Landing SLIM and Smartly on the Moon

A Japanese spacecraft is headed for the Moon, on course to attempt a pinpoint landing using technology that transforms the process of descending onto the lunar landscape from finding a place where it is easy to land to touching down where you want to land.

Japan’s Smart Lander for Investigating Moon (SLIM) was launched September 7, departing Earth atop an H-IIA launch vehicle from the Yoshinobu Launch Complex at the Japan Aerospace Exploration Agency’s (JAXA) Tanegashima Space Center.

SLIM — also called “Moon Sniper” — successfully transitioned into a lunar transfer orbit early last month, and the vehicle is slated to touchdown near Shioli crater sometime early in 2024.

Assuming the mission succeeds, Japan will become the 5th country to soft land a spacecraft on the surface of the Moon.

Technology-driven target

The SLIM project is led by members of the JAXA Institute of Space and Astronautical Science (ISAS), and researchers from universities and other institutions across the country.

The technology-driven mission includes a baseball-sized, spherical rover developed in Japan by space program experts and a toy manufacturer. Once it is deployed, this baseball-sized device will crawl across the harsh lunar surface, swinging from side to side to propel itself forward. But first, it must reach the surface intact.

The SLIM team is targeting a landing within roughly 240 feet of the ejecta blanket of Shioli crater. As it drops down over the site, the SLIM will descend almost vertically while detecting its altitude with landing radar. As it descends, it will begin to scan for obstacles at low altitude, just above the target zone.

SLIM is outfitted with high-tech gear and special software to process images captured by its onboard camera gear. The plan is to spot and identify craters, and then compare those features with a pre-loaded map embedded in the spacecraft’s memory to precisely compute the probe’s own position.

SLIM engineers have developed a dedicated image processing algorithm with high computational efficiency to achieve both accuracy and processing time.

SLIM’s landing is a sporty proposition.

Firing its main engine in the opposite direction of its travel direction around the Moon, SLIM will go into a powered descent phase. During that time, there will be a total of four “coasting” periods of about 50 seconds each.

During a coast period, SLIM will use camera imagery for quick processing to estimate its position and speed with high accuracy. At the same time, the onboard computer will recalibrate its trajectory toward the landing point.

At a little above 10 feet and with main engine shutdown, SLIM will auto-pilot itself to landing. SLIM is equipped with a mechanism to absorb the impact during landing at its five touchdown legs.

What all this adds up to is putting the smarts into a smart lander weighing a little over 460 pounds when it sits down on the Moon.

Running modes

The SLIM lander aims to achieve a small-scale, lightweight probe system and pinpoint landing technology – but that’s not all.

SLIM is to deploy a palm-sized Lunar Excursion Vehicle 2 (LEV-2) jointly developed with toy manufacturer, Takara Tomy, along with the Sony Group Corporation and Doshisha University.

The ball-shaped vehicle — SORA-Q — is equipped with a camera system and can transform its shape to traverse the lunar surface.

SORA-Q will be released from the lander as a sphere, then starts to transform, using its left and right wheels that either move separately or together. The tiny explorer can use its onboard imaging technology to take pictures of the lander and the surrounding moonscape.

SORA-Q’s wheels allow it to move in two types of running modes, “butterfly running” and “crawling running,” according to the Takara Tomy company.

The popular toy manufacturer is already offering a SORA-Q “Flagship Model” for public purchase.

Shioli: shining science

Once it reaches its destination, the rover will get a unique look at the Moon, including a rare glimpse of material brought from under the surface by an ancient impact.

Moon explorer Mark Robinson at Arizona State University’s School of Earth and Space Exploration notes that Shioli crater formed a little over 7 miles away from the rim of the 62-mile diameter Theophilus crater on ejecta emplaced during the Theophilus-forming impact.

In the case of Shioli crater, Robinson advised Parabolic Arc, the bright ejecta seen around the rim originated from over 60 feet beneath the Moon’s surface. “Since it comes from depth, the ejected material was protected from the effects of space weathering, which darkens and changes the color of the regolith. Over time, the bright ejecta will fade into the background as the surface weathers.”

Thus, all that tossed out material around Shioli crater originated from beneath the surface. “As a result, SLIM will provide an opportunity to explore a cross-section of the crust,” Robinson pointed out.

As JAXA and ISAS teams note, SLIM will be invaluable in honing future landing skills, not only for the Moon, but on other deep space locations.