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A Japanese spacecraft faceplanted on the Moon and lived to tell the tale

Japan, JAXA, moon, slim, Space / /
Japan's SLIM spacecraft is seen nose down on the surface of the Moon.

Japan’s SLIM spacecraft is seen nose down on the surface of the Moon.

Japan’s first lunar lander made an unsteady touchdown on the Moon last week, moments after one of its two main engines inexplicably lost power and apparently fell off the spacecraft, officials said Thursday.

About the size of a small car, the Small Lander for Investigating Moon (SLIM) landed on Friday, making Japan the fifth country to achieve a soft landing on the lunar surface. Shortly after landing, ground teams in Japan realized the spacecraft was not recharging its battery with its solar panels. The evidence at the time suggested that SLIM likely ended up in an unexpected orientation on the Moon, with its solar cells facing away from the Sun.

With the benefit of six days of data crunching and analysis, officials from the Japan Aerospace Exploration Agency (JAXA) briefed reporters Thursday on what they have learned about SLIM’s landing. Indeed, the spacecraft toppled over after touching down, with its nose planted into the lunar regolith and its rear propulsion section pointed toward space.

It turns out that SLIM overcame a lot to get to that point. In the final minute of Friday’s descent, one of SLIM’s two engines failed, leaving the craft’s sole remaining engine to bring the spacecraft in for an off-balance landing. Still, JAXA officials said the spacecraft achieved nearly all of its primary objectives. The roughly $120 million robotic mission made the most pinpoint landing on the Moon in history, just as it set out to do.

“From the spacecraft, we were able to acquire all the technical data related to navigation guidance leading to landing, which will be necessary for future pinpoint landing technology, as well as navigation camera image data during descent and on the lunar surface,” JAXA said in a statement.

One of two tiny robots released by SLIM just before landing relayed a remarkable image of the lander standing upside down a short distance away. This might be the first close-up view of a crash landing, however gentle, on another world.

One plucky bird

Based on the update JAXA released Thursday, it’s extraordinary that SLIM made it to the surface in one piece.

After launching in September and arriving at the Moon in December, SLIM lined up for a final descent to the lunar surface on Friday. Around 20 minutes before landing, the spacecraft ignited its two hydrazine-fueled rocket engines for a braking maneuver to drop out of lunar orbit.

JAXA officials said everything went according to plan in the initial phases of the descent. The spacecraft pitched over from a horizontal orientation to begin a final vertical descent to the surface. SLIM’s guidance computer was preloaded with a map of the landing zone, and an onboard navigation camera took pictures of the Moon’s surface throughout the landing sequence. The spacecraft’s computer used these images to compare to the map, allowing SLIM to autonomously correct its course along the way.

The SLIM spacecraft was built by Mitsubishi Electric under contract with JAXA.

Enlarge / The SLIM spacecraft was built by Mitsubishi Electric under contract with JAXA.

JAXA

But at an altitude of around 160 feet (50 meters), something went wrong with the spacecraft’s propulsion system. Less than a minute before touchdown, one of the engines suddenly lost thrust, and moments later, a down-facing navigation camera caught a glimpse of what appeared to be one of the engine nozzles falling away from the spacecraft. JAXA said engineers believe the engine failure was likely caused by “some external factor other than the main engine itself.” Officials are still investigating to determine the root cause.

The spacecraft continued descending on the power of its remaining engine, but it became more difficult to control the lander. The thrust from the single engine imparted a sideways motion to the spacecraft. Normally, SLIM would have used thrusters to tilt itself from the vertical orientation necessary for the final descent and into a position to plop itself on the lunar surface along the spacecraft’s long axis. SLIM had five crushable landing legs to absorb the force of the gentle impact.

While this two-stage landing sequence was the plan, JAXA said Thursday that the spacecraft “touched the ground in an almost straight standing position with lateral velocity.” The vertical speed at touchdown was about 3.1 mph (1.4 meters per second), slightly slower than the expected descent rate.

“Because the ground contact conditions such as lateral speed and attitude exceeded the specification range, a large attitude change occurred after touchdown, and the aircraft settled in a different attitude than expected,” JAXA said.

In other words, the squirrelly landing caused the spacecraft to tip over. SLIM settled in a bottoms-up position on a shallow slope rather than on its side. Its solar panel wasn’t facing up but was instead pointed toward the west, away from the Sun’s position in the eastern morning sky at the landing site.

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Japan becomes the fifth nation to land a spacecraft on the Moon

Japan, JAXA, lunar exploration, moon, slim, Space / /
Artist's illustration of the SLIM spacecraft on final descent to the Moon.

Enlarge / Artist’s illustration of the SLIM spacecraft on final descent to the Moon.

The Japanese space agency’s first lunar lander arrived on the the Moon’s surface Friday, but a power system problem threatens to cut short its mission.

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. About two hours after the touchdown, JAXA officials held a press conference to confirm the spacecraft made a successful landing, apparently quite close to its target. SLIM aimed to settle onto the lunar surface adjacent to a nearly 900-foot (270-meter) crater named Shioli, located in a region called the Sea of Nectar on the near side of the Moon.

But ground controllers at JAXA’s Sagamihara Campus in the western suburbs of Tokyo soon discovered the lander was in trouble. Its solar array was not generating electricity after landing, and without power, officials expected SLIM to drain its battery within a few hours.

In what could be the mission’s final hours, engineers prioritized downloading data from SLIM, including imagery taken during its descent, and potentially new pictures captured from the lunar surface. Official reported good communications links between SLIM and ground stations on Earth.

“Minimum success”

Even if SLIM falls silent, the mission has achieved its minimum success criteria, JAXA said. The SLIM mission is a technology demonstrator developed to verify the performance of a new vision-based navigation system needed for precision Moon landings.

“First and foremost, landing was made and communication was established,” said Hiroshi Yamakawa, JAXA’s president. “So a minimum success was made in my view.”

One of the core goals of the SLIM mission was to land within 100 meters (about 330 feet) of its bullseye. This accomplishment would be a remarkable improvement in lunar landing precision, which typically is measured in miles or kilometers. It would also 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.

Hitoshi Kuninaka, director general of JAXA’s Institute of Space and Astronautical Science, said it will take about a month for engineers to fully analyze data from SLIM and determine the precision of the landing.

“But as you saw on the real-time data livestream, SLIM did trace the expected course, so my personal impression is that we probably have been able to more or less achieve a high precision landing within 100-meter accuracy,” Kuninaka said. “So the solar cell state is unlikely to impact the full success criteria.”

Kuninaka said ground teams have seen no evidence of any damage to the solar array on SLIM. It’s possible the lander is sitting in an orientation with its solar cells facing away from the Sun. All other components of SLIM, including its propulsion, thermal, and communication systems, all appear to be functioning well.

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.

A milestone moment for Japan

The landing of SLIM made 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.

This artist's illustration shows the SLIM spacecraft descending toward the Moon and ejecting two deployable robots onto the lunar surface.

Enlarge / This artist’s illustration shows the SLIM spacecraft descending toward the Moon and ejecting two deployable robots onto the lunar surface.

Japan’s SLIM mission was primarily designed to test out 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 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.

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