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.