GOLDEN, Colo. — Scientists have successfully bounced a laser off the Soviet Union’s old Lunokhod 1 rover, which trekked across the Moon’s landscape more than four decades ago.
Lunokhod 1 was the first remote-controlled rover ever to land on another celestial body. The wheeled vehicle was carried to the lunar surface by a spacecraft called Luna 17, touching down Nov. 17, 1970 in the Sea of Rains.
Among its instruments, the rover carried a French-built laser retroreflector consisting of 14 corner cubes that can reflect laser light beamed from Earth.
Attempts to contact the rover after the lunar night that began in September 1971 were unsuccessful, apparently due to a component failure on the rover. Lunokhod 1’s days of roving the Moon formally ended on Oct. 4, 1971, after 11 lunar day-night cycles, or 322 Earth days.
Laser ranging observations
For the most part, those working in the laser ranging field have cautioned over the years not to spend time on Lunokhod 1, calling the rover a nearly impossible target.
“And during 30 years, nobody tried to range on Lunokhod 1,” Jean-Marie Torre, research engineer at the Côte d’Azur Observatory in France, said via email.
But Torre and his colleagues tried anyway, using the Côte d’Azur Observatory’s Grasse (MéO) laser-ranging station in Calern, France. In March, they received return signals from the Lunokhod 1 reflector.
The results were obtained over three nights, using a new instrumental configuration at the MéO station.
The historical difficulty of ranging on Lunokhod 1 may have been due to a number of factors. The reflector may have been dusty, its cover could have closed, or the rover may not have been parked in view of Earth, Torre said.
In the end, however, “it was more a problem of lack of confidence than [due] to a technical difficulty,” Torre said.
Poor weather conditions prevented the scientists from getting a good determination of the Lunokhod 1 reflector’s efficiency. Still, the results have buoyed the interest of Earth-based scientists to continue beaming their lasers at the long-dead rover.
A retroreflector array was also left on the Moon by the landing crew of NASA’s Apollo 11 mission in 1969, while two more retroreflector arrays were set up by Apollo 14 and Apollo 15 Moonwalkers.
In fact, Torre and a colleague were ranging successfully the Apollo 15 site when Torre suggested trying Lunokhod 1.
“Fortunately, we had immediate echoes … and if not, we might not have tried again for one or many years!” he said.
Pinpointing Lunokhod’s locale
The final end-of-mission location of Lunokhod 1 was uncertain until 2010. But thanks to images snapped by NASA’s Lunar Reconnaissance Orbiter, both the Luna 17 lander and Lunokhod 1 were spotted.
Lunokhod 1 came to its final stop on a site situated around 2.3 kilometers north of its point of landing.
The success in March by the Grasse station was not the first laser ranging effort targeting the “lost” Lunokhod 1 reflector.
In April 2010, specialists at the Apache Point Observatory Lunar Laser-ranging Operation in southern New Mexico used the Lunar Reconnaissance Orbiter images to first pinpoint the locale of Lunokhod 1, closely enough for laser range measurements.
Surprisingly, the Apache Point researchers reported that the craft’s retroreflector was returning much more light than other reflectors on the Moon.
“In normal conditions, the difficulty to range this [Lunokhod 1] reflector is the making of the corner cube [which is] silver coated. They are very good in the dark, from a few days after the new moon to the quarter. It is better to range the reflector from March to May,” when the on-the-moon reflector is the highest in the sky, Torre said.
A better understanding of the Moon
Lunar laser ranging has been made possible by combining advances in laser technology, data processing and precision timing via atomic clocks, according to the International Laser Ranging Service, a service of the International Association of Geodesy.
Lunar laser ranging uses short-pulse lasers and state-of-the-art optical receivers and timing electronics to measure how long it takes light beamed from ground stations to travel to retroreflector arrays on the Moon and back again.
It takes just two-and-a-half seconds for light to make this roundtrip trek, requiring use of an atomic clock.
Because the reflectors on the Moon are relatively small and a laser beam naturally loses its intensity with distance, only a tiny fraction of the signal makes it back. However, the information is sufficient for precise calculation of the Earth and Moon’s movement: speed of rotation, axial variation and orbital deviation.
Torre said that thanks to Lunokhod 1’s position on the Moon — closer to the Moon’s limb than any other reflectors — it allows researchers to detect more precisely small libration effects. Laser ranging on the rover can improve researchers’ understanding of the Moon’s internal structure and rotation, he added.