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NASA’s LRO (Lunar Reconnaissance Orbiter) twice transmitted a laser pulse to a cookie-sized retroreflector aboard JAXA’s (Japan Aerospace Exploration Agency) SLIM lander on the Moon and received a return signal.

When LRO passed 44 miles above SLIM (Smart Lunar Exploration Rover) during two consecutive orbits on May 24, 2024, it signaled the lander with its laser altimeter, as it had done eight times before. But on those two attempts, the signal bounced back to the LRO detector.

This was an important achievement for NASA because the device is not in an optimal position. Retroreflectors are usually attached to the top of the descent modules, giving LRO a 120-degree range of angles to aim at when sending laser pulses to the approximate location of a retroreflector. However, the SLIM dropper had settled on the surface with the top facing sideways, limiting the LRO’s range.

To increase the chances of reaching its target, the LRO team worked with JAXA to determine SLIM’s exact location and orientation. NASA engineers then predicted when LRO’s orbital trajectory would bring it to coordinates that would give it the best chance to reach SLIM’s retroreflector with the laser beams.

“LRO’s altimeter was not designed for this type of application, so the chances of determining a small retroreflector on the surface of the moon are already low,” said Xiaoli Sun, who leads the team that built the SLIM retroreflector at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, as part of a partnership between NASA and JAXA.

“That the LRO team has reached a retroreflector that looks sideways instead of skyward shows that these small devices are incredibly resilient,” Sun said.

SLIM touched down on the lunar surface on January 20. The retroreflector that went up with the lander, called the Laser Retroreflector Array, is one of six NASA has sent to the Moon aboard private and public landers, and the second to reflect a signal back to LRO’s altimeter.

The first time a laser beam was transmitted from LRO to a NASA retroreflector and back was on December 12, 2023, when LRO pinged ISRO’s (Indian Space Research Organisation) Vikram lander. Since then, LRO has exchanged laser pings with Vikram three more times.

The NASA retroreflector has eight quartz corner cubic prisms set in a domed aluminum frame that is 2 inches wide. With no need for power or maintenance, retroreflectors can last on the lunar surface for decades and thus provide reliable beacons for future missions.

Retroreflectors could, for example, guide Artemis astronauts to the surface in the dark or mark the locations of spacecraft already on the surface to help astronauts and unmanned spacecraft land near them.

LRO’s laser altimeter, the only laser instrument orbiting the moon so far, was designed to map the moon’s topography in preparation for missions to the surface — not to point within 1/100 of a degree of a retroreflector, which engineers of LRO trying to do with every ping.