For the first time at the Moon,a laser beam was successfully transmitted and reflected between NASA’s orbiting Lunar Reconnaissance Orbiter (LRO) and a compact device on ISRO’s Vikram lander. Positioned near the Manzinus crater in the Moon’s South Pole region, the Vikram lander, located 62 miles (100 kilometers) away from the LRO, became the focal point of the experiment.
On December 12, 2023, at 3 p.m. EST, the LRO utilized its laser altimeter to send pulses towards Vikram.The momentous success of the experiment, confirmed by the registration of reflected light from a NASA retroreflector on Vikram, heralds a groundbreaking approach for precision targeting on the lunar surface.
On August 23, 2023, ISRO’s Vikram lander, equipped with a NASA retroreflector, successfully landed on the Moon. NASA’s Lunar Reconnaissance Orbiter (LRO) captured a compelling image of the lander four days later, showcasing it at the image’s center with its distinct dark shadow against a luminous halo. The halo resulted from the interaction of the rocket plume with the fine-grained regolith, akin to soil, on the Moon’s surface. The photograph covers an expanse of 1 mile (1.7 kilometers), offering a glimpse into the lunar landing site’s unique features.
Utilizing laser pulses to gauge the time taken for light to reflect back is a standard method for tracking Earth-orbiting satellites from the ground. However, scientists highlight the innovative application of this technique in reverse: sending laser pulses from a moving spacecraft to a stationary one, determining its exact location. This reversal of the process holds significant promise for various applications on the Moon, providing a novel means of precisely locating targets and advancing our understanding of lunar dynamics.
Xiaoli Sun, leading the team at NASA’s Goddard Space Flight Center, emphasized the successful demonstration of locating the retroreflector on Vikram’s surface from the Moon’s orbit. Developed through collaboration between NASA and ISRO, the 2-inch-wide Laser Retroreflector Array, a compact yet robust device, comprises eight quartz-corner-cube prisms within an aluminum frame. The team envisions refining the technique to make it a standard practice for future missions utilizing these retroreflectors. This unpowered and low-maintenance retroreflector, capable of lasting for decades, reflects light from any direction back to its source, showcasing its simplicity and durability.
NASA’s Laser Retroreflector Array, measuring a mere 2 inches or 5 centimeters wide, features eight quartz-corner-cube prisms arranged within a dome-shaped aluminum frame. This design enables the device to efficiently reflect light from any direction back to its source. Retroreflectors, a technology with applications in science and exploration, have been integral since the Apollo era. Housed in suitcase-size packages on the Moon, these retroreflectors play a crucial role in revealing lunar dynamics. By reflecting light back to Earth, they unveiled the Moon’s gradual movement away from our planet, indicating a rate of 1.5 inches (3.8 centimeters) per year.
The latest iteration of compact retroreflectors boasts a broader range of applications compared to their larger counterparts. Currently employed on the International Space Station as precision markers facilitating the autonomous docking of cargo-delivery spacecraft, these tiny devices hold significant potential for future missions. Envisaged applications include guiding Artemis astronauts during lunar surface navigation in low-light conditions and marking the locations of existing spacecraft, aiding in the precise landing of astronauts or unmanned vehicles alongside them.
While the prospect of illuminating the Moon with retroreflectors is promising, there are significant challenges to overcome. The primary hurdle lies in the fact that NASA’s Lunar Reconnaissance Orbiter’s (LRO) altimeter, known as LOLA, is currently the sole laser instrument orbiting the Moon. Operating for an impressive 13 years beyond its original mission, LOLA was not designed for precision targeting. Since 2009, its role has focused on mapping the Moon’s topography in preparation for surface missions. Daniel Cremons, a NASA Goddard scientist collaborating with Xiaoli Sun’s team, acknowledges the difficulty in aligning LOLA with the Oreo-sized target consistently. Notably, it took eight attempts for the altimeter to successfully contact Vikram’s retroreflector, highlighting the complexity of the task.
The Lunar Reconnaissance Orbiter’s altimeter, LOLA, employs a mechanism involving the dispatch of five laser beams toward the Moon. The duration it takes for each beam to bounce back determines the distance between LOLA and the lunar surface, indicating elevation—shorter return times correlate with higher elevations. Covering a width of 32 feet or 10 meters, each laser beam operates from a 62-mile or 100-kilometer altitude. However, due to substantial gaps between these beams, the chances of a laser pulse contacting a retroreflector during each pass over the lander are limited, emphasizing the intricacies involved in achieving precise target alignment.
While altimeters excel in detecting features like craters and rocks, enabling the creation of comprehensive elevation maps of the Moon, their precision falls short when it comes to pinpointing retroreflectors with the required accuracy of within one-hundredth of a degree for consistent pings. Achieving this level of precision demands a future laser technology capable of a slow and continuous sweep across the lunar surface without gaps in coverage, providing optimal conditions for tiny retroreflectors. Presently, NASA’s miniature retroreflector team will persist in using the Lunar Reconnaissance Orbiter’s laser altimeter to enhance the positional accuracy of surface targets, particularly landers.
Numerous NASA retroreflectors are set to embark on lunar exploration missions, with plans for deployment on both public and private Moon landers. Among these, the Japan Aerospace Exploration Agency’s (JAXA) SLIM lander, scheduled to touch down on the Moon on January 19, 2024, will carry one such retroreflector. Additionally, Intuitive Machines, a private company participating in NASA’s Commercial Lunar Payload Services (CLPS) initiative, is slated to launch its spacecraft in mid-February, accommodating six NASA payloads, including a retroreflector. This concerted effort reflects a collaborative approach in advancing lunar exploration technology and capabilities.
