The SLIM Moon Landing: a Partial Success that Opens Up New Perspectives

On Jan. 19, 2024, at 3:30 pm UTC, the telemetry data indicates that the engines of the Japanese SLIM (Smart Lander for Investigating Moon) lander are turned off and that the vehicle is stationary.

Japan thus becomes the fifth nation in the world to succeed in placing its object on the lunar soil. 

However, no official statement was given after announcing that SLIM landed on the Moon. We had to wait until the press conference, two hours later, held by JAXA executives, to discover that the moon landing was a partial success.

What happened?

From the data received it emerged that the lander managed to land correctly and release the two small rovers, but that the solar panels were not charging the batteries correctly.

Probably, the vehicle landed on the ground and, due to the roughness of the terrain, it lay on its side, preventing the solar panels, positioned on the back, from receiving the correct solar radiation. We will know more in the next few days when the images of the two rovers LEV-1 and LEV-2 will arrive.

In the condition in which SLIM found itself operating with only battery energy, JAXA technicians decided to place the apparatus in low-consumption mode, trying to exploit the available time to maximize the results of the experiments to be conducted.

We will then wait for the next lunar dawn to understand if, with more favorable radiation, the mission can be resumed.

The long journey of SLIM

SLIM was launched from the Tanegashima space center on Sept. 6, 2023, on board a Mitsubishi H-II launcher together with the XRISM space observatory for lunar orbit.

The aim of this journey, which lasted four months, was to save fuel consumption by maintaining an adequate quantity of fuel until the end to allow the small Japanese lander, weighing only 590 kg, to carry out all the maneuvers necessary for a precision moon landing.

Landing where you want to land

This is the slogan of the JAXA space agency for the SLIM mission: a sophisticated terrain recognition system, very similar to the facial recognition systems in use on smartphones, allowed SLIM to verify the maps received from the SELENE satellite in lunar orbit and compare them with the terrain seen by the on-board cameras.

🧵1/n

After Chandrayaan 3 it's now time for 🇯🇵Japan's lunar lander which will be launching tomorrow (28 August) at 5:56am (IST).

The lander named SLIM(Smart Lander for Investigating Moon) will demonstrate high precision pinpoint landing for future missions like 🇮🇳🇯🇵LUPEX. pic.twitter.com/1WWIHegqlM

— TitaniumSV5 (@TitaniumSV5) August 27, 2023

This technique allowed a precision moon landing with a margin of error of only 100 meters. For a convenient comparison, the margin of error of the Apollo moon landings was a 20×5 km ellipse.

In the future, this system will allow us to carefully choose where an object will land on the surface of another celestial body using real-time mapping taken from an object orbiting around it.

Few tools, a lot of substance

Built by Mitsubishi, the same company that produces the H-II launcher, SLIM is 2.7 meters long, 1.7 meters wide, and 2.4 meters high.

Inside, little instrumentation: a multispectral camera, a magnetometer, and a miniaturized sensor for the detection of electrons and protons constitute the onboard instrumentation, completed by the moon landing system cameras, the communication antennas, and the moon landing radar.

Inside, two small rovers called LEV (Lunar Excursion Vehicle). LEV-1 will move in leaps on the lunar surface, and LEV-2, produced by a well-known Japanese toy manufacturer, is a rolling transformable robot the size of a baseball.

It was thanks to LEV-1’s ability to communicate autonomously with the Earth that it was confirmed that SLIM and its cargo were not lost. Twenty minutes after the moon landing, LEV-1’s radio signal was picked up from the Deep Space Network antennas in Spain.