Japanese moon lander SLIM safe on lunar surface, but …

Watch yesterday’s Japanese moon lander – SLIM – press conference from JAXA. Video via JAXA.

SLIM makes soft landing on the moon

The Japan Aerospace Exploration Agency (JAXA) safely landed its Smart Lander for Investigating the Moon – aka SLIM – on the lunar surface. Japan thus became the 5th earthly nation to reach the moon. So, Japan achieved its main objective, landing softly on the lunar surface. But all was not well with the uncrewed craft. Its solar array suffered a glitch. Last we heard, JAXA was rushing to gather data from the moon’s surface, before SLIM goes silent. Mission control is still assessing data to determine if the lander met the second goal, a pinpoint landing within 100 meters (330 feet) of its target.

JAXA issued a quick statement:

The Japan Aerospace Exploration Agency (JAXA) has confirmed that the Small Lunar Lander Demonstration Vehicle (SLIM) landed on the lunar surface at 0:20 a.m. on January 20, 2024 (Japan Standard Time/15:20 UTC on January 19). Communication has been established after landing.

However, the solar cells are not generating electricity, so we are currently prioritizing data acquisition from the lunar surface.

In the future, we will proceed with detailed analysis of the obtained data. We will keep you updated on the future situation.

That’s all.

They also provided a composite video of SLIM’s view of the lunar surface as it descended.

This is an animation made by connecting the images taken by the navigation camera during the Far Moon Descent Maneuver (ADM)!

This is what the landing should look like.

SLIM’s journey to the moon

The Japanese Aerospace Exploration Agency (JAXA) lunar lander reached the surface of the moon during a live broadcast on Friday, January 19, 2024. The Smart Lander for Investigating the Moon (SLIM) descended until circling only 9.3 miles (15 km) above the moon. Then at around 12 a.m. JST Saturday, January 20 (15 UTC or 9 a.m. CST on January 19), SLIM began its landing. Touchdown came 20 minutes later.

The lander entered orbit about a month ago, JAXA said on December 25, 2023:

The lunar orbit of SLIM will be inserted into an elliptical lunar orbit connecting the moon’s north and south poles with a period of approximately 6.4 hours, and altitude of about 370 miles (600 km) at the closest point to the moon (perilune), and 2,500 miles (4,000 km) at the farthest point from the moon (apolune). The orbit change proceeded as planned, and the spacecraft is currently in a normal condition.

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Against a background of black sky, a boxy golden spacecraft fires retro rockets in front of gray lunar hills.
This artist’s concept depicts the JAXA Special Lander for Investigating the Moon (SLIM) deploying the Lunar Excursion Vehicle (LEV). Image via JAXA.

It began its descent Monday

On Monday (January 15) JAXA reported the craft was ready to begin a slow downward creep:

It was therefore decided to move to the landing descent preparation phase on January 10, 2024, and the apolune (the farthest point from the moon) descent maneuver was successfully executed and completed on January 14, 2024, at 17:32 JST (08:32 UTC). The SLIM spacecraft is further confirmed to be inserted into a circular orbit at the planned altitude of approximately 370 miles (600 km).

Oval orbital paths around moon with colored lines and black background.
Artist’s concept of the JAXA SLIM lunar lander’s orbital path. The green line shows its previous circular orbit at 370 miles (600 km) above the surface. The yellow line represents its 370 x 95 mile (600 x 150 km) orbit. The red line shows how SLIM moved to an orbit that carried it to just 9.3 miles (15 km) over the moon. Image via JAXA.

Then came the big moment on Friday, the landing.

SLIM’s mission objectives are simple yet difficult

SLIM had basically two tasks to accomplish at the moon, and the first one was just making it to the lunar surface. But not just anywhere on the moon. SLIM’s navigation systems were designed to put the craft within 100 meters of its intended target in Mare Nectaris, the Sea of Nectar. Specifically, it landed near the small impact crater Shioli.

Previous landers were considered on target if they touched down within a few kilometers of their landing zone. JAXA’s SLIM aimed to bull’s-eye the moon using “vision-based navigation” and “navigation, guidance and control.” JAXA designed a three-step process:

1. Initiate the landing descent from lunar orbit and perform precise vision-based navigation to accurately estimate its own position. Utilizing navigation, guidance and control, it will approach the target location above the lunar surface.

2. From above the target location, precise measurements of altitude and terrain-relative velocity will be conducted using the landing radar, which will be integrated into the navigation and guidance system.

3. During the final approach, autonomous image-based obstacle detection and avoidance will be employed to ensure a safe landing, avoiding hazardous rocks and other obstacles.

The second objective was more of a proof-of-concept for SLIM’s small, lightweight design. It’s a compact vehicle, only about 6.5 feet (2 meters) tall and 5 feet (1.5 meters) wide, and weighing just 250 pounds (120 kg). The design is intended to allow more frequent landings on the moon and other planets. So far, the high-performance chemical propulsion system has worked perfectly, nudging SLIM along its way on this shakedown cruise.

A series of 5 images of a yellow spacecraft tipping over onto the gray lunar surface.
Artist’s concept shows the steps of JAXA’s SLIM landing on the surface of the moon. The small craft was meant to land on a slope, intentionally tipping over. Image via JAXA.

JAXA lander planned to tip over

SLIM has a unique approach to landing. It was planned to tip itself onto its side in what JAXA calls a two-step landing. The craft hovered toward the lunar surface, tipping itself to about a 45-degree angle before its main leg touches down. The craft then pitched forward onto a its “front” leg, located at what was moments earlier the top of the craft. We’re still uncertain if the craft followed these exact steps leading to its landing.

The unusual touchdown was meant to allow SLIM to land on a 15-degree slope. This will be an important ability for self-guided landers on future missions to the moon and other solar system destinations featuring uneven and sloping surfaces. In theory, JAXA says the method works well:

Especially for the case of a SLIM-scale spacecraft, the ‘two-step landing method,’ in which the main landing gear first touches the ground and then rotates forward to stabilize, has shown excellent reliable landing results through simulation.

JAXA will continue to share updates once it learns more about the health and operation of SLIM. The landing success was much needed after two private efforts – HAKUTO-R and Peregrine Mission One – failed.

Bottom line: The JAXA Special Lander for Investigating the Moon (SLIM) landed successfully on the lunar surface on January 19, 2024. But the solar panels aren’t working.


Read more: HAKUTO-R – 1st private moon lander – loses contact

Read more: Failed lunar lander hit Earth’s atmosphere on Thursday

January 20, 2024

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