Japan’s SLIM lands on the Moon, power issues cast doubt on lander’s survival

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Japan’s Smart Lander for Investigating Moon (SLIM) lunar lander appears to have achieved a soft touchdown on the Moon’s surface but is currently unable to generate electricity from onboard solar cells. With the spacecraft running only on internal battery power, controllers are working to balance the recording and downloading of data, utilizing the remaining battery life to maximum effect.

The reason for the lack of electrical generation is not currently known, but officials report that the solar cells do not appear to be damaged and that all other systems are working as designed. There is some optimism that sunlight may reach the solar cells and recharge the batteries, but this is by no means guaranteed. The mission has been officially marked as reaching its minimum success levels at this time, subject to further data analysis.

Four months after launching from Tanegashima Space Center, SLIM, led by the Japan Aerospace Exploration Agency (JAXA), made a landing near Shioli, an impact crater within the larger Mare Nectaris (“Sea of Nectar”) at 11:20 PM JST on Jan. 19, 2024 (15:20 UTC).

SLIM has been on a unique voyage involving two close approaches to the Moon and orbits, which took it out into deep space, traveling for 110 days before making a lunar insertion burn on Christmas Day into a Lunar Orbit of 15 x 600 kilometers. Further burns have slowly lowered and circularised the orbit. The long looping trajectory was designed to save vital fuel and mass for the landing phase, giving the spacecraft the best possible chance for a soft landing.

Japan was hoping to join the small group of countries that have soft-landed on the Moon, along with Russia, the United States, China, and, more recently, India. Japan has had two previous attempts at landing on the Moon — Omotenashi and Hakuto-R — both of which failed. There have also been several other notable failed attempts at a Lunar landing in recent years:

Omotenashi was a small 6u cubesat launched from Artemis 1, which had issues with solar cell orientation and was lost. Hakuto-R was a commercial lander designed by the Japanese company iSpace. This lander crashed into the Moon on April 25, 2023, following confusion between the various navigation systems leading to fuel exhaustion.

Previously, in 2019, Israel’s Beresheet lander was another failure, crashing onto the Moon’s surface after a gyroscope failed, resulting in loss of control of the craft. More recently, the Astrobotic Peregrine One lunar lander was unable to attempt a Moon landing following an anomaly in the propulsion system, which, in turn, caused problems keeping the spacecraft in a Sun-facing orientation. Propellant leaking from the system added to the issues, ruling out any landing attempt. The leak slowed down as the days passed, which allowed controllers to concentrate on allowing Peregrine to complete as much science as possible. Astrobotic was able to maintain transparent and informative communication with the public throughout the mission. Controllers steered the lander carefully and responsibly to avoid creating any space debris, bringing it onto a course to intercept Earth’s atmosphere, where it burned up safely over the Pacific on Jan.18, 2024.

SLIM’s primary mission was to demonstrate that advanced navigation and radar systems can provide a pinpoint landing within 100 meters of any given target. To achieve this vastly improved accuracy, the spacecraft carried several advanced instruments, including a laser range finder and a landing radar. During its descent to the lunar surface, the lander was able to compare the terrain below to high-resolution imagery gathered from Japan’s previous lunar orbiter, Kaguya, and NASA’s Lunar Reconnaissance Orbiter, to make autonomous real-time decisions about its speed and course using image processing algorithms developed by JAXA. Accurately targeted landings are seen as vital for ensuring optimum results from future landers, but the final position of this landing may not be known for some weeks, according to JAXA officials.

The target for landing, the Shioli crater, is an impact crater about 300 meters wide and has important scientific potential, not least for the suspected presence of the mineral olivine, which is conjectured to comprise part of the Moon’s mantle. The near-infrared Multi-Band Camera aboard SLIM will determine the composition of olivine by analyzing the spectra of sunlight reflected off the lunar surface. The data will further inform scientists about the early formation of the Moon.

The second major objective for this mission is the “realization of a lightweight lunar and planetary probe system to allow more frequent lunar and planetary exploration missions.” To this end, SLIM has been through thoughtful weight reduction, using modern construction techniques, and a powerful chemical-based thruster system.

The structural core of the spacecraft is the integrated fuel tank, a cylinder that holds both fuel and oxidizer using a common dome to save mass. The oxidizer portion has a specially developed form of Polytetrafluoroethylene lining to prevent reaction.

There are two main engines, built by Mitsubishi Heavy Industries, Ltd. KYOCERA Corporation. These feature ceramic combustion chambers, a very wide thrust range, and can fire using advanced pulsing techniques to aid precision positioning. The engines each provide around 500 newtons of thrust and are used for the main insertion burns during transit to the Moon and the descent/landing burn. Thrusters used to provide attitude control were built by IHI AEROSPACE Co. Ltd. There are 12 of these thrusters, each with a thrust rating of around 20 newtons. They use the same fuel/oxidizer as the main engines to aid in the lightweight design by avoiding the need for additional fuel tanks.

Graphic showing SLIM’s landing procedure. (Credit: JAXA)

The solar panel arrays for SLIM are built by the SHARP Corporation and are made of thin, light, and flexible film. These are designed to bend around some of the curved surfaces of the craft and are attached simply with velcro in some places.

SLIM was designed to land on a slope of approximately 15 degrees from the horizontal. As the spacecraft approached its landing with the main engines pointing downward to reduce the rate of descent, the thrusters were expected to tip the craft over to about 45 degrees, so that the main landing legs touch down first and then the craft completes the rotation into horizontal mode with the auxiliary legs touching down last. The legs all have a crush pad of 3D-printed aluminum alloy to absorb any impact stresses during touchdown.

Just before landing, SLIM ejected two small automata Lunar Excursion Vehicles (LEV), LEV-1 and LEV-2. These will explore and photograph the surrounding environment and the lander, each utilizing novel experimental propulsion techniques. LEV-1 is designed to hop frog-like around the Lunar surface, not only taking photos but also measuring slope, elevation, temperature, and radiation of the local lunar environment. This rover is also capable of direct communication back to Earth.

LEV-2 has been developed by JAXA in collaboration with Tomy, Sony, and Doshisha University, Japan. Weighing a mere 250 grams and only eight centimeters in diameter, this baseball-shaped vehicle has been inspired by co-developer Tomy’s Transformers toys. The initial design concept had to be reduced in size and weight to meet the limitations imposed by the mission’s lightweight ideals. On landing, the ball split apart to form two wheels and reveal pop-out cameras and a stabilizer. The method of forward movement, a waggling motion was inspired by that of the sea turtle. This collaboration with Tomy is intended to inspire children to dream big, and indeed a toy version of SORA-Q as LEV-2 is also known, is being sold in Japan.

LEV ejection

SLIM ejects two LEV’s onto the Moon. (Credit: JAXA)

LEV-1 will beam data from both LEVs back to the Deep Space Network stations on Earth.

Before the landing, the information available suggested that SLIM is a limited lifetime proof of concept lander, and was expected to function only until the Sun sets on the landing site after a maximum of 14 Earth days. At this point, the spacecraft would lose all power, and the electronics would likely be damaged by the extreme cold of the lunar night. However, since the issue with the electrical supply became known, the JAXA officials are suggesting that the lander could be put into sleep mode until the Sun lights the cells again.

There has been an upturn in interest in landing on the Moon, and there are several more landing attempts due in the near future. NASA’s Commercial Lunar Payload Services (CLPS) is providing incentives for commercial partners to demonstrate reliable cargo delivery capabilities to support the Artemis mission. CLPS has already produced the Peregrine One mission, which has provided data, and experience and generated public interest despite its failure to land.

Intuitive Machines is hoping to succeed with its first Moon landing attempt when its Nova-C lander launches as early as February 2024 on a SpaceX Falcon 9. Nova-C is described as being the size of a telephone box and carries 130 kilograms of payload, mostly instrumentation for NASA, but also a cubesat and a deployable camera called EagleCAM. Intuitive Machines has three missions for Nova-C under the CLPS contract.

Japan has a second Hakuto-R mission planned for NET Q4 2024. This mission, named Resilience, will launch on a SpaceX Falcon 9 and will feature a micro-rover.

(Lead image: Render of the SLIM Lander on the Moon. Credit: JAXA)



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