NASA’s LRO laser instrument successfully pings Indian lunar lander

For the first time on the Moon, a laser beam was transmitted and reflected between an orbiting NASA spacecraft and an Oreo-sized device on ISRO’s (Indian Space Research Organization) Vikram lander on the lunar surface. This successful experiment opens the door to a new style of precisely locating targets on the surface of the Moon.

At 3 p.m. EST on December 12, 2023, NASA’s LRO (Lunar Reconnaissance Orbiter) pointed its laser altimeter at Vikram. The lander was 62 miles, or 100 kilometers, from LRO, near Manzinus Crater in the moon’s south pole region, when LRO transmitted laser pulses toward it. After the orbiter recorded the light that bounced off a tiny NASA retroreflector aboard the Vikram, NASA scientists knew their technique had finally worked.

Sending laser pulses toward an object and measuring the time it takes for the light to bounce back is a commonly used way to track the location of satellites orbiting Earth from the ground. But scientists say using the inverse technique — to send laser pulses from a moving spacecraft to a stationary one to determine its precise position — has many applications on the Moon.

“We showed that we can locate our retroreflector on the surface from the Moon’s orbit,” said Xiaoli Sun, who led the team at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, that developed the retroreflector on Vikram as part of a partnership. between NASA and ISRO. “The next step is to improve the technique so that it becomes routine for missions that want to use these retroreflectors in the future.”

Just 2 inches, or 5 centimeters, wide, NASA’s tiny but powerful retroreflector, called the Laser Retroreflector Array, features eight cubic quartz wedge prisms placed in a dome-shaped aluminum frame. According to scientists, the device is simple and durable, requiring no power or maintenance, and can last for decades. Its configuration allows the retroreflector to reflect light coming from any direction towards its source.

Retroreflectors can be used for many applications in science and exploration and have indeed been used on the Moon since the Apollo era. By reflecting light back to Earth, the suitcase-sized retroreflectors revealed that the Moon is moving away from our planet at a rate of 1.5 inches (3.8 centimeters) per year.

This new generation of tiny retroreflectors has even more applications than their larger predecessors. On the International Space Station, they are used as precision markers that help cargo delivery spacecraft dock autonomously.

In the future, they could, for example, guide Artemis astronauts to the surface in the dark, or mark the location of spacecraft already on the surface, thereby helping astronauts or uncrewed spacecraft land alongside two.

But there is still work to be done before retroreflectors can illuminate the moon. The biggest obstacle to their immediate adoption is that LRO’s altimeter, which operated for 13 years beyond its primary mission, is currently the only laser instrument in orbit around the Moon. But the instrument was not designed to locate a target; since 2009, the altimeter – called LOLA – has been responsible for mapping the topography of the Moon to prepare missions to the surface.

“We would like LOLA to point at this Oreo-sized target and hit it every time, which is difficult,” said Daniel Cremons, a NASA Goddard scientist who works with Sun. It took eight attempts for the altimeter to contact Vikram’s retroreflector.

LOLA works by sending five laser beams towards the moon and measuring the time it takes for each one to bounce back (the faster the light returns, the less distance between LOLA and the surface, and therefore the higher the elevation in that zone is high). Each laser beam covers an area 32 feet or 10 meters wide, from an altitude of 62 miles or 100 kilometers. Due to the large gaps between the beams, there is only a small chance that the laser pulse could contact a retroreflector during each pass of the lunar orbiter over the lander.

Altimeters are great for detecting craters, rocks, and boulders to create global elevation maps of the moon. But they’re not ideal for pointing within a hundredth of a degree of a retroreflector, which is necessary to get ping consistently. A future laser that slowly and continuously rakes the surface without any gaps in coverage would help tiny retroreflectors reach their potential.

For now, the team behind NASA’s miniature retroreflectors will continue to use LRO’s laser altimeter to refine the position of targets on the surface, particularly landers.

Several NASA retroreflectors are expected to fly aboard public and private lunar landers, including one on the Japan Aerospace Exploration Agency (JAXA) SLIM lander, which is scheduled to land on the Moon on January 19, 2024, and another built by Intuitive Machines, a private company. is expected to launch its spacecraft to the Moon in mid-February. Intuitive Machines will carry six NASA payloads, including the Retroreflector, as part of NASA’s Commercial Lunar Payload Services (CLPS) initiative.