Japan’s Lunar Landing Was Lopsided—And Transformative
Japan’s SLIM lander has ushered in a new era of precision landings, with implications for lunar science and exploration.
By Jatan Mehta
An image of JAXA’s Smart Lander for Investigating Moon (SLIM) in its final landing position, its engine facing upwards, on the lunar surface. Data artifacts appear as horizontal bars and blocks of color. Although lopsided, SLIM’s lunar landing was the most precise in history.
JAXA/TOMY/Sony Group Corporation/Doshisha University
When Japan’s solar-powered SLIM spacecraft made a lopsided-but-successful touchdown on the moon in mid-January, most news coverage focused on the feat as a historic first for the nation—only the fifth after the U.S., the former Soviet Union, China and India to ever achieve a soft lunar landing. But the most historic aspect of SLIM—which stands for Smart Lander for Investigating Moon—wasn’t the mere fact of its landing but rather its remarkable circumstances. Designed by the Japanese Aerospace Exploration Agency (JAXA), the mission used autonomous navigation to demonstrate an unprecedentedly precise landfall in unusually treacherous lunar terrain, coming to rest close to the center of its boulder-strewn and crater-pocked target site despite losing one of its two main engines during its descent.
The landing is a milestone in lunar exploration that will undoubtedly lead to more as future missions use lessons from SLIM to pull off precise landings of their own to quickly access points of scientific or technological interest upon touchdown.
“With SLIM, we have shown that access to express questions of clinical interest on the Moon can now also be achieved with small missions, instead of relying on traditional and more expensive missions to transport giant rovers that travel long distances to be successful. in the desired areas,” says Masaki Fujimoto, deputy director general of the JAXA Institute of Space and Astronautical Sciences.
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SLIM’s achievements are best understood through what engineers call the “landing ellipse,” which is the internal domain in which a spacecraft must land. In an ideal world, a lander would succeed in the middle of this ellipse, heralded as the target landing. place. In reality, even the smallest navigation, orientation, and errors make it almost certain that the landers will miss the target. The distance between a lander and the center of the ellipse defines the accuracy obtained.
JAXA set an ambitiously tight landing ellipse of 100 by 100 meters for SLIM, and the spacecraft managed to touch down about 55 meters from its center. This makes SLIM the most precise planetary landing in history by a considerable margin. The runner-up is China’s Chang’e 3 lander, which in 2013 landed some 90 meters off-center from its landing ellipse; but Chang’e 3’s landing ellipse also spanned an area 3,600 times as large as that of SLIM, notionally giving the Chinese spacecraft vastly more room for error—a distinction that matters for mission planners.
“Precision landings allow you to reliably examine more types of rocks and soils, making planes an optimal landing point. This way you can land, for example, in a fusion region and then sail smoothly to a marine basalt,” says Benjamin Farcy, a postdoctoral researcher at NASA’s Goddard Space Flight Center. “Without precision landings, a typical rover with a diversity of kilometers might not cover either lithology. “
At the center of SLIM’s precision landing capability is a generation that JAXA calls “vision-based navigation. “To determine its location and descent trajectory, SLIM took pictures of the moon’s surface to compare them in near real time with the orbit. maps preloaded into your memory. These maps come from NASA’s Lunar Reconnaissance Orbiter (LRO), the Indian Space Research Organization’s (ISRO) Chandrayaan-2 orbiter, and JAXA’s SELENE spacecraft. Each set of cards presented its own benefits for the other stages of SLIM. descent. For example, Chandrayaan-2’s High Resolution Orbital Camera (OHRC) presented the highest space solution and was therefore very useful for the final descent phase of SLIM, as well as for the pre-launch variety of a landing site.
“JAXA sent us a request in 2021 to image SLIM’s landing site, and so we planned an orbital pass-over of the Chandrayaan-2 orbiter to acquire images for [the agency],” says Amitabh Singh, head of ISRO’s Planetary and Space Science Data Processing Division and a lead scientist on the OHRC team. Later in 2023 ISRO provided JAXA with more images of the landing site from different sun angles, which helped engineers tune SLIM’s algorithms to instantly identify boulders and other hazards. Even though modern robotic landers, including India’s own Chandrayaan-3, can self-identify a good landing spot during the final phase of a landing, the lack of an apt last-mile map affects the landing precision.
When SLIM was about 50 meters above the lunar surface, it hovered for a moment and checked for landing hazards beneath it using vision-based navigation with photographs from Chandrayaan-2. But at this point, one of the two main engine nozzles mysteriously indifferent and the off-center thrust caused SLIM to drift. SLIM’s steering formula learned of this anomaly and descended using photographs captured during the hover phase while frequently seeking to stabilize its orientation with the other main engine and smaller thrusters. While SLIM’s vertical velocity of 1. 4 meters was consistent with the timing of landing being within appropriate limits, the eastward lateral motion and landing orientation were not nominal, so it was employed to tilt over the surface with its solar panels facing the sun. Even when affected, SLIM can simply see and aim for a safe position to land. and avoid an accident.
Combined with evolving tools for planetary missions, precision landings can help answer some basic questions that in the past required complex and expensive pattern return missions. In a 2021 paper published in Planetary Science Journal, Farcy and his team suggested that by using precision landing techniques, even cheap missions like those of NASA’s Commercial Lunar Cargo Services program can reap enormous rewards by targeting sites very specific of wonderful clinical interest. where in situ measurements (rather than expensive acquisition of specimens for research on Earth) can simply explain some difficult-to-understand chapters of lunar history.
But that’s not to say that precision landings don’t also gain advantages in the pattern of previous missions. Such efforts tend to be undertaken without a reason to lessen the complexity and burden of missions, as recently observed with China’s Chang’e 2020. Fifth mission, which brought home 1. 7 kilograms of geologically young lunar volcanic material. A precise landing can generate certain high-fidelity clinical effects for desktop landers by allowing them to reach and collect exactly what they came for.
A sample return project to the South Pole Aitken Basin (SPA), the largest and innermost basin on the Moon, has been assessed as one of the most sensible priorities for exploration over the past three years. of planetary science and astrobiology. Surveys. This report is produced every 10 years through the US clinical network to advise NASA’s long-term projects. Scientists believe they have an effect on the hollow, scattered component that created the SPA of the moon’s interior. “The SPA is the only position where we believe the lunar mantle is exposed to the surface,” says Farcy. “If you need to examine those materials, a precision landing will allow you to better target them. “
Sampling key SPA spaces will help reveal the exact conditions under which the moon formed and evolved. To this end, China will launch the Chang’e 6 project this year to recover approximately two kilograms of SPA samples. There’s also NASA’s Artemis program, which aims to send humans to the lunar south pole in late 2026. Although none of the recently planned crewed landings are targeting the SPA, scientists have learned of a few Artemis applicant landing sites that they could only accommodate mantle curtains. expelled through impacts. in the SPA. Since the first two crewed Artemis landings, Artemis III and IV, will not bring a rover, the lack of a targeted precision landing capability may limit astronauts to collecting gadgets within a few steps of their spacecraft. However, starting with Artemis V, plans call for crews to operate the Lunar Ground Vehicle, potentially allowing them to reach sites up to 20 kilometers away.
Even then, exact landings could prove imperative for Artemis missions to access water ice within the permanently shadowed regions of the South Pole, one of the program’s main ones. The terrain here is rocky with steep slopes, with only small, remote, flat spaces on which a lander can land. There, deviating from the target by a few meters can also mean the difference between a triumphant landing near the rim of a crater that harbors water ice or a fatal fall from its walls. .
Of course, Artemis astronauts will most likely be able to descend their lander, just like their Apollo-era predecessors, but even then, automated features would help reach vital milestones. In addition, NASA recently asked SpaceX and Blue Origin to get started. running dispatch versions of its manned lunar landers for long-duration Artemis missions. These will require SLIM-type automated precision landings to reliably deliver materials to astronauts, starting with Artemis VII in the 2030s. SLIM’s descent into a rock-prone region and over a non-trivial slope was thus a useful emulation of the facets of a polar landing.
In fact, for its next moon mission, which is planned to launch before the end of the decade, JAXA is partnering with ISRO to have its Lunar Polar Exploration Mission (LUPEX) rover directly study the nature, abundance and accessibility of water ice at a polar landing site. To safely and precisely land LUPEX amid treacherous polar terrain, ISRO will build the lander with input from both Chandrayaan-3’s success and that of SLIM.
For ISRO, the SLIM case also shows how Chandrayaan-2 can provide complex orbital insights to NASA to better clear candidate Artemis landing sites, especially as the LRO, introduced in 2009, ages gracefully and becomes obsolete. According to Singh, the OHRC stereo photographs from Chandrayaan-2 may prove especially important in ensuring the fate of the Artemis landings, as they offer “a point-height solution four times higher than the LRO photographs,” allowing Incoming spacecraft better discern topography. of the land. .
At last year’s annual meeting of the NASA-backed Lunar Exploration Analysis Group in September 2023, LRO project scientist and science lead for Artemis III, Noah Petro, reaffirmed this collaborative value by saying, “[The Chandrayaan-2 orbiter] data is helping build on the LRO foundation by filling important needs, and we’re very much looking forward to more data from the mission.”
Jatan Mehta is a globally published independent space exploration writer and author of Moon Monday, a newsletter dedicated to covering lunar exploration developments from around the globe.
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