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Last Saturday, the Japan Aerospace Exploration Agency's (JAXA) Smart Lander for Investigating Moon (SLIM) reached the lunar surface. However, not everything went as planned: according to images from one of its autonomous probes, the ship unfortunately landed upside down. The incident, believed to be due to an anomaly in one of its main engines, would explain why the company is having difficulty generating electricity.
Japan officially became the fifth country in the world to reach the lunar surface (after the United States, Russia, China and India) at 12:20 a.m. (Tokyo time) on January 20. Its SLIM lander landed just 55 meters east of its intended landing point. This shows impressive precision, as the target area was only about a hundred meters in diameter. In comparison, previous lunar probes typically targeted landing zones about ten kilometers in diameter.
The two probes LEV-1 and LEV-2 (LEV for Lunar Excursion Vehicle) were successfully separated before SLIM reached the lunar surface. However, engineers could not immediately confirm whether the lander's solar batteries were working properly. The machine had significant difficulties generating electricity. As a result, power was cut off almost three hours after landing. “The battery was disconnected at 12% remaining charge in accordance with our procedures to avoid a situation where a restart would be hindered,” the agency said in a statement.
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By analyzing technical data and images transmitted by LEV-2 (also called SORA-Q), experts determined that the SLIM landed upside down, preventing its solar panels from being exposed to direct light. This would explain the difficulty of generating electricity. Unfortunately, the lander has no way to correct its position.
Estimated current location of SLIM and target landing site. © Chandrayaan-2/ISRO/SLIM/JAXA
A loss of thrust from one of the main engines
During the descent, SLIM was able to collect technical and visual data to provide navigation guidance for the landing. The onboard Multiband Spectroscopy Camera (MBC) was also used experimentally to capture images until the power was turned off. In turn, SORA-Q also collected images that were relayed to LEV-1 to send back to Earth. The latter has an autonomous device that allows it to maintain communication with the mission control center. Communication with Earth was correctly established 20 minutes after the moon landing. In total, the probes were able to record and transmit 275 images of their immediate surroundings.
In the images captured by SORA-Q, we can clearly see that SLIM landed in a position that almost completely obscured its solar panels. According to JAXA, “an anomaly in the main engine affected the spacecraft’s landing performance.” The technical data showed in particular that at a height of 50 meters, shortly before the start of the obstacle avoidance maneuver, the thrust of one of the two main engines had apparently been lost.
In an attempt to adjust the trajectory, SLIM's navigation software identified the anomaly and attempted to control its position as best as possible. The remaining descent was carried out by the second engine, which gradually brought the lander towards the east. The speed at the moment of contact with the ground was around 1.4 m/s, which is lower than initially expected. According to engineers, this may also have affected the SLIM's landing position.
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A chance to restart
However, the agency estimates that it will still be possible to restart the lander when lighting conditions on the moon improve to partially power the solar panels. The pictures show that the solar panels face west. “If sunlight hits the Moon from the west in the future, we believe there is a possibility of power generation and we are currently preparing for recovery,” they explain.
In addition, the engineers believe that the entire mission will remain a success despite the landing problems. Disaster landings and communications failures are commonplace on space missions. The success of LEV-1's jumping movements, communication synergy with SORA-Q, and fully autonomous operation represent, in their opinion, a “revolutionary achievement.” In fact, LEV-1 is the smallest and lightest direct data transfer robot ever to set foot on has set the lunar surface. His jumping movements are also a technical masterpiece in themselves.
In the near future, JAXA plans to conduct a more in-depth study of the data collected as well as the anomalies that occurred in the SLIM engine. External factors are also taken into account to determine the actual causes of the landing error in detail. The SLIM activities should initially only last a few days. Data collection will resume as soon as it can be restarted.