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The Sun Sets on SLIM – What Next?

After a troubled journey to the lunar surface and subsequent recovery, the sun has set on JAXA’s Smart Lander for Investigating Moon (SLIM) at 1:57 PM, February 1st, Japan Standard Time. Officially, SLIM has entered a dormant period, and the intent to attempt continued operations once the sun begins shining on SLIM’s solar cells again has been declared on the official page of the SLIM mission on the site formerly known as Twitter. Details on the precise nature of the difficulties faced during landing and the fruits of its limited operation on the lunar surface have been disclosed in the time since Space Scout’s last report on the mission. While the mission is in its dormant period and the chances of the mission continuing remain unclear, SLIM’s mission has accomplished its goals, and has wide reaching implications for JAXA’s future ambitions at the Moon and beyond.

An artistic rendition of SLIM in its nominal landing orientation. Note the solar arrays facing upwards and twin main engine nozzles facing down-slope.
Credit: JAXA

JAXA held a press conference on the 25th of January which detailed the events of SLIM’s descent to the lunar surface. Telemetry shown live during the landing displayed the SLIM spacecraft remaining remarkably on-course, the lander even seemed to adjust itself onto course after starting slightly offset in the early phases of landing, and no problems were indicated by mission commentators until after the spacecraft finally touched down. However, imagery released during this conference showed that one of SLIM’s main engine nozzles fell off while the spacecraft was making its way down to the surface. This occurred during the final descent phase, around 50 meters above the surface.

A pair of images taken during SLIM’s descent shown during the January 25th Press Conference. The images show the progression of lunar terrain as SLIM approaches its landing site. The second image notes the addition of an engine nozzle and three smaller pieces of debris falling away from the lander.
Credit: JAXA

While SLIM was able to compensate for the issues it encountered during powered descent, a power issue befell SLIM after it had landed and began communicating back to Earth. As many had speculated, this was confirmed by JAXA to be due to SLIM landing in the incorrect orientation. SLIM landed “nose-down” and with its solar panels facing away from the Sun. Because battery power was limited, SLIM only had a short timeframe in which to conduct as much of its planned operations as it could on the lunar surface. 

Both of SLIM’s Lunar Excursion Vehicles (LEV), LEV-1 and LEV-2 were deployed and both operated autonomously on the lunar surface. LEV-2, developed by the Tomy Company, Sony, and Doshisha University, is a small ball-shaped rover. After being deployed from SLIM, LEV-2 unfolded into a motile configuration which allowed it to crawl through the sandy lunar regolith and expose its two cameras, and captured imagery of the surrounding landscape. Working with a projected battery life span of two hours by design, LEV-2 maximized what it could accomplish in its limited time by automatically capturing images and deducing which images are most valuable to send back. According to JAXA researcher and designer of LEV-2, Daichi Hirano, the rover was able to determine which of its images contained the SLIM spacecraft, and prioritize its return.

SLIM as photographed by the LEV-2 rover shortly after landing on the Moon. Note the presence of only one large engine bell facing upwards.
Credit: JAXA

The process to transmit the sensational image of SLIM on the lunar surface was in and of itself an impressive technology demonstration, as LEV-2 does not carry a powerful enough transmitter to send images directly to Earth. Instead, LEV-2 transmitted the imagery to its larger counterpart, LEV-1, which carried communication equipment powerful enough to return the image directly to Earth. LEV-1 is a larger rover which moves by hopping across the surface and also has the capacity to automatically capture and transmit imagery. Reportedly both rovers together managed to capture and transmit 275 images from the lunar surface, though most of these images are unreleased, and no images from LEV-1 have been released at time of writing.

While the pair of rovers managed to complete their short missions, at the same time power on the main spacecraft continued to drain, and when battery power reached 12% the spacecraft automatically switched into hibernation. Knowing that SLIM was functional, and the power problem was only a matter of incorrect orientation, the possibility of continuing the mission presented itself. Enough power was left in SLIM’s batteries to enable the spacecraft to reactivate once the Sun began shining on the other side of the spacecraft and power could be generated again. This occurred nine days after landing on the 28th of January, 2024. 

SLIM is largely a technology demonstration, and as such carries only a light scientific payload primarily consisting of a multiband camera with the goal of identifying olivine, as the material is believed to originate from the lunar mantle. Olivine found by SLIM could shed light on the history and nature of lunar volcanism. This was used to capture imagery of the rubbly-slope on which SLIM landed, and identified several notable rocks. SLIM also carries a laser retroreflector array provided by NASA’s Goddard Spaceflight Center, which is used by NASA’s Lunar Reconnaissance Orbiter to reveal SLIM’s precise location on the Moon.

SLIM’s imagery of its landing site, the notable rocks featured have been named after breeds of dog.
Credit: JAXA

The major question however, was in regards to SLIM’s primary mission: achieving a precise landing on the lunar surface. To perform its landing on the lunar surface, SLIM determined its location based on maps generated by both the Japanese KAGUYA and Indian Chandrayaan-2 spacecraft, both orbiters. Comparing imagery from the Chandrayaan-2 spacecraft with navigational photos taken by SLIM during its descent allowed mission team members to roughly determine SLIM’s final landing site. SLIM targeted a landing ellipse with a radius of 100 meters, and despite the loss of an engine resulting in a high lateral velocity, SLIM managed to touch down roughly 55 meters away from its target site. This level of accuracy according to JAXA is considered a successful demonstration of the spacecraft’s precise landing technology, had the loss of an engine not occurred it’s entirely possible the landing may have been even more precise.

A comparison between images taken by SLIM to guide itself to the surface (overlayed) and Chandrayaan-2 imagery used to construct SLIM’s internal map of the site (underlaid).

SLIM continued to use its multi-spectral camera to seek out signs of olivine, focusing on the rock named “Toy Poodle” by mission team members, until sunset three days later on February 1st. JAXA’s stated intent to continue the SLIM mission should it survive the lunar night mirrors statements made by ISRO last year regarding their Chandrayaan-3 lander, Vikram. Neither craft were explicitly designed with the Moon’s harsh nighttime conditions in mind, and while it remains to be seen if SLIM will indeed survive – Vikram did not, which casts doubt on the small lander’s prospects.

SLIM’s final image at time of writing. The image shows the progression of dark shadows across SLIM’s landing site as the Sun begins to set.
Credit: JAXA

SLIM’s successful mission to the Moon in spite of multiple challenges raises another appropriate analogy, this time to JAXA’s own Hayabusa mission which launched back in 2003. The mission collected and returned samples from asteroid Itokawa in 2010, and overcame many setbacks ranging from mechanical and software failures to simple bad luck. However, following in Hayabusa’s footsteps was Hayabusa2, which performed a much more ambitious mission to return samples from Ryugu and operate several rovers on its surface flawlessly. The spacecraft continues to operate to this day as Hayabusa2# (Hayabusa2-”SHARP,” Small Hazardous Asteroid Reconnaissance Probe) on an extended mission to investigate two more near-earth asteroids. With this in mind the question becomes this; what missions will be flowing in SLIM’s footsteps?

As described by Hirdy Miyamoto from the University of Tokyo at the 30th meeting of the Small Bodies Assessment Group in late January, JAXA’s Martian Moons eXploration (MMX) mission will be utilizing the pinpoint landing technique proven by SLIM to perform precise landings on Mars’ moons Phobos. MMX aims to answer questions regarding the origins of Mars’ two small moons by returning samples from Phobos. MMX is planned to launch in 2026, and hopefully kickstart a line of Japanese explorers sent to the red planet. Additionally, the Lunar Polar Exploration Mission (LUPEX), a joint mission between JAXA and ISRO will see SLIM’s precise landing technology applied to an Indian moon lander, which will deliver a Japanese rover to the lunar south pole in search of water ice. LUPEX is tentatively set to launch in 2025. 

For JAXA to fully realize these missions in the wake of SLIM’s success, the new H3 launch vehicle must enter operation. The MMX mission was originally set to launch in 2024, but was delayed due to the failure of H3’s first launch. H3’s vital return to flight is currently scheduled for Thursday, February 15th.

An artistic rendering of MMX on the surface of Phobos, with Mars behind it.
Credit JAXA

JAXA also has long-term commitments to delivering large payloads, namely a pressurized rover, to the lunar surface for use by NASA’s Artemis Program, in return for Japanese astronauts on Lunar Gateway and the lunar surface. SLIM represents both a step towards the system that would deliver such a rover to the surface, and a complimentary ability to access high-risk lunar terrain that such a rover could not safely access. Though SLIM has once again entered dormancy on the surface of the Moon, and even if the chances of it reawakening are low, the mission has already potentially demonstrated the spark of a new era of space exploration for Japan, the Moon, and the international community.

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