Japan’s SLIM Successfully Lands on Moon, But Will Have Limited Lifetime

Japan became the fifth country to successfully soft land on the Moon today. The Small Lander for Investigating Moon, SLIM, touched down as scheduled, but its solar cells are not delivering power. That means the lander’s useful lifetime is limited to how long the battery lasts. Nonetheless it is transmitting data back to Earth as are two tiny probes that were ejected just before landing.

Developed by the Japan Aerospace Exploration Agency (JAXA), SLIM was launched on September 6, 2023 and entered lunar orbit on Christmas Day. Since then it’s been waiting for the right time to descend to the surface. That was today. Landing was at 10:20 am Eastern Standard Time (12:20 am January 20 in Japan).

JAXA webcast the landing and all appeared to go well. JAXA’s webcast hosts looked somber, however, and they asked viewers to remain patient while they assessed the spacecraft’s health.

The lander ejected two tiny rovers, Lev1 and Lev2, when it was 2 meters above the surface. Lev1 can communicate directly back to Earth and relay signals from Lev2. Experts and amateurs on X who were monitoring communications with SLIM and Lev1 reported signals were being received so JAXA’s delay in declaring success was mystifying.

At a press conference almost two hours later, JAXA President Hiroshi Yamakawa declared SLIM was on the Moon, but then turned to Hitoshi Kuninaka, head of JAXA’s Institute for Space and Astronautical Sciences (ISAS), to provide the disappointing news that the solar cells are not sending power to the battery.

That means SLIM’s useful lifetime is limited to the life of the battery, which is only several hours. Kuninaka said they had turned off the heater to conserve power so the battery would last as long as possible and permit critical data and images to be transmitted.

SLIM’s main objective is to demonstrate pinpoint landing accuracy using optical navigation. The spacecraft compares previously acquired images from Japanese and U.S. lunar orbiters stored in its memory with what its sensors were seeing in real time.  The lander chose its own route to the landing site in the SHIOLO crater near the Sea of Nectar, stopping to hover several times to make sure it knew where it was and avoid rocks. Some of the data JAXA is eager to get back is exactly what path SLIM followed.

JAXA set the criteria for assessing whether SLIM is successful in advance and published it in the SLIM press kit.

Kuninaka and Yamakawa said they met the criteria for  “Minimum” success.

The pinpoint accuracy criteria was to land within 100 meters of the predetermined spot. It will take some time for them to analyze data to find out how close they got, but Kuninaka said he’s pretty confident they did. That would give them “Full” success. He acknowledged “Extra” success seems out of reach now.

Lev1 and Lev2 were working at least initially. Lev1 can move on the surface by hopping. Lev2 is a baseball-sized “transformer” built by toy company Takara Tomy. When it opens, the sides become wheels and it can roam around.

JAXA said they will have another press conference next week with an update on the accuracy of the landing.

Kuninaka said they do not know why the solar cells are not supplying power to the batteries. They do not think they were damaged in the landing and have not determined whether the spacecraft’s orientation is the problem — if it landed upside down, for example. He holds out hope that in the course of time as the sun’s angle to the lunar surface changes, it just might be possible for some rays of sunlight to reach SLIM and rejuvenate it.