Nasa’s LRO pings Vikram, opens door to new style of locating targets on Moon

BENGALURU: For the first time on the Moon, Nasa transmitted and reflected a laser beam between an orbiting spacecraft it operates and a device —Laser Retroreflector Array (LRA)— on Isro’s Vikram lander on the lunar surface, opening the door to a new style of precisely locating targets on the Moon’s surface.
“At 3pm (EST) on Dec 12, 2023, Nasa’s Lunar Reconnaissance Orbiter (LRO) pointed its laser altimeter instrument toward Vikram.The lander was 100km away from LRO, near Manzinus crater in the Moon’s South Pole region, when LRO transmitted laser pulses toward it. After the orbiter registered light that had bounced back from a tiny Nasa retroreflector aboard Vikram, Nasa scientists knew their technique had finally worked,” Nasa said.
Isro said the LRA has begun serving as a fiducial point (precisely located markers for reference) on the Moon.
“...The ranging carried out by LRO utilised its onboard Lunar Orbiter Laser Altimeter (LOLA). The observation occurred during lunar night time, with the LRO ascending to the east of Chandrayaan-3.”
The achievement “opens the door to a new style of precisely locating targets on the Moon’s surface,” said Xiaoli Sun, who led the Nasa team that developed the reflector. While using lasers to track satellites from Earth is common, flipping the technique to locate surface craft from space could enable safer Moon landings and simpler rendezvous between vehicles.
To make it work, Nasa equipped Vikram with a durable, no-fuss retroreflector containing eight quartz prisms. Small enough to fit in a pocket, it passively bounces back light from any direction, making it an ideal lunar locator beacon. After years of work, the modest device finally proved itself by registering in LRO’s ageing laser altimeter instrument.
“We showed we can locate our retroreflector on the surface from orbit. The next step is routine use for future missions,” said Sun.
So far, LRO’s altimeter has struggled to regularly connect with the tiny reflector as the orbiter races over the landing site. But with tweaks to the system and new spacecraft carrying similar laser gear, pinpoint locations could soon be standard.
According to Nasa, future applications are numerous, like guiding astronauts down or automatically landing supply ships near bases. Retroreflectors may even reveal the Moon’s retreat from Earth and aid lunar construction projects.
Several reflectors will launch soon, riding on vehicles under Nasa’s CLPS (commercial lunar payload services) programme, which contracts private companies for delivery services. Engineers will test improvements as new data trickles in from these forthcoming laser lighthouses sprinkled across the Moon.
With perseverance, Nasa aims to birth a new lunar capability, ushering an age of precision targeting to open up the long-neglected frontier that is Earth’s nearest neighbour.
TOI had reported in August 2023 that LRA, the fourth payload Vikram took to the lunar surface, will start its work only once the rest of its onboard instruments and two on Pragyan (the rover) finish their job.
LRA is designed to use reflected laser light from orbiting spacecraft laser — typically a laser altimeter or light detection and ranging (lidar) — to precisely determine the location of the lander, as a fiducial (assumed as a fixed basis of comparison) marker, and the distance to that point on the lunar surface with respect to the orbiter.
The retroreflectors reflect any light striking them directly back to the source. They can be tracked by an orbiting laser altimeter or lidar from a few hundred kilometres.
The LRA consists of eight circular 1.27-cm diameter corner-cube retroreflectors mounted on a 5.11cm diameter, 1.65cm high hemispherical gold-painted platform. Each of the retroreflectors points in a slightly different direction, and each has a maximum useful light incidence angle of about +-20 degrees. The Total mass of the LRA is 20 grams, it requires no power.