SMART LANDER FOR INVESTIGATING MOON (SLIM)

• SLIM, the Smart Lander for Investigating Moon, is a spacecraft developed and launched by the Japan Aerospace Exploration Agency (JAXA) on September 7, 2023, from the Tanegashima spaceport.
• Weighing only 590 kg at launch, SLIM is considerably lighter than Chandrayaan-3, which weighed 3,900 kg at launch, albeit with a more extensive suite of instruments.
• It was launched alongside XRISM, a next-generation X-ray space telescope, using an H-2A rocket. Delays in the readiness of XRISM led to SLIM's launch date being shifted from 2021 to 2023.
• SLIM successfully entered an elliptical orbit around the moon, with an apogee (farthest point) of 4,000 km and perigee (closest point) of 600 km above the lunar surface.
• Noteworthy is the timing of SLIM's launch, occurring just two weeks after the successful surface component of India's Chandrayaan-3 mission and the failure of Russia's Luna 25 spacecraft.
• This mission also represents Japan's second attempt this year to soft-land on the moon, following the unfortunate crash of the HAKUTO-R M1 lander, developed by Japanese company ispace, in late April due to engine shutdown issues during landing.

3. SLIM's Journey to the Moon

SLIM's distinct path to the moon reflects its lighter design, necessitating a more fuel-efficient approach compared to Chandrayaan-3. While Chandrayaan-3, with its propulsion module weighing 2.1 tonnes, followed a rapid Hohmann transfer orbit, SLIM's four-month journey was dictated by the principles of weak-stability boundary theory.

1. SLIM's reduced weight is attributed to carrying less fuel. Chandrayaan-3, with its 3.9-tonne mass, was propelled by a 2.1-tonne propulsion module. Launched on July 14, Chandrayaan-3 reached the moon in less than a month through the Hohmann transfer orbit, a more direct route.
2. SLIM's journey involved multiple orbits around Earth, strategically building kinetic energy during each swing. This fuel-thrifty approach allowed SLIM to gradually accelerate before ascending towards the moon's orbit. This trajectory, rooted in weak-stability boundary theory, contributed to the extended travel time of four months.
3. Once in Earth's orbit, SLIM utilized a series of swings around the planet to accumulate kinetic energy. This gradual acceleration prepared the spacecraft for the subsequent trajectory towards the moon.
4. Unlike Chandrayaan-3, which applied brakes to slow down and be captured by the moon's gravity, SLIM adopted a different strategy. As SLIM approached the moon, it allowed itself to be deflected in the moon's direction, even shooting past lunar orbit and deeper into space. This deflection was orchestrated by the combined gravitational forces of the Earth and the Moon.
5. The deflection technique employed by SLIM echoes principles established in the late 1980s during the JAXA mission 'Hiten,' showcasing the continuity of scientific approaches in space exploration.

4. SLIM's Lunar Mission Objectives

1. After following a larger, more extended orbit, SLIM positioned itself strategically for a lunar encounter in December. This approach, sacrificing time for fuel efficiency, culminated in orbital capture on Christmas Day.
2. By adopting a more loopy path, SLIM maximized fuel efficiency, a critical factor in its mission design.
3. SLIM's standout feature is its moniker as the "moon sniper," earned through its ambitious objective on January 19. It aims to achieve a remarkable feat by attempting to soft-land within an exceptionally tight tolerance of 100 meters from its chosen landing site.
4. In comparison to historical moon-landing missions, SLIM's precision goal is unprecedented. While the Chandrayaan-3 Vikram lander targeted an elliptical area of 4 km x 2.5 km, SLIM aims to set a new record with the smallest-ever area tolerance for a soft landing on the moon.
5. SLIM's chosen landing site is near the Shioli Crater, located at 13.3º S and 25.2º E. To guide its descent, SLIM will leverage data from JAXA's SELENE orbiter, a precursor mission that concluded in 2009.
6. SLIM's attempt to soft-land with a precision of 100 meters surpasses previous achievements. Notably, China's Chang'e 3 spacecraft held the record for the most precise moon landing, touching down 89 meters away from its designated spot in the Mare Imbrium plain in 2013.
7. SLIM's lower mass, excluding fuel at only 120 kg, enhances its manoeuvrability during the descent. Its compact size serves as a testament to its economical design, being a critical factor in achieving the ambitious landing precision.
8. Before landing, SLIM will deploy two small rovers, Lunar Excursion Vehicles (LEV)-1 and Lunar Excursion Vehicles (LEV)-2, to explore the lunar surface near the landing point. These rovers will collect data on temperature, and radiation, and aim to study the moon's mantle, contributing valuable insights to lunar exploration.

5. SLIM's Impact on Chandrayaan-4: Advancing Lunar Exploration

• Scientists are particularly intrigued by the moon's South Pole region due to permanently shadowed craters containing water ice. This area, shielded from sunlight and diurnal temperature variations, holds the potential for water extraction. Chandrayaan-4, as part of its Lunar Polar Exploration (LUPEX) mission, aims to delve deeper into this region.
• The successful soft landing of Chandrayaan-3 marked the conclusion of the second phase of India's lunar exploration program. The subsequent mission, Chandrayaan-4, represents the first endeavour of the program's third phase, focusing on the Lunar Polar Exploration.
• LUPEX is envisioned as a collaborative effort between India and Japan, with JAXA (Japan Aerospace Exploration Agency) having approved the mission. The launch is anticipated in 2026, with Japan providing the launch vehicle and lunar rover, while India contributes the lander module.
• Chandrayaan-4 will target an area even closer to the moon's South Pole than Chandrayaan-3, offering the potential for groundbreaking discoveries. The challenging terrain characterized by rocky surfaces, numerous craters, and steep slopes demands advanced landing technologies.
• The technologies tested by JAXA through SLIM, particularly the feature-matching algorithm and navigation systems, will be pivotal for Chandrayaan-4's success in navigating the intricate lunar terrain. SLIM's achievements and learnings will contribute to refining the capabilities required for precise landings.
• Near the moon's poles, identifying suitable landing spots for larger modules or rovers is challenging. With rocky and crater-filled terrains, the downrange and cross-range limits for landing will be more restrictive. Chandrayaan-4 will need to land as close as possible to the designated site, emphasizing the importance of advanced landing capabilities.
• The feature-matching algorithm and navigation systems tested by SLIM will play a crucial role in overcoming the challenges posed by the rocky lunar terrain. These technologies will be essential for Chandrayaan-4 to land with precision and efficiency in the targeted region near the moon's South Pole.
• While JAXA is expected to provide the launch vehicle and lunar rover, India will contribute the lander module for Chandrayaan-4. The exact landing site is yet to be determined, but it is anticipated to be closer to the South Pole than the previous Vikram lander, which landed 600 km away.