SLIM attempted to land on the Moon with a precision of less than 100 meters.
STEPHEN CLARK – UPDATED 1/19/2024, 5:42 AM
The Japanese space agency’s first lunar lander appeared to reach the Moon’s surface Friday, but officials were still “checking the status” of the spacecraft more than an hour after it touched down.
Japan’s robotic Smart Lander for Investigating Moon (SLIM) mission began a 20-minute final descent using two hydrazine-fueled engines to drop out of orbit. After holding to hover at 500 meters and then 50 meters altitude, SLIM pulsed its engines to fine-tune its vertical descent before touching down at 10:20 am EST (15:20 UTC).
The Japan Aerospace Exploration Agency (JAXA), which manages the SLIM mission, streamed the landing live on YouTube. But the stream of updates stopped after SLIM’s landing, and the webcast hosts did not provide any update on the status of the spacecraft before ending live coverage about 10 minutes later.
JAXA officials will hold a press conference later Friday. The SLIM spacecraft targeted a landing adjacent to a nearly 900-foot (270-meter) crater named Shioli in a region called the Sea of Nectar on the near side of the Moon.
SLIM launched September 6 on top of a Japanese H-IIA rocket, riding to orbit alongside an X-ray astronomy telescope. The spacecraft took a long route to get to the Moon, trading time for fuel to preserve propellant for Friday’s landing attempt. SLIM entered orbit around the Moon on December 25, then completed several maneuvers to settle into a low-altitude orbit in preparation for the descent to the surface.
If successful, the landing of SLIM would make Japan the fifth country to soft-land a spacecraft on the Moon, following the Soviet Union, the United States, China, and India. But landing on the Moon is a hazardous thing to do. Three commercial landers similar in scale to SLIM failed to safely reach the lunar surface over the last five years.
One of those was developed by a Japanese company called ispace. Most recently, the US company Astrobotic attempted to send its Peregrine lander to the Moon, but a propellant leak cut short the mission. After looping more than 200,000 miles into space, Peregrine reentered Earth’s atmosphere Wednesday, where it was expected to burn up 10 days after its launch.
A Russian lander crashed into the Moon in August, and India’s first lunar lander failed in 2019. India tried again last year and made history when Chandrayaan 3 safely landed.
Japan’s mission is a pure technology demonstrator to test our new guidance algorithms and sensors, rather than pursuing scientific objectives. The technologies riding to the Moon on SLIM could be used on future spacecraft bound for the Moon. SLIM cost the Japanese government approximately 18 billion yen ($121 million) to design, develop, and build, according to JAXA.
The 20-minute descent on Friday was the the riskiest part of the entire mission. The spacecraft is modest in size, measuring nearly 8 feet (2.4 meters) tall and nearly 9 feet (2.7 meters) across. Without propellant in its tanks, SLIM has a mass of roughly 660 pounds (200 kilograms).
“The start of the deceleration to the landing on the Moon’s surface is expected to be a breathless, numbing 20 minutes of terror!” said Kushiki Kenji, sub-project manager for the SLIM mission, before the landing.
A new way to land
First, the spacecraft ignited its two engines for a retrograde burn to slow the lander’s velocity enough to be pulled to the surface by lunar gravity. As it slowed down, SLIM flipped into a configuration to enable a top-down vertical final approach. During this process, SLIM’s advanced guidance system received data from a landing radar and an optical rangefinder. This told the spacecraft’s control system how high it was above the surface.
The lander’s vision-based navigation system was designed to autonomously guide SLIM toward its target landing zone. Engineers loaded SLIM’s computer with maps of craters surrounding the landing site, using imagery collected by orbiters flying overhead. SLIM’s down-facing cameras were expected to take pictures of the Moon’s charcoal-gray surface, and the spacecraft’s computer was to compare the locations of craters to the pre-loaded orbiter imagery. Working autonomously, SLIM’s computer was also supposed to look for hazards and obstacles such as boulders or steep craters.
Theoretically, this method should have allowed SLIM to settle onto the lunar surface within about 300 feet (100 meters) of its bullseye. If it worked according to plan, the accomplishment would be a remarkable improvement in lunar landing precision, which typically is measured in miles or kilometers. This would be an enabling capability for future Moon missions because it lays the foundation for future spacecraft to land closer to lunar resources, such as water ice.
The precision navigation technology tested on SLIM caused some Japanese space officials to nickname this spacecraft the “Moon sniper.”
Thanks to observations from orbiting spacecraft like NASA’s Lunar Reconnaissance Orbiter, India’s Chandrayaan, and Japan’s Kaguya, scientists have created high-resolution lunar maps showing topography, interesting geologic features, and resources like water ice that astronauts could harvest.
“Therefore, interest in lunar science and resource exploration has shifted from ‘somewhere on the Moon’s surface’ to ‘that rock next to this specific crater!’” Kushiki said last year.
In order to get to more intriguing locations on the Moon, future landers must be able to navigate themselves to safe touchdown zones near craters, ridges, or mountains. Many previous lunar landing missions have targeted locations far away from these rugged features to ensure they had a wide expanse to safely settle onto the surface.
followed by a 90-degree slip as the spacecraft settles onto the Moon’s surface.
In the final moments before touchdown, SLIM was expected to release two small robots to drop onto the lunar surface. If everything worked, these robots could have been in position to take pictures of the lander just as it reached the Moon. JAXA calls the robots Lunar Excursion Vehicles (LEVs). One has a hopping mechanism designed to travel short distances across the lunar landscape.
The other is shaped like a ball, with a diameter of about 3 inches (80 millimeters). Once on the Moon, this robot will change its shape and move across the surface. Its design resembles that of a Transformer toy, and guess what? Japanese toymaker Tomy Company, founder of the Transformers brand, partnered with Japan’s space agency on this little gadget.
Just 10 feet (3 meters) over its landing site, SLIM was supposed to briefly hold in a hover position before shutting off its main engines and using smaller thrusters to rotate itself. This would allow SLIM to drop to the surface on the weight of five crushable landing legs—two under SLIM’s upper body, two at the rear, and one extending toward the back of the lander.
“As SLIM descends vertically downwards, it is this back leg that first touches the lunar surface,” JAXA said in a mission overview posted before the launch of SLIM. “In the second step, the spacecraft tips forwards, rocking onto the upper support legs before stabilizing on the lunar surface. In a nominal case, the rear support legs do not touch the Moon, but can prevent SLIM from tumbling sideways in the case of a more tricky landing.”
This novel two-step landing sequence will be useful for safely landing future spacecraft on steep slopes. Mission managers expected the landing site for SLIM to have an incline of about 6 to 7 degrees.
SLIM was not designed to survive a cold, Sun-free lunar night, so if it remains healthy after the landing, the mission will operate for the rest of the 14-day-long lunar day. Then, starved of power and heat, it will shut down for good.
This story was updated on January 19, 2024, after the landing of the SLIM spacecraft on the Moon.