following principle: Once we plug in the batteries, the M-42 is in 'sleep mode'. This means that it is not measuring the radiation environment. However, every few seconds the M-42 processor asks the accelerometer: "Are you seeing increased acceleration for a sustained period of time?" If not, everything stays in sleep mode. But if the acceleration increases, the sensor wakes up the electronics, so the M-42 switches to 'measurement mode'. It continues measuring until the batteries are empty, saving the resulting data every five minutes.
You might ask: "That is all very well, but have tested your accelerometer and the process of switching from sleep to measurement mode?" The answer is yes. Our M-42 has already made three flights as a 'piggyback payload', accompanying experiments conducted by our colleagues at the Gravitational Biology Department of the DLR Institute of Aerospace Medicine, as part of the DLR MAPHEUS missions (MAPHEUS-7, -8 and -10) and has proven that switching from sleep to measurement mode when acceleration is detected works without any problems.
To read about this in more detail please refer to our publication about the M-42 instrument (Open Access): Berger, T., Marsalek, K., Aeckerlein, J., Hauslage, J., Matthiä, D., Przybyla, B., Rohde, M., Wirtz, M. (2019). The German Aerospace Center M-42 radiation detector - a new development for applications in mixed radiation fields. Review of Scientific Instruments, 90, 125115 (see Chapter IV B: The MAPHEUS campaigns).