Somewhere on the near side of the Moon, in a zone referred to as the Sea of Rains, a small Soviet robot has remained still in the lunar dust for the past 55 years.
It is known as Lunokhod 1. It landed in November 1970, brought by the Luna 17 mission, and traversed approximately seven miles across the lunar landscape over the next eleven months before its onboard heating system failed, leaving the machine inactive. Attached to its front is a small glass tray — a laser retroreflector built in France, comprising fourteen corner-cube prisms intended to reflect incoming light directly back to its source.
For almost forty years following the rover’s silence, that reflector was essentially lost.
Not lost in a physical sense. It remained at the location where Lunokhod 1 had ceased to function. What was lost was its exact position. The Soviet mission control had an approximate idea of where the rover had come to rest, within a few kilometers, but due to the data-sharing practices of the Cold War, the precise coordinates were not published in a way that Western scientists could utilize. A handful of range measurements were taken within the first three months after landing. Those measurements existed somewhere within Soviet archives but were not available to the global community.
By the 1990s, teams focused on laser-ranging had stopped attempts to locate the reflector. It was impossible to aim accurately at a target that couldn’t be pinpointed within about a kilometer. The reflector was, for all intents and purposes, considered lost.
Then, in March 2010, NASA’s Lunar Reconnaissance Orbiter captured an image of the rover from above.
The significance of lunar laser ranging
The reason the search for an old Soviet mirror in 2010 was noteworthy requires explanation, as the science it facilitates is truly significant and infrequently covered in mainstream discussions.
Lunar laser ranging involves emitting very short laser pulses from ground-based telescopes directed at retroreflectors on the Moon, catching the tiny fraction of returning photons, and using the round-trip travel time to gauge the distance between Earth and the Moon. When executed correctly, this measurement boasts an accuracy of about one millimeter. Over an average distance of 384,400 kilometers, that’s roughly one part in four hundred billion.
That accuracy is fundamental to the science. Ranging measurements accumulated over time demonstrate, for instance, that the Moon is currently moving away from Earth at approximately 3.8 centimeters per year, influenced by tidal interactions. They enable extremely precise testing of Einstein’s general relativity — the theory forecasts specific minute deviations in the Moon’s orbit that other measurement methods cannot clarify. They provide direct insights into the Moon’s internal structure, including indications of a liquid outer core. They contribute to monitoring Earth’s rotation and the calibration of international time standards utilized by GPS and other systems requiring ultra-accurate timekeeping.
Currently, there are only five retroreflectors available for this purpose. Three were deployed by Apollo astronauts (Apollo 11, 14, and 15). Two were installed by the Soviet Union (Lunokhod 1 and 2). Combined, these five mirrors form the entirety of the foundational data for one of the most challenging measurement endeavors in modern physics.
The loss of one of them — as occurred with Lunokhod 1 — meant a reduction of about twenty percent of the geometric baseline necessary for calculations.
Insights from the LRO images
The Lunar Reconnaissance Orbiter had been orbiting the Moon since 2009, systematically capturing high-resolution photographs of the surface as a part of a larger mapping initiative. Its onboard camera, LROC, was managed by a team led by Mark Robinson at Arizona State University.
In March 2010, the team directed the camera toward the area where Lunokhod 1 was thought to have ceased operation. The rover was distinctly visible in the images — a small dark object at the terminus of a set of tracks that had been untouched for nearly four decades. The team was able to determine its coordinates with an accuracy of approximately 100 meters.
That was sufficient. Tom Murphy of UC San Diego, who headed the APOLLO laser-ranging program at Apache Point Observatory in New Mexico, utilized the new coordinates and aimed his team’s 3.5-meter telescope at the target.
On April 22, 2010, they emitted the first laser pulse.
The results
The signal was not only present; it was astonishingly intense.
The APOLLO team was experienced in working with Lunokhod 2, whose reflector had been discovered and utilized for many years. The best returns they had received from Lunokhod 2 were around 750 photons per session. However, when they targeted Lunokhod 1’s site, they recorded approximately 2,000 photons in return.
Subsequent analysis indicated that Lunokhod 1’s reflector was returning signals roughly four to