Japan’s SLIM Lunar Lander Notches More Successes

Japan’s SLIM Lunar Lander Notches More Successes

The Japan Aerospace Exploration Agency confirmed today that the Smart Lander for Investigating Moon, or SLIM, achieved its primary objective of demonstrating pinpoint landing accuracy. By JAXA’s criteria, that alone makes SLIM fully successful even though an engine malfunction caused it to land upside down in a position where the solar cells cannot recharge the battery. It operated for only for a few hours, but they are holding out hope operations may be restored when the sun angle changes.

JAXA’s Institute for Space and Astronautical Science (ISAS) held a press conference today to update SLIM’s status and later issued English-language press releases summarizing the results. [UPDATE January 26: An English version of the slides used in the press conference is now available.]

Launched on September 6, 2023, SLIM landed on the Moon on January 19, 2024 Eastern Standard Time (January 20 in Japan). JAXA knew it soft-landed and was transmitting data back to Earth, but didn’t understand why the solar cells were not recharging the battery. That meant it could operate only for several hours instead of days. They used that time to get back as much data and imagery as possible enabling them to determine what happened.

Illustration of JAXA’s Smart Lander for Investigating Moon (SLIM) on the lunar surface. Note the solar cells on top of the lander. Credit: JAXA

SLIM landed 55 meters east of the targeted landing site, well within the 100 meter accuracy they hoped to achieve. JAXA added that the accuracy was even better — 10 meters or less — when SLIM was 50 meters above the surface before beginning obstacle avoidance maneuvers.  It might have been as good as 3-4 meters.

However, it landed in the wrong attitude with the solar cells pointing away from the sun. Studying the data, JAXA determined that one of the two engines failed just before the obstacle avoidance maneuver began. Onboard software compensated and was able to control SLIM’s horizontal position, but “lateral velocity and attitude were outside the design range, and this is thought to have resulted in a different attitude than planned.”

An image taken by one of two tiny Lunar Excursion Vehicles, LEV-2, ejected from SLIM before landing shows that it is upside down.

The solar cells are facing west “suggesting that there is the possibility for power generation and thus recovery of SLIM as the sunlight illumination condition improves with time.”

SLIM landed at 12:20 am Japan Standard Time on January 20. JAXA deliberately shut it down at 2:57 am JST.  In a thread on X on January 22, they said they disconnected the battery when it reached 12 percent to preserve the opportunity for it to recharge.

JAXA’s press kit for SLIM published the criteria they would use to determine if the mission met “minimal,” “full” or “extra” success. Demonstrating pinpoint landing accuracy was the main objective.

JAXA said they had achieved minimal success after soft landing.

With precision landing now confirmed, SLIM is at least a “full” success.

SLIM is equipped with scientific instruments and cameras. JAXA released an image from one of the two navigation cameras used during descent.


Image of the lunar surface captured by the SLIM onboard navigation camera after landing. Note: the image is rotated to align with the direction of gravity. Credit: JAXA

SLIM also carries a Multi-Band Camera or MBC to collect data on the surface. JAXA, Ritsumeiken University and the University of Aizu released an MBC image “created by synthesizing 257 low-resolution monochrome pictures” and began assigning names to various features for further study. They hope to obtain higher resolution images if SLIM resumes operations.

Source: JAXA press release

SLIM ejected two tiny Lunar Excursion Vehicles, LEV-1 and LEV-2, just before landing. LEV-1 was designed to move on the surface by hopping. LEV-2, also called SORA-Q,  is a “transformer” built by toy company Takara Tomy. Once opened, the sides became wheels allowing it to roll on the surface. LEV-1 has its own system to communicate with Earth independently of SLIM and also was a relay to LEV-2. As noted above, LEV-2 sent back the image showing SLIM is upside down.

JAXA reported that LEV-1 “executed planned leaping movements” and communicated with Earth and LEV-2, but acquiring surface imagery from LEV-1 “has not been confirmed as of now.”  LEV-1 has “completed its planned operational period on the lunar surface, depleted its designated power, and is in a standby state on the lunar surface.” They will continue to try to contact it.

“Both LEV-1 and LEV-2 have become Japan’s first lunar exploration robots. Additionally, the small LEV-1 with a mass of 2.1 kg (including a 90g communication device), achieved successful direct communication with Earth from the moon. This is considered as the world’s smallest and lightest case of direct data transmission from approximately 380,000 kilometers away.” — JAXA

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