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More specifically, they fired lasers at a book-sized satellite dish so that reflected light would fly towards Earth. After almost ten years, they finally made it. This is the first time that photons have been successfully reflected from a satellite flying around the Moon. And not only does it provide new opportunities for lunar measurements, it can also help us understand the conditions on the lunar surface that are damaging equipment brought here more than 50 years ago.
Under the Apollo program, astronauts visited the Moon from 1969 to 1972. However, they not only visited there for a short time, but also left some equipment for continuous monitoring, such as seismometers and three laser reflectors. During the Soviet space program, the searchlights were transported to the Moon and mounted on two lunar moons. In total, both space agencies sent five of these searchlights.
Why laser reflectors? Well, if you send a really powerful laser beam to the Moon and you see how long it will take for the beam to “bounce off”, then it will be possible to determine the distance between two points very precisely, depending on the speed of light. Thus, we can determine the distance to the Moon to the nearest millimeter.
Over time, these measurements can provide more information about how the Moon rotates. This is how we know that the Moon has a liquid core (based on its rotation). And if there is solid matter in that liquid core, then it can tell us where the Moon got its energy from its magnetic field.
These precise measurements also gave us the information that the Moon is slowly moving away from Earth by about 3.8 cm per year. Distance measurements can tell us a lot, as long as we are patient.
“Because we’ve been collecting data for 50 years, we’re seeing trends that we wouldn’t otherwise notice,” said Erwan Mazarico, a scientist at NASA’s Goddard Space Flight Center. “The science of measuring distance is a long game.”
But there is a problem. Over time, the amount of light reflected by the lunar reflectors decreased and eventually decreased to just 10 percent. It is not clear why this is so.
What is really on the Moon is dust. Although there is no atmosphere on the Moon and therefore no wind to carry that dust, the effects of the tiny micrometeorites are enough to gradually build up a layer of dust on the reflectors.
And that’s where the LRO role comes in. If we can get the reflected signals from its reflector, scientists can compare the results obtained with those obtained from surface reflectors.
With the help of modeling, this would help determine the cause of the decreased efficiency of the surface reflector, as well as the extent to which the Moon is bombarded by micrometeorites and the amount of dust displaced by such bombardments.
But It’s easier to say it than do it. It is already difficult to hit the reflector on the surface of the Moon with a laser due to the atmospheric effects of the Earth and electromagnetic attenuation. Hitting the LRO reflector is even more difficult. It is a small, fast-moving target in dimensions of 15, 18 and 5 cm. And it is an average of 384,400 km from Earth.
The team’s initial attempts to reach the reflector using visible green light were unsuccessful. However, they later began collaborating with the University of the Côte d’Azur in France, whose researchers developed an infrared laser. Its light penetrates through gas and clouds much more efficiently.
September 4, 2018 The laser distance measuring station in Grase, France, captured the reflection of infrared laser light from the LRO for the first time.
Then the result was repeated on August 23 and 24, 2019, during two sessions of the year. This time, however, the team rotated the moon so that the reflector pointed at Earth and demonstrated how to create the possibilities for bi-directional laser distance measurement, rather than waiting for the LRO to rotate at the correct angle.
The return of light was minimal and consisted of only a few photons. This is still not enough to determine what is blocking reflectors on the Moon’s surface. But over time, even a few photons can draw a larger picture and tell us more.
However, it is not only the LRO measurements that are interesting in this case. This team’s work shows that improvements can be made by using an infrared laser rather than an optical one, as its light spreads further, potentially allowing the use of much smaller reflectors.
“This experiment offers a new method to test theories about the accumulation of dust on the lunar surface over decades,” the researchers said in their findings.
The findings of this study were published in the journal Earth, Planets and Space.
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