The LRO probe SLIM hit the lunar lander's laser retroreflector twice

On May 24, 2024, while orbiting about 70 km above the lunar surface, NASA's Lunar Reconnaissance Orbiter (LRO) sent two laser pulses to a small retroreflector on the Japanese SLIM lander. He had already tried eight times before that moment, but only on these two occasions, The signal is returned to the LRO detector.

LRO's laser altimeter was not designed for this type of application. This made it difficult to use a small retroreflector to detect the lunar surface. Also, the retroreflector was not optimally positioned because SLIM landed “bent” on the moon. However, despite these difficulties, it was possible to jump the signal from the surface to orbit.

From the laser altimeter…

LRO's laser altimeter, called LOLA (Lunar Orbiter Laser Altimeter), is currently orbiting the Moon.

LOLA was designed to map the lunar landscape. Not for that Mark a retroreflector with an accuracy of 1/100 of a degreeThis is what LRO engineers try to do with each signal.

In fact, over the years, NASA has sent six different retroreflectors to the Moon, both on private and public landers. When LRO sent a pulse to LRO's (Indian Space Research Organisation's) Vikram lander on December 12, 2023, it was the first time a laser beam was sent from LRO to an agency retroreflector and received a response. After that, LRO exchanged laser pulses with Vikram three more times.

…for a little retrospect

Japanese space agency JAXA's SLIM (Smart Lander for Investigating Moon) lander landed on the lunar surface on January 20, 2024. A retroreflector on board, called Laser retroreflector arrayThis is the second time the LRO altimeter has bounced a signal.

Retroreflectors are usually attached to the top of the landers, giving LRO 120 degree angles to send laser pulses to the approximate location of the retroreflector. SLIM, however, was not positioned vertically on the lunar surface, which posed an additional difficulty.

The LRO team worked with JAXA to determine SLIM's exact location and orientation to maximize the chances of hitting the target. Then, NASA engineers predicted when LRO's orbital path would take it to optimal coordinates to reach the laser retroreflector array.

An important conclusion: Why?

Despite its lateral position, achieving it was a major achievement for NASA. The Laser Retroreflecor Array consists of eight angular cubic quartz prisms, inserted into a dome-shaped aluminum structure approximately 5 centimeters wide. Retroreflectors with no electricity or maintenance required They can last for decades on the lunar surface. Therefore, act as “beacons” for future missions.

For example, retroreflectors could guide Artemis astronauts to the surface or mark the location of existing probes on the Moon to help astronauts and unmanned probes land near them.

Currently, we already have retroreflectors on board satellites such as the American GPS and the European Galileo. For a time, an Italian retroreflector was active on the lunar surface Chinese Song 6 lander: His name stands for INRRI, Instrumentation for Landing-Roving Laser Retroreflector Surveys. Built by the National Institute of Nuclear Physics at Frascati, INRRI consisted of two passive laser retroreflectors used to measure the lander's laser range.

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