One year since SLIM landed on the Moon "Thanks for the great achievement!": Cause of engine shutdown also clarified

January 20 marked one year since Japan's first Moon landing by the demonstrator lander SLIM. Not only did it achieved the world's first highly accurate "pinpoint landing" but also survived three nights when it could not generate power, exceeding its designed capabilities and repeatedly recovering, leaving its mark on the history of Japanese science and technology. A representative from the Japan Aerospace Exploration Agency (JAXA) said, reflecting on the spacecraft, which now rests on the lunar surface after completing its mission, "We are very grateful for your hard work and for the great results you have achieved." The cause of the abnormal shutdown of one main engine just before landing was clarified. The project team was disbanded on New Year's Day.

Error of less than 10 m achieved

SLIM was developed with two main objectives: to achieve high-precision landing on the lunar surface and demonstrate exploration technology using a small, lightweight lander. The lander has a height of 2.4 m, a width of 2.7 m, and a thickness of 1.7 m. Weighing approximately 700 kg, including fuel, it is lighter than conventional landers, which weigh over one ton or even several tons. The name "SLIM" stands for "Smart Lander for Investigating Moon."

It departed from Earth in September 2023 on the large H2A rocket. On January 20th of last year, it landed on a slope near a crater in Mare Nectaris, a southern sea on the Moon. Landing was approximately 60 m east of the original target, successfully achieving pinpoint landing with an error of 100 m. Excluding the automatic avoidance of obstacles, such as rocks, on the lunar surface and effects of abnormal engine shutdown, the error was less than 10 m. This can be interpreted as a substantial error.

During descent, the spacecraft took pictures of the lunar surface with a camera and compared them with the images of the lunar surface taken by the Indian orbiter in the past to determine the spacecraft's position and altitude in real time as it aimed for the landing site. The principle of this "image-matching navigation" is the same as that of the face recognition function of a digital camera. Craters and other topographic features are used as landmarks rather than faces.

One of the two main engines stopped abnormally at an altitude of approximately 50 m. Because of this, the lander could not land as planned and came to rest in a prone position with the solar cells facing west instead of directly upward. The batteries were not exposed to sunlight, and the lander soon became inactive. Later, when the sunlight direction changed, the batteries began generating electricity, and the lander made its first comeback on January 28. Scientific observations with a spectroscopic camera and the activity of a small robot separated during landing were also successful. Subsequently, the lander suspended its activity every time night fell on the lunar surface, reviving on February 25th, March 27th, and April 23rd. There was no response after late May, and the operation was terminated on August 23.

At a press conference held on December 26th before the dissolution of the team, Professor Shinichiro Sakai, then SLIM Project Manager at the JAXA's Institute of Space and Astronautical Science (ISAS), expressed his deep emotion, saying, "SLIM really did a great job. The lander traveled 380,000 km, and as it descended for landing, we were completely helpless, just watching from the Earth. However, SLIM made numerous decisions on its own. Even when the main engine broke down, it realized that 'something strange was going on,' and it even switched modes. It achieved great results."

Success amidst growing expectations for science and development use

With SLIM, Japan became the fifth country to land on the Moon, following the former Soviet Union (Russia), the United States, China, and India. The former Soviet Union achieved this feat in 1966, and India followed suit in August 2023, but Japan's achievement should not be seen as a mere follow up. The most remarkable attribute of this project is that it has achieved pinpoint landing, which has the potential to lead to future lunar and planetary explorations using a low-cost lander. The development cost was approximately 14.9 billion yen (including part of the launch cost and initial operating costs).

As lunar research progressed, there was a demand for more site-specific explorations. The spacecraft is expected to land with pinpoint accuracy at the targeted location, rather than just on a large safe plain, as in the past, with a margin of error of a few to a dozen kilometers in its landing position. Furthermore, there is a theory that water ice exists in abundance in places not exposed to sunlight, such as in the shadows of craters in the polar regions. The possibility that this water can be used in the future as drinking water for astronauts or as a source of fuel for transport spacecrafts has been discussed (note that some believe that there is not enough water to use). In this case, it is necessary to land in a limited area, close to ice and with access to sunlight to generate electricity.

With such high expectations for science and development use, SLIM succeeded in pinpoint landing. In other words, the mission followed the motto, "From where you can land to where you want to land." This technology will be used in the future explorations of Mars and other planets and their satellites.

Fuel accumulated and ignited, causing excessive shock

The process of the abnormal shutdown of one main engine was found to be as follows. First, (1) the timing of the main engine's firing coincided with the firing of the 12 auxiliary engines for attitude control. This reduced fuel supply pressure to the two main engines. One of the engines failed to ignite, and the fuel remained in this engine without burning. (2) Approximately 1 s later, the auxiliary engines finished firing all at once, and fuel supply pressure to the main engine was restored. (3) As a result, the main engine that had been shut down finally ignited. However, the accumulated fuel ignited all at once, causing excessive shock from ignition. This shock damaged the nozzle of the main engine, resulting in a significant loss of thrust. The SLIM's navigation camera spotted the nozzle that fell off onto the lunar surface.

While probes generally use a "pressure-regulating system" that employs a device to maintain fuel supply pressure, SLIM utilizes a "blowdown method" in which pressure decreases as fuel is used to reduce weight. Failure occurred when 98% of the fuel had been burned during the process of landing; this might have decreased pressure, making ignition difficult. Until then, the lander had demonstrated specified performance, and no trouble was encountered during combustion tests at the development stage. There was no possibility of a manufacturing error.

At the press conference, Sakai said, "I kept wondering if I could have foreseen the failure. Because the tests were performed satisfactorily, I think there are some aspects where consideration is lacking. The lander design was bit 'aggressive' to reduce the weight and was normal up to 98%; however, this event occurred at the end. We gained new knowledge regarding what to look out for (in the blowdown method) in the future."

As a result of engine failure, SLIM could not demonstrate the unique "two-stage landing" that was planned. This involves first touching down on the lunar surface using one main leg and then using the remaining four legs to collapse onto a slope and come to a rest. Because the flight up until the trouble had been going according to plan, Sakai believes that if the failure has not occurred, the two-stage landing may have been successful.

Small robot captures images that "anyone can tell are from the Moon"

SLIM's onboard electronics were not designed to withstand the harsh coldness of a lunar night, which can reach 170℃ below zero. However, in the end, it managed to survive three nights and made four recoveries. During this process, data, such as those regarding temperature trends in various parts of the lander were obtained, which is a major technological gain. "Temperatures rose by a little following the night. By investigating what changes occurred, we might gather clues as to why SLIM eventually stopped working as well as important knowledge for future lunar and planetary explorations. I want to continue this work," said Sakai.

When communication was not possible in late June when power should have been generated, it was considered possible that the onboard program had been overwritten by a "solar flare," arising from a massive explosion occurred on the sun surface in May. Efforts to establish communication, including sending the correct program, were fruitless. Although it is difficult to say for sure, it is believed that there is no effect from the flare.

SLIM carried two small robots: "LEV-1" and "LEV-2." They separated from SLIM at an altitude of approximately 5 m and worked on the lunar surface. LEV-1, developed by Chuo University and others, hopped around across the lunar surface and successfully communicated with Earth. LEV-2 from Takara Tomy and others could transform and move around and took pictures of SLIM on the lunar surface. The plan was to take the pictures of LEV-1 as well, but software-related problems arose, and it did not happen.

LEV-2 is the smallest and lightest lunar exploration robot in history, nicknamed "Sora-Q." JAXA was commissioned by the Japan Science and Technology Agency (JST) to conduct the "Innovation Hub Construction Support Project (an open innovation hub aimed at expanding the sphere of survival and domain of human activity through developing the solar system frontier)" and made this happen by signing a contract with Takara Tomy to collaborate on small robot technology and control technology. This robot captured the image of SLIM on the lunar surface, and JAXA Director General Hitoshi Kuninaka, director of the Institute of Space and Astronautical Science, who worked hard to make this happen, said, "Anyone who sees that photo can tell that SLIM is on the Moon. I'm really glad it worked out."

Long-cherished dream of landing on the Moon comes true

Japan had considered landing on the Moon in the past but could not achieve it. The "Kaguya (Selene)" probe, which orbited the Moon from 2007 to 2009, was planned to carry a lander in the early stages of development, but this was shelved due to the need to prioritize observation equipment. SLIM was developed through many twists and turns, based on the concept of the "Selene B" lander, which had been under consideration since around 2000 as a successor to the previous mission. Japan's long-cherished dream has finally come true.

In 2022, JAXA failed to reach the lunar surface with a parcel-sized ultra-compact lander called "Omotenashi." However, even if this mission was successful, it would be classified as hard landing, rather than soft and slow landing, which is what is generally considered to be a successful landing. It was a "semi-hard landing" with some sizeable velocity.

SLIM's achievements include the submission of approximately 20 academic papers in the field of engineering. In science, a spectroscopic camera was used to examine the "olivine" composition, which is exposed on the lunar surface from the subsurface mantle near the crater. Olivine is found in igneous rocks formed when magma cooled and solidified. Through comparisons with those found on the Earth and other factors, it may lead to an understanding of the origin and history of the Moon. A related paper is currently being submitted, and we look forward to learning more about the findings that can only be gained from pinpoint landing.

The SLIM surface also carried a mirror provided by the National Aeronautics and Space Administration (NASA). It reflects light in the direction it came from. Although SLIM itself is no longer in operation, the mirror will continue to be used for ranging and other purposes by spacecrafts orbiting the Moon.

Meanwhile, the space venture I-Space Inc. of Tokyo launched its second lunar lander from the United States on 15th of this month. The first lander misjudged its altitude and crashed onto the lunar surface just before landing in April 2023, and the company will make a second attempt at the end of May this year. SLIM's Sakai said, "As someone who was also attempting Moon landing, I am fully aware of the difficulty. I believe that success will lead to big things in a different sense for a private company. I hope that they will be successful this time and continue to take on new challenges," he said with encouragement.

Results are "treasures for the future"

Kuninaka, known for his dry evaluations, when asked to grade SLIM, said, "The landing was 60 points. LEV-1, LEV-2, and the spectrographic camera produced good results, scoring 1 point each. It made through three nights, so that's three points. We could come up with a tentatively plausible explanation for the shutdown of the ceramic thruster (main engine), and we also gained knowledge that can be passed on to future generations, such as data regarding the lander temperature after landing. These achievements are a treasure for the future, and I want to add three more points to make it 69," he smiled. How many points would you readers give it?

At the end of the conference, he was questioned about the significance of SLIM. When asked, Kuninaka compared it to the progress and prospects of solar system exploration: "In the Artemis program (an international collaboration to explore the Moon), a major theme is to use the Moon as a stepping stone to go to Mars. We have traveled to microgravity celestial bodies using Hayabusa and Hayabusa 2 (asteroid probes), and this time we have entered the world of the Moon, where gravity is one-sixth that of Earth. If we can refine this technology, knowledge, and experience, we can operate freely on the Moon, and we are gradually seeing the possibility of operating on the main planet, Mars (not a Moon of Mars, but one with a higher gravity). SLIM pioneered it."

Sakai noted, "We can leave behind many things, but if I have to pick one thing, it will be human resources." "Many of our mid-career and junior employees could experience the entire process of making things on-site, managing them, launching them, operating them, and landing them on the Moon. I look forward to seeing them do great work in various challenges ahead," he said with high expectations.

Since ancient times, Japanese people have admired the Moon, enhancing their imagination and honing their delicate sensibilities. "I am not from this country. I am from the Capital of the Moon" - The Moon has been depicted in countless literature works, including Taketori Monogatari (The Tale of the Bamboo Cutter (Princess Kaguya)), said to be the oldest story in Japan. It is also said that a rabbit is pounding rice cakes on the Moon. Although the truth of this is uncertain, it is said that Soseki Natsume translated "I love you" as "The Moon is beautiful, isn't it?" The fact that Japan has managed to achieve pinpoint landing in the current era, which requires great precision, seems to be the result of sensitivity cultivated while gazing at the Moon being somehow linked to technology. In the future, we will look back on the success of SLIM as an event that deepened the relationship between the Moon and Japanese people through the power of science and technology.