Dozens of times in the past decade NASA Scientists have launched the laser beam at a reflection the size of a paperback about 240,000 miles (385,000 km) away from Earth. Today, in collaboration with their French colleagues, they announced that they were getting the signal back for the first time, an encouraging result that could enhance the laser experiments used for research. Save the physics of the universe.
The reflector targeted by NASA scientists is mounted on the Orbiter Reconnaissance Moon (LRO), a spacecraft that has been studying the Moon from its orbit since 2009. One reason engineers set the mirror the reflection on the LRO is so that it can act as a pristine target helping to test the reflected strength of the sheets left on the lunar surface about 50 years ago. These older reflectors are returning a weak signal, which makes it more difficult to use them scientifically.
Scientists have been using reflectors on the Moon since the Apollo era to learn more about our nearest neighbor. It’s a pretty simple experiment: Direct a beam of light at the reflector and count the time it takes for the light to return. Decades of taking this one measurement have led to great discoveries.
One of the biggest revelations is that the Earth and Moon are slowly moving apart at the rate of nail growth, or 1.5 inches (3.8 centimeters) per year. This widening distance is the result of the gravitational interaction between two objects.
“Now that we have been collecting data for 50 years, we can see trends that,” said Erwan Mazarico, a planetary scientist from NASA’s Goddard Space Flight Center in Greenbelt, Maryland. we wouldn’t be able to see it otherwise. The LRO experiment was described August 7 in the journal Earth, Planets and Space.
“The science of laser long-range is a long game,” says Mazarico.
But if scientists continue to use the panels in the future, they need to find out why some of them return only a tenth of the expected signal.
There are five reflectors on the Moon. The two planes were delivered by the crew of Apollo 11 and 14 respectively in 1969 and 1971. Each mirror is made of 100 mirrors that scientists call “angular cubes”, because they are the corners of a glass block; The benefit of these mirrors is that they can reflect light back in any direction it comes in. Another panel with 300 corners was dropped by the Apollo 15 astronauts in 1971. Soviet robots called the Lunokhod 1 and 2, landed in 1970 and 1973, carrying two reflectors. addition, with 14 mirrors each. In general, these reflectors include the last working scientific experiment from the Apollo era.
Some experts suspect that dust may have deposited on these reflectors over time, possibly after being kicked up by the effects of asteroids on the lunar surface. As a result, dust can block the light reaching the mirror and can also insulate the mirror, causing them to overheat and perform less efficiently. Scientists hope to use the LRO’s reflector to determine if that’s true. They calculated that if they detect a difference in the light returned from the LRO reflector compared to the reflector on the surface, they can use computer models to check if there is any liability. dust or something else or not. Whatever the cause, scientists could then explain it in their data analysis.
Despite their first successful laser range experiments, Mazarico and his team have yet to solve the dust question. Researchers are refining their technique so that they can collect more measurements.
The art of sending a beam of photons to the Moon… and bringing it back
Meanwhile, scientists continue to rely on surface reflectors to learn new things, despite weaker signals.
By measuring the time the laser light is bounced back – on average about 2.5 seconds – researchers can calculate the distance between Earth laser stations and the lunar reflector drops below a few millimeters. This is the thickness of the orange peel.
Besides the Earth-Moon drift, such measurements over a long period of time and over a number of reflectors have revealed that the Moon has a liquid core. Scientists can tell by tracking the slightest vibrations as the Moon rotates. But they want to know if there is a solid core inside the liquid, said Vishnu Viswanathan, NASA’s Goddard scientist, who studies the inner structure of the Moon.
“Knowing about the inside of the Moon has a greater significance in regards to the Moon’s evolution and explains the timing of its magnetic field and how it died,” Viswanathan said.
The magnetic measurements of the Moon samples returned by the Apollo astronauts reveal something no one would have expected when the Moon is so small: our satellite has had a magnetic field for billions of years. before. Scientists have been trying to find out what is inside the Moon that might have been creating it.
Laser experiments could help reveal whether there is solid material in the Moon’s core that helps power the now extinct magnetic field. But to learn more, scientists first need to know the distance between the Earth station and the lunar reflector at a higher level. accuracy more than several millimeters at present. “The accuracy of this measurement is likely to improve our understanding of gravity and gravity,” said Xiaoli Sun, a planetary scientist with Goddard who helped design the LRO’s reflector. development of the solar system.
For example, sending more photons to the Moon and going back and better calculating those photons lost to dust are a few ways to improve accuracy. But it is an extremely large task.
Examine the surface plates. Scientists must first determine the exact position of each one, which always changes according to the orbit of the Moon. The laser photons must then travel twice through Earth’s thick atmosphere, tending to scatter them.
Thus, what begins as a beam of light about 10 feet wide, or several meters wide, above ground can spread more than 1 mile or 2 kilometers, by the time it hits the Moon’s surface and is much wider when it bounces back. That means one in 25 million chances that a photon from Earth will reach the reflective surface of Apollo 11. According to some estimates, for a few photons to reach the Moon, the chance is low. Furthermore, for every 250 million they will come back, according to some estimates.
If these ratios seem difficult, reaching the LRO reflector is even more difficult. For one, it is a 10order the dimensions of the Apollo 11 and 14 plates are smaller, with only 12 cube mirrors. It is also attached to a compact car-sized fast-moving target, 70 times more away from us than Miami, Seattle. Weather at the laser station also affects the light signal, as well as the alignment of the Sun, Moon and Earth.
That is why, despite many efforts over the past decade, NASA Goddard scientists were unable to access the LRO’s reflector until they collaborated with French researchers.
So far, their success is based on the use of advanced technology developed by the Géoazur team at Côte d’Azur University for a laser station in Grasse, France, able to pulses infrared light wavelengths at LRO. One benefit of using infrared light is that it penetrates the Earth’s atmosphere better than the visible wavelengths of green light that scientists normally use.
But even with infrared light, the Grasse telescope received only about 200 photons out of the tens of thousands of pulses generated at the LRO for a few days in 2018 and 2019, Mazarico and his team reported in the paper. their.
It may not seem like much, but even a few photons over time can help answer the question of dust that is reflected on the surface. A successful return to the laser beam also shows promise to use infrared lasers to accurately monitor the orbits of the Earth and the Moon, and to use many small reflectors – perhaps installed on board. NASA commercial moon landing – to do so. This is why some scientists want to see new and improved reflectors sent to more regions of the Moon, something NASA is planning to do. Others are calling for more facilities around the globe equipped with infrared lasers that can send pulses to the Moon from different angles, which could further improve the accuracy of the measurements. range. The scientists say that new approaches to lasers like these could ensure that the legacy of these fundamental studies continues.
Reference: “The first bidirectional laser to orbit the moon: infrared observations from Grasse station to LRO’s inverse reflector array” 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, August 6, 2020, Earth, planets and space.
DOI: 10.1186 / s40623-020-01243-w