SLIM, Japan’s precision lunar lander

Highlights

• SLIM aims to test highly accurate landings that can boost the science value of missions
• This JAXA mission uses a long, looping route to the Moon to save fuel and mass.
• A successful landing would make Japan the fifth country to land on the Moon

Japan has its sights set on the Moon. And thanks to the innovative navigation and landing systems, these sights are set on a very particular area with the aim of demonstrating “pinpoint” landings. The experimental mission could be proof of concept of a lightweight, accurate Moon landing spacecraft. And if it does succeed, it will make Japan the fifth country to soft-land on the Moon, just months after India became number four with Chandrayaan-3.

The Japan Aerospace Exploration Agency’s (JAXA) Smart Lander for Investigating Moon (SLIM) launched late on Sept. 6, 2023, aboard a H-IIA rocket from the Tanegashima spaceport in Japan. Instead of making a direct shot for the Moon, SLIM entered an initial low Earth orbit as part of a joint launch with the XRISM X-ray space observatory, starting an unusually long, but worthwhile voyage to the Moon.

Ground teams then conducted a range of checkouts before the spacecraft used its engines to raise its orbit. On Sept. 30 the spacecraft made its translunar injection burn, setting it on a unique course to our celestial neighbor. Instead of making the large braking burn made by most Moon-bound spacecraft, SLIM made a lunar flyby, imaging the Moon on Oct. 4 on its way past. SLIM’s smart, fuel-conserving trajectory sees it fly out into deep space on a long, looping orbit that will bring it back close to the Moon at the end of December.

This lunar approach will allow SLIM to return to and slip into orbit around the Moon with a smaller braking burn. This, crucially, allows SLIM to save fuel, mass, and cost. SLIM will spend around one month in lunar orbit as it prepares for landing.

The SLIM spacecraft cost an estimated 18 billion yen ($120 million) to develop. It weighs 200 kilograms (440 pounds) dry, or roughly 700 kilograms (1,540 pounds) when loaded with fuel at launch. This fuel will be used for the descent and landing, as well as the earlier maneuvers to get it into lunar orbit. 

The target landing site is the rim of Shioli crater. SLIM intends to land within 100 meters (330 feet) of its target point, as opposed to the much larger traditional landing ellipses (areas of probable landing) that are on the order of kilometers in terms of length and breadth.

Descent will start from 15 kilometers (9.3 miles) with SLIM traveling at 1,700 meters per second or 3,800 miles per hour.

The spacecraft will attempt this precision landing using an optical navigation system loaded with maps built using data from Japan’s Kaguya, or SELENE, lunar orbiter, which launched in 2007. The preloaded images will be matched with data collected during SLIM’s descent, allowing it to locate itself and guide the spacecraft to its target landing site. This process needs to be done speedily and autonomously, for which JAXA has developed special image processing algorithms.

The spacecraft also has a landing radar for determining its altitude during descent. A laser range finder will measure the SLIM’s altitude during the final few meters of descent before landing. The spacecraft is furnished with five crushable, 3D-printed aluminum lattice landing legs to aid its landing. These will absorb much of the impact of landing, and also help it land with a correct orientation on the crater slope.

While impressive in their own right, this mission’s landing ambitions are also key to the future of scientific lunar exploration. Global interest in the Moon is growing, with many nations and commercial entities entering the field. As lunar exploration advances, so will the need to target specific sites to address salient science questions.

SLIM’s mission architecture hopes to shift the standards of lunar landing missions, from touching down where it’s easy to setting down exactly where desired. The main objective of the mission is to demonstrate these landing capabilities.

But while SLIM is flying light, it still packs payloads for carrying out intriguing science on the lunar surface. Shioli is an impact crater about 300 meters (984 feet) wide, within the larger Mare Nectaris (“Sea of Nectar”). It is a mid-latitude site, offering good lighting, but has been chosen for its science potential.

SELENE data suggests that the mineral olivine is present and exposed on Shioli’s slopes. This is noteworthy as it may be material ejected from within the Moon’s mantle, which could hold clues to the mysteries relating to the Moon’s formation and evolution. The Multi-Band Camera (MBC) aboard SLIM will determine the composition of olivine by analyzing the spectra of sunlight reflected off the olivine.

SLIM is also carrying a couple of small, unconventional rovers. Lunar Excursion Vehicle 1 (LEV-1) moves using a hopping mechanism and comes equipped with wide-angle optical cameras and direct-to-Earth communications equipment. Its science payloads are a thermometer, radiation monitor, and an inclinometer for measuring slopes and elevation.

Also aboard is the baseball-sized, spherical rover named Lunar Excursion Vehicle 2 (LEV-2), developed by JAXA in collaboration with Tomy, Sony, and Doshisha University. The two halves of the 250-gram (0.55-pound) spacecraft will separate, revealing a pair of cameras and a stabilizer. The rover will crawl across the harsh lunar surface, swinging from side to side to propel itself forward.

SLIM is expected to operate for a single lunar day on the Moon. Without radioisotope heater units to provide warmth, as deployed on earlier lander and rover missions from NASA, Roscosmos, and the China National Space Administration, the intense cold of the very long lunar night is expected to render its electronics inoperable — as experienced recently by India’s historic Chandrayaan-3 Vikram lander and Pragyan rover.

However, if it sticks the landing and gets to carry out its science, SLIM will make history and provide new insights into the history of the Moon. The tech and expertise gained will be inherited by future missions such as JAXA’s Martian Moon eXploration (MMX) mission, which aims to collect samples from Phobos and deliver them to Earth.