The LASER Retroreflector Array on the Vikram lander
Credit: NASA's Goddard Space Flight Centre
Bengaluru: In a technique demonstration that could significantly improve the tracking of locations on the moon’s surface, NASA's Lunar Reconnaissance Orbiter (LRO) has achieved a laser range measurement using the 'tiny but mighty' LASER Retroreflector Array (LRA) on the Chandrayaan-3 lander.
On December 12, 2023, a laser beam was transmitted and reflected between the orbiting NASA spacecraft and the LRA on the Chandrayaan-3 lander Vikram. The experiment is a demonstration of capabilities to locate the retroreflector, which is on the moon’s surface, from the moon’s orbit.
NASA said the successful experiment could inspire a new style of precisely locating targets on the lunar surface.
The Indian Space Research Organisation (Isro) said the observation occurred during lunar night, with the LRO ascending to the east of Chandrayaan-3.
Accommodated on Vikram as part of a NASA-Isro collaboration, the LRA comprises eight retroreflectors on a hemispherical support structure. This array facilitates laser ranging from various directions by any orbiting spacecraft with suitable instruments. The two-inch-wide LRA is designed to last for decades on the lunar surface. It weighs only about 20 grams and does not require power or maintenance.
Since its lunar landing, Vikram has been accessible for measurements from the altimeter on LRO, called LOLA.
Opening new possibilities
At 3 pm EST on December 12, the altimeter was pointed toward Vikram which was about 100 km away from the LRO, near the Manzinus crater in the moon’s south pole region. The LRO transmitted laser pulses toward Vikram and registered the light that had bounced back from the retroreflector onboard the lander.
NASA noted that sending laser pulses toward an object and measuring how long it takes the light to bounce back is a common technique to track the locations of earth-orbiting satellites from the ground. But using the technique “in reverse”, or sending laser pulses from a moving spacecraft to a stationary one to determine its precise location has many applications at the moon.
“The next step is to improve the technique so that it can become routine for missions that want to use these retroreflectors in the future,” Xiaoli Sun, who led the team at NASA’s Goddard Space Flight Centre in Greenbelt, Maryland that developed the LRA, said.
The Chandrayaan-3 lander touched down near the moon’s south pole on August 23, 2023. Four days later, a camera onboard the LRO captured the lander on the lunar surface.
Isro said the LRA on Chandrayaan-3 is the only one currently available near the lunar south pole and its applications as a location marker would benefit current and future lunar missions. Measurements recorded using the LRA will help in the precise determination of the spacecraft’s orbital position and reveal insights into the moon's dynamics, internal structure, and gravitational anomalies, the space agency said.