Sometimes space exploration doesn’t go as planned. But even in failure, engineers can learn, adapt, and try again. One of the best ways to do that is to share the learning, and allow others to reproduce the work that might not have succeeded, allowing them to try again. A group from MIT’s Space Enabled Research Group, part of its Media Lab, recently released a paper in Space Science Reviews that describes the design and testing results of a pair of passive sensors sent to the Moon on the ill-fated Rashid-1 rover.
The rover, which was developed by the Mohammed bin Rashid Space Centre (MBRSC) in the United Arab Emirates (UAE), was launched aboard the Hakuto-R 1 mission, the first private mission to the Moon’s surface, operated by a Japanese start-up called iSpace. When it launched in December 2022, it took a low energy transfer route to the Moon, eventually coming to a “hard landing” when its sensors incorrectly estimated that it had already landed when it was passing over the rim of a crater and cut its engines, despite still being 5 km above the surface.
Despite that mistake, many of the technologies on the 10kg Rashid-1 rover were cutting edge - and the passive regolith sensors were definitely that category, despite being simple by design. The two sensors, which were part of the Material Adhesion/Abrasion Detection (MAD) experiment the rover planned to carry out, were designed to be completely passive - no power and no moving parts. Both were designed to fit physically on the wheels of the rover, and would utilize another feature the rover had - its camera.
One sensor, known as the Passive Regolith Sampler (PRS), was an aluminum tray covered by a plate with a series of perforated holes of different orientation and sizes. There were actually two on the Rashid-1 rover - one on each front wheel. As the wheel it was attached to turned, small samples of regolith would be deposited through the holes and onto the tray. Its intention was to determine whether the spacing and sizing of the holes significantly impacted regolith collection and retention.
Even with such a passive sensor there were still lots of complications. To test the sensor, in lieu of an actual rover wheel, the researchers simply pressed the sensor into some regolith simulant. However, they weren’t able to find any statistically significant information about the difference size or space made, so they believe they were just doing it wrong - manually stamping the sensor into the dirt isn’t the same as rolling it on a wheel.
They did eventually get their chance to test it on a real wheel, though only after the mission had failed. The research team got access to the sandbox environment at the MBRSC with lunar regolith simulant, where they could attach their sensor to an engineering model of the rover to test. However, as of the time of publication of the paper, the results from that experiment aren’t yet available.
Part of that reason might be because of the difficulty in analyzing the data in the sensor. the engineering team had to use advanced image processing algorithms, combined with simplified “lookup tables” to understand where the Sun was in the lunar sky and how that might affect the shadows, and therefore the results of the PRS experiment. While the methodology is applicable no matter where on the Moon the sensor lands, the lookup tables would have to be adjusted based on factors like the position and time of year.
The other sensor, known as the Passive Wax Thermometer (PWT), was also heavily reliant on the rover’s camera and advanced image processing algorithms. It was designed to act as a thermometer by housing capsules of different waxes that would change from solid to liquid at different temperatures. Essentially, each wax sample would provide a binary yes/no check of whether it was below or above the wax’s melting temperature. Since the waxes were selected based on the fact they turned clear when liquid and were opaque when solid, the camera could then pick out whether any of the samples were liquid at any given time.
The waxes selected were chosen to read between 9°C (Pentadecane) and 87.5°C (Tetratetracontane). The samples also included two capsules of natural beeswaxes and one of a commercially available candle wax. Originally, this experiment would have had the perfect testing opportunity as an eclipse was expected during its mission time, allowing it to observe a rapid and dramatic drop in temperature, but given the mission’s failure that opportunity was lost.
But the point of publishing this paper is to make sure it is not lost forever. Other research teams can pick up the mantle of updating and adapting these two simple, passive sensors for use on other missions. That’s how science (and engineering) make progress - by standing on the shoulders of those who came before, despite, or maybe even because of, whatever failures they might have experienced along the way.
Learn More:
K. J. Stober et al. - The Passive Regolith Sampler: From Concept to Delivery to the Lunar Surface
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