On January 19th 2024, JAXA’s Smart Lander for Investigating Moon (SLIM) successfully touched down on the surface of the Moon. This makes Japan the fifth nation in history to conduct a successful soft landing on the surface of the Moon, following Russia, the United States, China, and India. 135 days after launching alongside the XRISM X-Ray Space Telescope, SLIM has demonstrated a new method for landing on the surface of the Moon. However, unexpected power problems have occurred, and it will take further time to confirm if the lander’s precision goals were met.
SLIM is primarily a low-mass technology demonstration. SLIM’s goal was to test a method for high-precision lunar landings using real-time image-based guidance software. The system works by matching images taken during descent to maps created by JAXA’s KAGUYA Moon orbiter and NASA’s Lunar Reconnaissance Orbiter in order to determine SLIM’s exact position relative to the Lunar surface. SLIM targeted a 100 meter landing ellipse location near Shioli Crater, which is itself southwest from Mare Nectaris, on the eastern side of the Moon’s Earth-facing hemisphere. The region is believed to contain exposed material from the lunar mantle, potentially yielding valuable insights into the Moon’s formation. However, the site is somewhat treacherous, specifically for its 15 degree slope. Additionally, SLIM weighs in at only 560 kg fully fueled, around half the weight of Astrobotic’s Peregrine lander. Not only did SLIM aim for a precise landing, but a mass-efficient one as well. To reach Lunar orbit with minimal fuel costs, SLIM did not take a direct path to the Moon. Rather, they used a Lunar flyby to place themselves on a distant post-Lunar arc, that then fell back and reencountered the Moon in December. This allowed SLIM to efficiently enter Lunar orbit on Christmas Day.
After its final orbit-lowering maneuver the day prior, SLIM began its 20 minute descent to the Moon’s surface on January 19th, 10:00 EST. SLIM’s descent to the surface consisted of three phases. The first was a powered descent from 15 kilometers followed by a brief coast where SLIM began its image-based navigation. Telemetry showed SLIM was very close to its predicted trajectory going into the next phase, a vertical descent which killed the last bit of horizontal velocity and placed SLIM on-course for its landing site. All of this was executed as expected and was followed by a brief final coast. SLIM then navigated to a safe position and assumed its expected orientation and slowly brought itself down to a hover just 2 meters above the Lunar surface, at which point the first Lunar Excursion Vehicle (LEV-1) was deployed. For the final touchdown, to save weight, SLIM does not employ traditional landing legs, but instead has five crushable “feet” made of metallic mesh. These are attached directly to SLIM’s body and are made to absorb the force of touchdown. SLIM fell the short final distance to the surface of the Moon, landing on its back foot and then tipping onto its front feet. This planned “two-step” approach to landing allowed SLIM to lean into the slope it landed on, and if all went to plan hopefully avoid tipping over into the wrong orientation.
Following the landing, commentators stated that while SLIM was confirmed to be on the Lunar surface, it was taking time to assess the health of the lander, and landing coverage would be ending, but a press conference regarding the status of the spacecraft would occur later on the same stream. Meanwhile, NASA’s DSN Now website showed downlinking from SLIM and LEV-1 occurring in Madrid Spain. A YouTube livestream from the private Bochum Observatory in Germany was able to independently verify signals from both SLIM and its first rover, and even detect some amount of activity from the spacecraft.
It took two hours for a press conference to begin, wherein it was quickly stated that SLIM had successfully landed on the Moon, established two-way communication with Earth, and deployed both rovers on the surface. According to experts, however, SLIM is not generating power from its solar panels. JAXA has stated they intend to maximize the time they have while operating on limited battery power. Further information on the cause of the problem, possible solutions, and activities conducted in the meantime are expected to be disclosed in an additional press conference later this week. It is possible, though speculative, that SLIM did not land in the correct orientation for its solar arrays to receive power from the Sun, however this cannot be confirmed at this time. SLIM has deactivated its heating elements to conserve power, and is prioritizing work to download its descent telemetry and imagery in order to better assess the nature of the problem. Battery power is expected to last for “several hours” according to Sasaki Hiroshi, JAXA Vice President and Director of Human Spaceflight Technology.
The Bochum Observatory observed a loss of SLIM’s signal at around 17:50 UTC, the LOS was also seen at the DSN. It is unclear if this was an intentional break in transmission, or if the SLIM spacecraft has already lost power. Space Scout will provide updates on SLIM’s condition and conclusion when more information is disclosed at next week’s conference. For now, it can be stated, regardless of the duration, that Japan has become the fifth nation to successfully land and operate spacecraft on the lunar surface, and their concept for precise low-mass landings on the Moon has proven itself as a solid basis.
The guidance technology SLIM demonstrated will see use on an Indian-built Moon lander as part of the Lunar Polar Exploration (LUPEX) mission, a joint mission between JAXA and the Indian Space Research Organization, that will deliver a Japanese ice-hunting rover to the Lunar south pole, comparable to NASA’s VIPER. Several more Moon landings are scheduled for 2024. The next will be Intuitive Machines’ NOVA-C lander set to launch for its first mission in February.