Dozens of times over the last decade NASA scientists have emitted a laser beam at a reflector the size of an unclean novel about 240,000 miles (385,000 kilometers) away from Earth. They announced today, in collaboration with their French colleagues, that they received the signal for the first time, an encouraging result that could strengthen the laser experiments used to study the physics of the universe.
NASA scientists’ reflector intended to be mounted on the Lunar Reconnaissance Orbiter (LRO), a spacecraft that has been studying the Moon from its orbit since 2009. One reason the engineers placed the reflector on the LRO was so it could serve as a pristine target to help test the reflective power of the remaining panels on the Moon’s surface about 50 years ago. These old reflectors are restoring a weak signal, which is making it more difficult to use them for science.
Scientists have been using reflectors on the Moon since the Apollo era to learn more about our nearest neighbor. It’s a fairly straightforward experiment: Aim for a beam of light on the reflector and clock the amount of time it takes for light to return. Decades of making this measurement have led to great discoveries.
One of the biggest discoveries is that the Earth and Moon are slowly moving away at the rate that nail polish is growing, or 1.5 inches (3.8 centimeters) a year. This expansion gap is the result of gravitational interactions between two bodies.
“Now that we’ve been collecting data for 50 years, we can see trends we would not be able to see otherwise,” said Erwan Mazarico, a planetary scientist from NASA’s NASA Space Flight Center in Greenbelt. , Maryland who coordinated the LRO Experiment described on August 7 in the journal Earth, Planets and Space.
“Laser science is a long game,” Mazarico said.
But if scientists are to continue to use surface panels in the future, they need to understand why some of them are only returning one-10th of the expected signal.
There are five reflective panels on the Moon. Two were delivered by the Apollo 11 and 14 crews in 1969 and 1971, respectively. They are each of the 100 mirrors that scientists call “corner cubes,” as they are the corners of a glass cube; the benefit of these mirrors is that they can reflect light in any direction it comes from. Another panel with 300 corner cubes was shot down by the Apollo 15 astronauts in 1971. The Soviet robot rover called Lunokhod 1 and 2, which landed in 1970 and 1973, hold two additional reflectors, with 14 mirrors each. Collectively, these reflectors constitute the latest labor science experiment from the Apollo era.
Some experts suspect that dust may have settled on these reflectors over time, possibly after being raised by micrometeorological impacts on the Moon’s surface. As a result, dust can block light from reaching the mirrors and also insulate the mirrors and causing them to overheat and become less efficient. The scientists hoped to use the LRO reflector to determine if it is true. They realized that if they found a discrepancy in the light returned by the LRO reflector with the surface ones, they could use computer models to test whether dust, or something else, is responsible. Whatever the cause, scientists could then calculate it in the analysis of their data.
Despite their first successful laser-scale experiments, Mazarico and his team did not solve the dust question alone. Researchers are refining their technique so they can collect more measurements.
The Art of Sending a Photon Ray to the Moon … and Its Return
Meanwhile, scientists continue to rely on surface reflectors to learn new things, despite the weaker signal.
By measuring how long it takes for laser light to bounce back – on average about 2.5 seconds – researchers can calculate the distance between the Earth’s laser stations and the Moon’s reflectors in less than a few millimeters. This has to do with the thickness of an orange peel.
In addition to the Earth-Moon displacement, such measurements over a long period of time and through several reflectors have revealed that the Moon has a fluid nucleus. Scientists can show by monitoring the smallest fluctuations as the Moon rotates. But they want to know if there is a strong core inside this liquid, said Vishnu Viswanathan, a scientist from NASA Goddard who studies the internal structure of the Moon.
“Knowledge of the Moon’s interior has greater implications that include the evolution of the Moon and the explanation of the time of its magnetic field and how it died,” Viswanathan said.
Magnetic measurements of lunar samples returned by Apollo astronauts revealed something no one had expected given how small the Moon is: our satellite had a magnetic field billions of years ago. Scientists have been trying to figure out what might have generated inside the Moon.
Laser experiments could help discover if there is a strong material in the Moon’s core that would help power the now-disappearing magnetic field. But to learn more, scientists must first know the distance between the Earth stations and the Moon reflectors at a higher degree of accuracy than a few millimeters of current. “The accuracy of this measurement has the potential to refine our understanding of the gravity and evolution of the solar system,” said Xiaoli Sun, a Goddard planetary scientist who helped design the LRO reflector.
Taking more photons to the Moon and returning and better calculating for those lost due to dust, for example, are some ways to help improve accuracy. But it is a Herculean task.
Consider surface panels. Scientists must first determine the exact location of each, which is constantly changing with the orbit of the Moon. Then, the laser photons have to travel twice through the Earth’s thick atmosphere, which tends to scatter them.
Thus, what starts as a ray of light that is about 10 meters, or a few meters, wide on the ground, can spread to more than 1 mile, or 2 kilometers, by the time it reaches the surface of the Moon, and much more wide when released supported. This translates into a one in 25 million chance that a photon emitted from Earth will reach the Apollo 11 reflector. For some photons that manage to reach the Moon, there is an even lower chance, one in 250 million, that they will return them, according to some estimates.
If these odds seem daunting, reaching the LRO reflector is even more challenging. For one, it’s a 10th the size of the smaller Apollo 11 and 14 panels, with only 12 cubic mirrors. It’s also attached to a fast-moving target, the size of a compact car that is 70 times farther from us than Miami is from Seattle. The weather at the laser station also affects the light signal, as well as the alignment of the Sun, Moon and Earth.
This is why despite several efforts over the last decade NASA Goddard scientists had not been able to reach the LRO reflector until their collaboration with French researchers.
Their success so far is based on the use of advanced technology developed by the Géoazur team at the Universit Côte d’Azur for a laser station in Grasse, France, that can pulse an infrared wavelength of light in the LRO. One benefit of using infrared light is that it penetrates the Earth’s atmosphere better than the length of green light that scientists have traditionally used.
But even with infrared light, the Grasse telescope only took about 200 photos back from the tens of thousands of pulses thrown at the LRO over several dates in 2018 and 2019, Mazarico and his team report on their work.
It may not seem like much, but even some photons over time can help answer the question of surface reflector dust. A successful return of the laser beam also shows the promise of using an infrared laser to accurately monitor the orbits of the Earth and the Moon, and to use many small reflectors – possibly installed on NASA’s commercial lunar land – for to do so. This is why some scientists will want to see new and improved reflectors sent to more regions of the Moon, which NASA is planning to do. Others are calling for more facilities around the globe equipped with infrared lasers that can pulsate on the Moon from different angles, which could further improve the accuracy of distance measurements. New approaches to lasers starting like these could ensure that the legacy of these basic studies continues, scientists say.
Reference: “The first two-way laser starting from a lunar orbiter: infrared observations from the Grasse station to the LRO retro-reflective group” by Erwan Mazarico, Xiaoli Sun, Jean-Marie Torre, Clément Courde, Julien Chabé, Mourad Aimar, Hervé Mariey, Nicolas Maurice, Michael K. Barker, Dandan Mao, Daniel R. Cremons, Sébastien Bouquillon, Teddy Carlucci, Vishnu Viswanathan, Frank G. Lemoine, Adrien Bourgoin, Pierre Exertier, Gregory A. Neumann, Maria T. Zuber and David E. Smith, 6 August 2020, Earth, planets and space.
DOI: 10.1186 / s40623-020-01243-w