NASA’s Lunar Reconnaissance Orbiter spacecraft has now twice sent a laser pulse to a retroreflector on the Japanese SLIM lander on the Moon, and received a return signal.
Quite a feat, considering the retroreflector is about 2-inches wide and the orbiter is roughly 50km (30 miles) from the lunar surface.
The successful transmission and return between the Lunar Reconnaissance Orbiter and JAXA's (Japan Aerospace Exploration Agency) SLIM lander shows there is potential for the technique to help guide future astronauts to the surface of the Moon in the dark, or reveal the locations of lunar spacecraft to crewed and uncrewed landing missions.
The Lunar Reconnaissance Orbiter passed 44 miles (70km) above the SLIM (Smart Lander for Investigating Moon) twice on 24 May 2024.
As it did so, it used its laser altimeter instrument to beam a laser to a retroreflector - about the size of a cookie, says NASA - that's attached to SLIM.
The orbiter had done this two times before and not received a return transmission from the retroreflector, but on both these two attempts, it worked.
Previous attempts had proven challenging because the JAXA SLIM lander, which landed on the Moon on 19 January 2024, touched down on the lunar surface on its side.
Solving SLIM's retroreflector problem
Ideally, a retroreflector would be on top of a lander, giving any orbiting spacecraft an optimal chance of making contact.
With SLIM on its side, the Lunar Reconnaissance Orbiter had struggled to receive a return transmission from the retroreflector.
The Lunar Reconnaissance team worked with JAXA to determine the location and orientation of SLIM.
NASA engineers calculated the moment that LRO’s orbit trajectory would provide the best chance of reaching SLIM’s retroreflector with its laser beams.
"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," says 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."
Why use retroreflectors on the Moon?
The retroreflector, called a Laser Retroreflector Array, is one of the six that NASA has sent to the Moon on private and public landers, and it's the second to bounce a signal back to the orbiter's altimeter.
The first was on 12 December 2023, when LRO transmitted to the Indian Space Research Organisation's Vikram lander.
LRO has since exchanged laser transmissions with Vikram lander three more times, NASA says.
The retroreflectors are about 2 inches wide and do not require any power or maintenance, NASA says, meaning they can last on the surface of the Moon potentially for decades, providing beacons that can be used for navigation.
They could, for example, help future astronauts such as the Artemis crews land on the Moon.
The retroreflectors could also help crewed and uncrewed spacecraft land near probes, rovers and other instruments on the lunar surface.
