How to Measure the Earth–Moon Distance to the Last Micrometer
High-precision measurements of the Earth–Moon distance play a key role in studies of lunar geology and in tests of general relativity. State-of-the-art systems use pulsed lasers and achieve 5–10 mm accuracy, but now a researcher has proposed a pathway to significantly greater precision by replacing laser pulses with a continuous high-power beam [1]. He believes the new method could achieve submillimeter accuracy in the next few years, offering a technique to probe hidden details of the lunar interior or even to detect ultralow-frequency gravitational waves.
The best current technique for measuring the Earth–Moon distance involves sending a laser pulse to the lunar surface. From there, it reflects from a so-called corner-cube reflector, which sends photons back in the direction from which they came. Timing the pulse’s round trip gives the distance.
The beam can be kilometers wide by the time it reaches the Moon, so the reflector size determines the amount of reflected light. A smaller reflector increases accuracy because it reduces several sources of error, including errors stemming from the Moon’s wobbling. But there’s a trade-off: A smaller reflector returns fewer photons, which reduces the signal-to-noise ratio. The latest reflectors—some were placed on the Moon in March by the Blue Ghost mission—are as small as 10 cm in diameter, so they return perhaps 1% as many photons as previous, meter-scale models.
“Today’s pulsed systems return only a few photons per pulse, limiting ranging precision to 5–10 mm,” says Slava Turyshev of Caltech. As an alternative, Turyshev now proposes using a different measurement technique with a high-power continuous-wave laser. He estimates that such a system, employing a kilowatt laser, could allow the collection of light over an interval as long as 100 seconds. It could increase the number of returned photons by a factor of 10,000, potentially reaching sub-millimeter or even tens-of-micrometer accuracy.
A continuous-wave ranging system would measure the distance by encoding a signal using techniques familiar from radio waves—amplitude modulation (AM) or frequency modulation (FM). By comparing the reflected signal with a local “reference” laser, standard signal-processing techniques enable estimation of the time delay experienced by the light reflected from the Moon.
Realizing optimal precision in such a system will require overcoming a series of technical obstacles but seems eminently plausible, Turyshev notes. One key challenge is overcoming the distortions coming from atmospheric turbulence, which become worse over longer light-collecting times. Turbulence in Earth’s atmosphere can alter the lengths of light paths by tens to hundreds of micrometers. Overcoming this problem, says Turyshev, will require corrections of these errors, in part through improved real-time monitoring of temperature, pressure, and humidity gradients in the atmosphere. He also calculates that simultaneous laser measurements at multiple wavelengths—which could help researchers monitor the atmosphere’s optical properties—could reduce errors by a factor of 10 or so.
Another challenge is managing mechanical drifts and thermal expansion. Vibrations and small temperature changes can shift telescopes, mirrors, or optical benches by micrometers, which can introduce errors in the AM or FM optical signal encoding. Overcoming these problems, Turyshev argues, should be feasible with systematic use of extremely stable optics and mounts built from thermally nonsensitive materials. Alternatively, the temperature of optical equipment could be actively controlled to limit variations to within 0.1 °C, for example, using chilled‐water cooling.
Finally, the advanced optics and electronics involved in achieving such accuracy will require extreme precision in timing and high frequency stability for the reference lasers. “These frequency references will have to maintain a fractional stability of less than one part in 10¹³ over the entire measurement period, which could be as long as 100 seconds, to get an accuracy of a few tens of micrometers,” Turyshev says. Challenges to achieving this accuracy include real-time compensation for the Moon’s motion toward or away from Earth, which is roughly 1 kilometer per second.
Achieving the challenges outlined by Turyshev would make a big difference to future science, says Jürgen Müller of Leibniz University Hannover, Germany, a specialist in Earth-Moon dynamics. “If lunar tracking could be improved down to the submillimeter or even tens of micrometer [level],” he says, “one could do some nice new science.” In particular, Müller says his research team has shown in simulations that the boundary between the lunar core and the mantle and the rotation of the core could be much more precisely determined. Turyshev expects the improved accuracy of lunar ranging to lead to more stringent tests of the equivalence principle of general relativity and to opportunities to detect the very-low-frequency stochastic (random) gravitational-wave background.
–Mark Buchanan
Mark Buchanan is a freelance science writer who splits his time between Abergavenny, UK, and Notre Dame de Courson, France.
References
- S. G. Turyshev, “Lunar laser ranging with high-power continuous-wave lasers,” Phys. Rev. Appl. 23, 064066 (2025).