NASA Lunar Probe Plays Laser Tag With Japanese Lunar Lander

NASA’s Lunar Reconnaissance Orbiter (LRO) is playing an interesting game of hide-and-seek with Japan’s Smart Lander for Investigating Moon (SLIM), which landed on the lunar surface on January 19.

LRO tried to ping the SLIM retroreflector, a domed, cookie-sized structure attached to the back of the lander, just below its solar panels. The first eight times LRO fired its laser at SLIMfailed to hit the retroreflector. But on May 24, during two subsequent orbits, LRO succeeded: the SLIM retroreflectors were found.

Retroreflectors are fairly simple devices. They’re basically just arrays of mirrors that bounce light back to their source. Hit them with a laser and it bounces light back, providing a way to make incredibly precise distance measurements based on the speed of light. And they don’t require any power to operate; SLIM is probably broken on the lunar surface several months ago, but its reflector can still bounce laser light into space.

close-up of a spacecraft in a clean room, showing a small spherical instrument equipped with many small mirrors.

NASA’s Laser Retroreflector Array, seen on JAXA’s SLIM lunar lander prior to launch. (Photo source: JAXA)

The problem in this case was that SLIM landed Moon it wasn’t perfect. SLIM has landed on the edge of the Shioli crater, not far from the intended landing site, but the landing was not as gentle as expected. Images sent back from the mission show that SLIM landed face downthe tip of his nose stuck in the gray soil.

Related: NASA’s lunar retroreflector network could make lunar landings much easier

The awkward landing made it difficult to use the SLIM’s solar panels because they were facing away from sun (Though the mission team solved that problem by keeping SLIM alive through many cold, dark lunar nights.) It also meant that SLIM’s retroreflector was in a difficult position to ping. This problem was compounded by the fact that LRO is not designed to find and ping retroreflectors. Its laser altimeter was built to reveal topographic detail on the lunar surface, providing a map of the lunar craters and plains.

“The LRO altimeter was not built for this type of use, so the chances of locating a small retroreflector on the moon’s surface are slim,” said Xiaoli Sun, an engineer at NASA’s Goddard Space Flight Center in Maryland. in the press release. Sun currently leads a team developing and deploying small retroreflectors on the Moon for lunar exploration.

But the LRO’s ability to ping a retroreflector has recently been proven. The Indian Space Research Organisation’s Vikram lander, which landed on the moon in August 2023, the Chandrayaan 3 mission also carried a NASA retroreflector. In December 2023, NASA scientists attempted to ping the retroreflector from lunar orbit for the first time.

From an altitude of 100 kilometers (62 miles), the LRO directed laser pulses toward the Vikram landing site and a response was recordedThe same technique was used to make contact with SLIM, although the orbiter was only 44 miles (71 km) above the lunar surface at the time.

The key design feature of the tiny retroreflectors is their eight quartz-horn-cube prisms. They provide the ability to receive a laser signal and send it directly to its source. NASA suggests that these devices could one day aid in lunar exploration, perhaps helping the agency Artemis missions land precisely on the lunar surface.

Lunar laser experiments are not limited to the LRO, however. For example, the Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) in New Mexico is firing laser pulses at suitcase-sized retroreflectors left behind by Apollo lunar missions. This type of work can determine the distance between the moon and Earth to within a few millimeters — an extraordinary level of precision that has allowed scientists to show that the moon is moving away from Earth by about 1.5 inches (3.8 centimeters) each year.