NASA and JAXA exchange laser signals between SLIM lander and LRO in lunar orbit

NASA and JAXA exchange laser signals between SLIM lander and LRO in lunar orbit
by Clarence Oxford
Los Angeles CA (SPX) Jul 30, 2024

NASA's Lunar Reconnaissance Orbiter (LRO) has successfully transmitted a laser pulse to a small retroreflector on JAXA's (Japan Aerospace Exploration Agency) SLIM lander on the Moon, receiving a return signal on two occasions.

During two orbits on May 24, 2024, LRO passed 44 miles above the SLIM (Smart Lander for Investigating Moon) and used its laser altimeter instrument to send signals to the lander. Unlike previous attempts, the signals were returned to LRO's detector this time.

This achievement is notable because the retroreflector's position is not ideal. Normally, retroreflectors are placed on the top of landers, allowing a 120-degree range for LRO to target. However, SLIM landed with its top facing sideways, which limited the range for LRO's laser pulses.

To improve their chances, the LRO team collaborated with JAXA to pinpoint SLIM's exact location and orientation. NASA engineers then calculated when LRO's orbit would best align with SLIM's retroreflector for optimal signal transmission.

"LRO's altimeter wasn't built for this type of application, so the chances of pinpointing a tiny retroreflector on the Moon's surface are already low," said Xiaoli Sun, who led the team that built SLIM's retroreflector at NASA's Goddard Space Flight Center in Greenbelt, Maryland, as part of a partnership between NASA and JAXA.

"For the LRO team to have reached a retroreflector that faces sideways, instead of the sky, shows that these little devices are incredibly resilient," Sun said.

SLIM landed on the Moon on January 20. The Laser Retroreflector Array, a device carried by SLIM, is one of six NASA retroreflectors sent to the Moon. This instance marks the second time a retroreflector has returned a signal to LRO's altimeter.

The first successful laser transmission from LRO to a NASA retroreflector and back occurred on December 12, 2023, when LRO communicated with ISRO's (Indian Space Research Organisation) Vikram lander. LRO has since exchanged laser pings with Vikram three more times.

NASA's retroreflector, featuring eight quartz corner-cube prisms in a dome-shaped aluminum frame, measures 2 inches wide. These retroreflectors require no power or maintenance and can last for decades on the Moon's surface, providing reliable markers for future missions.

The retroreflectors could assist Artemis astronauts in navigating the lunar surface in the dark or mark spacecraft locations to aid in landing operations for both crewed and uncrewed missions.

LRO's laser altimeter, currently the only laser instrument orbiting the Moon, was initially designed to map lunar topography for surface missions. Despite this, LRO engineers are working to achieve the precision necessary to target retroreflectors within 1/100th of a degree.