On December 25, Japan’s Smart Lander for Investigating Moon (SLIM) reached the moon. It had been travelling for months. It plans to try landing on the moon on January 19. If it works, Japan will be the fifth country to softly land a robot there. This will happen months after India’s successful Chandrayaan-3 mission in August. SLIM’s success or failure could also impact India’s upcoming Chandrayaan-4 mission also.
Understanding SLIM
SLIM is a spacecraft from Japan. The Japan Aerospace Exploration Agency (JAXA) built and launched it. It was launched from Tanegashima spaceport on September 7, 2023. It was lighter than most other spacecraft, weighing in at just 590 kg. In contrast, Chandrayaan-3 weighed 3,900 kg.
- Launch Partners and Delays: SLIM was launched with XRISM, an advanced X-ray space telescope. They both set off on an H-2A rocket. The pair were supposed to be launched together. However, problems with XRISM caused a delay. SLIM’s launch was postponed from 2021 to 2023.
- SLIM’s Lunar Orbit: SLIM entered a special lunar orbit on December 25. It’s an elliptical orbit. This means it varies in distance from the moon. At its farthest, it is 4,000 km away. At its closest, it’s just 600 km away.
- Competition and Attempts: SLIM’s launch came right after some notable events. India’s Chandrayaan-3 mission had just succeeded. Russia’s Luna 25 spacecraft had just failed. Furthermore, this was Japan’s second attempt in the same year to soft-land on the moon. The first attempt was with the HAKUTO-R M1 lander. Unfortunately, it crashed in April due to issues with its engines.
SLIM’s Journey To The Moon: A Lighter Approach
SLIM is lighter than Chandrayaan-3 because it carries less fuel. Out of Chandrayaan-3’s total weight of 3.9 tonnes, 2.1 tonnes was just the propulsion module. Thanks to this lighter weight, the mission began on July 14 and reached the moon in less than a month. This quick journey was possible due to the Hohmann transfer orbit route.
- The Longer, Fuel-efficient Route: Unlike Chandrayaan-3, SLIM took a more extended, fuel-saving route based on weak-stability boundary theory. This route made the journey four months long. After the launch, SLIM circled Earth multiple times. Each orbit increased its kinetic energy. Once it reached the necessary speed, SLIM rushed towards the moon’s orbit. Chandrayaan-3 followed a similar path until this point.
- The Final Approach: A Crucial Difference: As Chandrayaan-3 neared the moon, it used its brakes. In space, slowing down requires fuel. This process allowed the spacecraft to be captured by the moon’s weaker gravity. SLIM, however, took a different approach. Instead of slowing down near the moon, it used the deflection caused by the combined gravitational pull of both the Earth and moon. This allowed SLIM to continue its journey deeper into space, even as it passed the lunar orbit. This deflection theory was first applied in a late 1980s JAXA mission, known as ‘Hiten’.
Landing Precision
In the history of space exploration, precision in landing is crucial. Downrange and cross-range distances are used to measure deviation from the planned path. NASA’s ‘Curiosity’ rover had a mission to land in Mars’ Gale Crater within a 20 km x 7 km ellipse. It deviated by 2.4 km from the center.
- Record-Breaking Moon Landings: China’s Chang’e 3 spacecraft holds the record for the most precise moon landing with a deviation of only 89 meters. However, it still landed within a permissible 6 km x 6 km ellipse.
- SLIM’s Moon Endeavour: SLIM is all set to break records on January 19. It aims for the smallest area tolerance ever for a soft-landing on the moon. The chosen site is near the Shioli Crater. In its mission, it will use data from JAXA’s SELENE orbiter, similar to how ‘Vikram’ lander of the Chandrayaan-3 mission used data from Chandrayaan 2.
- Design and Manoeuvrability: Due to its lower mass (120 kg excluding fuel), SLIM is highly manoeuvrable. This feature, along with its compact design, will be put to the test.
- Lunar Excursion Vehicles: Before landing, SLIM will deploy two small rovers, LEV-1 and LEV-2. These Lunar Excursion Vehicles, along with SLIM, will gather data on temperature and radiation, and study the lunar surface near the landing point and the moon’s mantle.
Understanding SLIM’s Impact On Chandrayaan-4
The moon’s South Pole region holds great interest for scientists. The reason is some parts of the craters in this region never receive sunlight. This lack of sunlight allows for very low temperatures and protects these areas from daily temperature changes. These parts are known to contain water-ice.
- Chandrayaan-3: A Successful Leap: On August 23, ISRO successfully landed a robotic craft on the moon’s surface. This marked the successful execution of the Chandrayaan-3 mission. It also signalled the end of the second phase of ISRO’s lunar exploration programme.
- Chandrayaan-4: A New Venture: Next on the agenda is the Lunar Polar Exploration (LUPEX) mission. Also known as Chandrayaan-4, this will be the first mission of ISRO’s third phase. Notably, Chandrayaan-4 will be a collaboration between India and Japan. Although JAXA, the Japanese space agency, has greenlit LUPEX, India’s approval is still pending. If everything goes as planned, the earliest launch date will be in 2026. This mission will take us closer to the moon’s south pole than Chandrayaan-3 did, which is particularly significant.
- Lunar Poles Terrain: The area near the moon’s poles is challenging. It is filled with rocks, multiple craters, and steep slopes. Large landing modules or rovers may struggle to find an adequate landing spot, a situation different from the Chandrayaan-3 mission.
- Importance of SLIM Technologies: The technologies tested by JAXA with SLIM will play a critical role in LUPEX. The feature-matching algorithm and navigation systems, in particular, are essential for landing closer or directly at the selected site.
- JAXA and India’s Contribution: Presently, JAXA is set to provide the rocket and the lunar rover. India, on the other hand, will supply the lander module.
- Landing Site: The final decision on the landing site is pending. For comparison, the ‘Vikram’ lander touched down 600 km away from the South Pole.
Source: The Hindu