Lunar Locating at Light Speed: NASA’s Precision Laser “Pings” Indian Moon Lander

NASA and ISRO’s laser experiment on the Moon opens up new possibilities for precise lunar exploration and the use of innovative retroreflectors in upcoming missions.

For the first time at the Moon, a laser beam was transmitted and reflected between an orbiting NASAEstablished in 1958, the National Aeronautics and Space Administration (NASA) is an independent agency of the United States Federal Government that succeeded the National Advisory Committee for Aeronautics (NACA). It is responsible for the civilian space program, as well as aeronautics and aerospace research. Its vision is "To discover and expand knowledge for the benefit of humanity." Its core values are "safety, integrity, teamwork, excellence, and inclusion." NASA conducts research, develops technology and launches missions to explore and study Earth, the solar system, and the universe beyond. It also works to advance the state of knowledge in a wide range of scientific fields, including Earth and space science, planetary science, astrophysics, and heliophysics, and it collaborates with private companies and international partners to achieve its goals.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]” tabindex=”0″ role=”link”>NASA spacecraft and an Oreo-sized device on ISRO’s (Indian Space Research Organisation) Vikram lander on the lunar surface. The successful experiment opens the door to a new style of precisely locating targets on the Moon’s surface.

Precise Lunar Locating Technique

At 3 p.m. ESTEST is an abbreviation for Eastern Standard Time, the time zone for the eastern coast of the United States and Canada when observing standard time (autumn/winter). It is five hours behind Coordinated Universal Time. New York City, Washington, D.C., Boston, and the Kennedy Space Center are in the Eastern Time Zone (ET).” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]” tabindex=”0″ role=”link”>EST on December 12, 2023, NASA’s LRO (Lunar Reconnaissance Orbiter) pointed its laser altimeter instrument toward Vikram. The lander was 62 miles, or 100 kilometers, 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.

The Pragyaan rover captured this image of the Vikram Lander on August 30th, before lunar night enshrouded it in its frigid darkness. Credit: ISRO

Sending laser pulses toward an object and measuring how long it takes the light to bounce back is a commonly used way to track the locations of Earth-orbiting satellites from the ground. But using the technique in reverse – to send laser pulses from a moving spacecraft to a stationary one to determine its precise location – has many applications at the Moon, scientists say.

Revolutionary Retroreflectors

“We’ve shown 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 can become routine for missions that want to use these retroreflectors in the future.”

Only 2 inches, or 5 centimeters, wide, NASA’s Laser Retroreflector Array has eight quartz-corner-cube prisms set into a dome-shaped aluminum frame. This configuration allows the device to reflect light coming in from any direction back to its source. Credit: NASA’s Goddard Space Flight Center

Only 2 inches, or 5 centimeters, wide, NASA’s tiny but mighty retroreflector, called a Laser Retroreflector Array, has eight quartz-corner-cube prisms set into a dome-shaped aluminum frame. The device is simple and durable, scientists say, requiring neither power nor maintenance, and can last for decades. Its configuration allows the retroreflector to reflect light coming in from any direction back to its source.

Versatile Applications and Future Potential

Retroreflectors can be used for many applications in science and exploration and, indeed, have been in use at the Moon since the Apollo era. By reflecting light back to Earth, the suitcase-size 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 StationThe International Space Station (ISS) is a large spacecraft in orbit around the Earth that serves as a research laboratory and spaceport for international collaboration in space exploration. It was launched in 1998 and has been continuously occupied by rotating crews of astronauts and cosmonauts from around the world since 2000. The ISS is a joint project of five space agencies: NASA (USA), Roscosmos (Russia), JAXA (Japan), ESA (Europe), and CSA (Canada). It orbits the Earth at an altitude of approximately 400 kilometers (250 miles), and provides a unique platform for scientific research, technological development, and human space exploration.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]” tabindex=”0″ role=”link”>International Space Station, they’re used as precision markers that help cargo-delivery spacecraft dock autonomously.

In the future, they could guide Artemis astronauts to the surface in the dark, for example, or mark the locations of spacecraft already on the surface, helping astronauts or uncrewed spacecraft land next to them.

Artist’s rendering of NASA’s Lunar Reconnaissance Orbiter. Credit: NASA’s Goddard Space Flight Center

Challenges and Next Steps

But there’s more work to do before retroreflectors can light up the Moon. The biggest hurdle to their immediate adoption is that LRO’s altimeter, which has operated for 13 years beyond its primary mission, is the only laser instrument orbiting the Moon for now. But the instrument wasn’t designed to pinpoint a target; since 2009, the altimeter – called LOLA – has been responsible for mapping the Moon’s topography to prepare for missions to the surface.

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

LOLA works by dispatching five laser beams toward the Moon and measuring how long it takes each one to bounce back (the quicker the light returns, the less distance between LOLA and the surface, and thus the higher the elevation in that area). Each laser beam covers an area 32 feet, or 10 meters, wide, from a 62-mile, or 100-kilometer, altitude. Because there are large gaps between the beams, there is only a small chance that the laser pulse can 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 aren’t ideal for pointing to within one-hundredth of a degree of a retroreflector, which is what’s required to consistently achieve a ping. A future laser that slowly and continuously rakes the surface without any gaps in coverage would help tiny retroreflectors meet their potential.

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

Expanding Lunar Exploration Tools

Several NASA retroreflectors are slated to fly aboard public and private Moon landers, including one on JAXA’s (Japan Aerospace Exploration Agency) SLIM lander, due to land on the Moon on Jan. 19, 2024, and one built by Intuitive Machines, a private company scheduled to launch its spacecraft to the Moon in mid-February. Intuitive Machines will carry six NASA payloads, including the retroreflector, under NASA’s Commercial Lunar Payload Services (CLPS) initiative.