Why the Moon's Darkest Craters Are Ideal for Next-Gen Laser Navigation Systems

The Moon's south polar region hides some of the most extreme environments in the Solar System—craters that never see sunlight and remain in eternal shadow. These frigid, dark cavities might seem inhospitable, but physicist Jun Ye and colleagues have identified them as the perfect locations for building a critical component of an ultrastable laser system. Such lasers could revolutionize navigation on the lunar surface, offering pinpoint accuracy for future rovers, habitats, and astronauts. Here we answer key questions about this intriguing proposal.

What Makes Lunar Craters at the South Pole So Unique?

The Moon's south pole contains hundreds of craters whose interiors never receive direct sunlight. Because the Moon's axis is tilted only about 1.5 degrees from the ecliptic plane, sunlight strikes these deep depressions at such a grazing angle that their floors remain in permanent shadow. This results in surface temperatures as low as -240°C (-400°F), making them among the coldest places in the Solar System. The lack of sunlight also means no diurnal temperature swings, providing an incredibly stable thermal environment—a key requirement for sensitive scientific instruments like ultrastable lasers.

Why the Moon's Darkest Craters Are Ideal for Next-Gen Laser Navigation Systems — Source: phys.org

Why Are Ultrastable Lasers Important for Moon Navigation?

Navigation on the Moon poses unique challenges. Radio signals from Earth suffer from a two-second round-trip delay, and GPS-like satellite networks are sparse. Ultrastable lasers can provide highly precise distance measurements using interferometry, allowing spacecraft and rovers to determine their positions to within millimeters. They could also serve as laser ranging beacons for landing guidance or formation flying. Without extreme frequency stability, these lasers would drift over time, making them unreliable for long-duration missions. The permanent cold and darkness of certain lunar craters offers a solution to maintain that stability.

How Could Permanently Shadowed Craters Help Stabilize Lasers?

Ultrastable lasers rely on locking their frequency to a reference cavity—typically a pair of highly reflective mirrors separated by a spacer. The spacer's length must remain constant because even tiny thermal expansions change the resonant frequency. In a shaded crater, temperature fluctuations are nearly zero, so the spacer experiences no thermal drift. Additionally, the absence of direct sunlight eliminates stray light that could heat the optics. The deep cold also reduces thermal noise in the mirrors and electronics. By placing the laser's reference cavity inside such a crater, scientists can achieve frequency stability orders of magnitude better than on the surface or on Earth.

Who Proposed This Idea and What Is Their Background?

The concept was introduced by Jun Ye, a physicist at JILA (a joint institute of the National Institute of Standards and Technology and the University of Colorado Boulder). Ye is a world-renowned expert in precision measurement and laser stabilization, having pioneered techniques for ultrastable lasers with record-breaking frequency stability. His team's work on optical atomic clocks and cavity-stabilized lasers made them natural candidates to suggest using lunar craters as ultra-stable platforms. They published the proposal in a recent paper, arguing that the Moon's unique thermal environment could enable laser systems far more stable than anything achievable on Earth.

What Are the Challenges of Building Lasers in Lunar Craters?

The obvious difficulty is constructing instruments in a place that never receives sunlight. Power generation would require either long cables from sunlit areas, nuclear sources, or batteries. The extreme cold also poses engineering challenges—materials become brittle, and lubricants freeze. Dust from the lunar surface (regolith) is abrasive and could damage sensitive optics. Additionally, deploying equipment into a deep crater requires advanced robotics or crewed missions. Communication links might need relay satellites due to the crater's shadowed horizon. Despite these hurdles, the payoff in stability could justify the investment for high-priority scientific and navigational applications.

How Does This Compare to Current Moon Navigation Methods?

Current navigation relies on Earth-based radio tracking, which has limited accuracy (meters to tens of meters) and a time lag of about 2.5 seconds round-trip. On the lunar surface, inertial navigation systems can drift over time. The proposed ultrastable laser system could provide real-time, centimeter-level positioning without needing constant contact with Earth. It would be especially valuable for autonomous rovers exploring the far side or for human missions requiring precise landing zones. The craters' stability means the laser reference cavity would need recalibration far less often, reducing operational complexity.

What Future Missions Could Benefit from This Technology?

Several upcoming NASA and ESA missions aim to establish a sustained human presence on the Moon, such as Artemis. Lunar rovers like VIPER, which will search for water ice in permanently shadowed regions, could use ultrastable lasers for precise navigation in the tricky terrain. Additionally, formation-flying satellites in lunar orbit could maintain relative positions with laser ranging. The same cavity technology could also support a future lunar gravity experiment or a test of general relativity. In the longer term, an ultrastable laser network could form the backbone of a lunar positioning system analogous to GPS.

Are There Other Potential Uses for Ultrastable Lasers on the Moon?

Beyond navigation, ultrastable lasers can drive optical atomic clocks—the most accurate timekeeping devices ever built. On the Moon, such clocks could enable tests of Einstein's theory of relativity by comparing time dilation effects due to the Moon's lower gravity. They could also serve as precise frequency references for radio astronomy or deep-space communications. The same cavity-stabilized laser design could be used to search for variations in fundamental constants. In essence, the cold, dark craters offer a one-of-a-kind laboratory for precision physics, with applications that extend far beyond navigation.