SLIM

Mission objectives

SLIM had two primary objectives set by JAXA. The first was to demonstrate high-precision ('pin-point') lunar landing technology — reducing the landing ellipse from the several-to-tens-of-kilometres typical of past landers to a target accuracy of approximately 100 metres. Achieving this required vision-based navigation using onboard cameras to match real-time descent imagery against pre-loaded maps derived from the SELENE (Kaguya) lunar orbiter, a laser landing radar for final altitude measurement, and an advanced autonomous navigation, guidance and control system. A two-stage landing sequence and metal shock absorbers at the contact points completed the landing architecture.

The second objective was to prove that a lightweight, compact lander architecture could support precise lunar and planetary landings. SLIM was built to show that small, high-performance chemical propulsion and reduced-mass subsystems — covering tanks, engines, structures, power, and navigation — could enable frequent, cost-effective missions beyond Earth orbit.

Spacecraft design

SLIM is a compact, box-like lander optimised for mass reduction and autonomous guidance. Its wet (fuelled) mass at launch was approximately 700–730 kg, reducing to around 200 kg after propellant consumption. Main propulsion came from two ~500 N class bipropellant thrusters burning hydrazine (N₂H₄) and MON-3 oxidiser. Attitude control was provided by twelve 22 N thrusters.

Power was generated by thin-film solar cells emphasising lightweight design. Communications used an S-band antenna. The navigation suite comprised dedicated navigation cameras (CAM-PX) for image-based guidance and a landing radar for final-phase altitude measurement. The guidance algorithm applied image-processing techniques derived from facial-recognition technology to identify and match lunar craters against the onboard Kaguya-derived map, enabling continuous autonomous position updates during descent.

Landing site: Shioli crater

SLIM targeted the rim of Shioli crater, on the western edge of Mare Nectaris (Sea of Nectar), at approximately 25.2°E, 13.3°S. The site was selected because the crater rim is believed to expose olivine-rich material from the lunar mantle, making it scientifically valuable for studying the Moon's internal structure. The topographically complex terrain also made it an appropriate test for precision landing — a conventional large landing ellipse would have been scientifically and operationally impractical there.

During its autonomous descent, SLIM recognised craters and terrain features in real time, updated its estimated position relative to the pre-loaded Kaguya maps, and selected a safe touchdown point close to the designated spot. NASA's Lunar Reconnaissance Orbiter (LRO) imaged the SLIM lander on 24 January 2024, documenting its position from orbit.

Landing accuracy and engine anomaly

SLIM's vision-based navigation performed to a high standard throughout the powered descent. JAXA's evaluation at approximately 50 metres altitude recorded hover-point offsets of 3.4 metres and 10.2 metres from the target, indicating a demonstrated guidance accuracy of approximately 10 metres or better, with best performance in the 3–4 metre range.

In the final phase of descent, one of SLIM's two main engines appears to have shut down, causing the spacecraft to drift. As a result, the actual touchdown point was approximately 55 metres from the originally intended landing site. This remained well within the 100-metre mission requirement. JAXA stated that without the engine anomaly, SLIM could have landed as close as 3–4 metres from its target.

SLIM also came to rest at an abnormal attitude — essentially nose-down — leaving its solar panels misoriented and unable initially to generate power. The lander operated for several hours on battery power before being intentionally powered down with approximately 12% battery remaining, to preserve the possibility of later recovery. The abnormal attitude was confirmed by imagery from the deployed LEV-2 rover.

Power recovery and surface operations

After landing, JAXA determined that SLIM's solar cells were facing west and that power generation might resume when sunlight reached them from that direction. On 28 January 2024 — more than a week after touchdown — JAXA re-established contact and confirmed the spacecraft had resumed operations as changing illumination geometry allowed sunlight to reach the panels.

SLIM subsequently survived multiple lunar day and night cycles, continuing imaging and spectral measurements of surrounding rocks. Contact was lost again by late May 2024 as lighting conditions worsened. JAXA officially concluded the mission on 23 August 2024. Total surface operations spanned approximately 3 months and 10 days (intermittent, constrained by power and attitude).