For nearly forty years, scientists aimed lasers at the Moon and got nothing back from the Soviet rover Lunokhod 1 — then in 2010, a pulse bounced straight back from a reflector that had been sitting in the lunar dust the whole time.
This story is a fascinating tale of perseverance and the power of modern technology. It highlights the importance of precision and the unexpected ways in which scientific discoveries can be made.
The Lunokhod 1 rover, delivered to the Moon by the Soviet Luna 17 mission in 1970, carried a French-built laser retroreflector. This device was designed to send incoming light back in the direction from which it came, allowing scientists on Earth to measure the Earth-Moon distance with remarkable accuracy.
However, the reflector was effectively lost for decades. Scientists knew roughly where the rover had been, but the vast distance and the rover's final parking spot made it difficult to pinpoint the reflector's location. The reflector's usefulness was interrupted by geography and uncertainty, making it a piece of functioning hardware that could not be used.
The turning point came in 2010 when NASA's Lunar Reconnaissance Orbiter (LRO) photographed Luna 17 on Mare Imbrium, the plain where it had landed decades earlier. The image, catalogued as PIA12967, showed the lander still on the lunar surface where it delivered Lunokhod 1 in November 1970. This new orbital view helped researchers narrow the rover's location enough for a laser ranging attempt.
In the paper 'Laser Ranging to the Lost Lunokhod 1 Reflector', Thomas Murphy Jr. and colleagues reported that LRO images in March 2010 positively identified the rover and fixed its coordinates to within about 100 meters. This was good enough for the Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) to acquire a signal quickly.
The lunar retroreflector is a simple yet ingenious object. It contains corner-cube prisms that return light toward its source, even if the light arrives from a slightly different direction. On Earth, observatories send laser pulses toward reflectors left on the Moon by Apollo astronauts and Soviet rovers. A small fraction of the light comes back, and the timing reveals the distance.
Lunar laser ranging has been instrumental in testing gravity, refining the Moon's orbit, studying Earth's rotation, and investigating the Moon's interior. Over decades, repeated measurements have become a record of how the Earth-Moon system behaves.
The Apollo missions left reflector arrays on the lunar surface, and the Soviet Lunokhod 2 rover also carried one. Lunokhod 1 had one as well, but its usefulness was interrupted by geography and uncertainty. Without a reliable final position, a reflector is almost like a lock without an address.
The rediscovery of Lunokhod 1's reflector was a significant achievement. It was not only a recovered curiosity but also became part of the working apparatus scientists use to measure the Moon. The reflector's position near the lunar limb made it especially useful for certain measurements of lunar rotation and orientation.
The story of Lunokhod 1 is a testament to the power of modern technology and the importance of precision. It highlights the unexpected ways in which scientific discoveries can be made and the value of perseverance in the face of seemingly insurmountable challenges.
In conclusion, the rediscovery of Lunokhod 1's reflector has provided a new lever arm to a decades-long experiment. It has added a new dimension to our understanding of the Moon and the Earth-Moon system, demonstrating the enduring value of scientific exploration and the importance of precision in achieving scientific breakthroughs.
