Artist rendering of NASA’s Lunar Reconnaissance Orbiter. Credit: NASA’s Goddard Space Flight Center
Dozens of times in the last decades NASA Scientists have launched laser beams at a reflector the size of a paperback novel, about 240,000 miles (385,000 kilometers) from Earth. She announced today, in collaboration with her French colleagues, that she was receiving for the first time a signal, an encouraging result that could improve laser experiments that were used to study the physics of the universe.
The reflector intended for NASA scientists is mounted on the Lunar Reconnaissance Orbiter (LRO), a spacecraft that has studied the moon since its orbit since 2009. One reason engineers had placed the reflector on LRO so that it could serve as an incredible goal to help test the reflectivity of panels about 50 years ago on the moon’s surface. These older reflectors give off a weak signal, which makes it harder to use them for science.
Scientists have been using reflexes on the moon since the Apollo era to learn more about our nearest neighbor. It’s a fairly simple experiment: Direct a beam of light toward the reflector and clock how much time it takes for the light to return. Decades of making this one measurement has led to great discoveries.
One of the biggest revelations is that the Earth and the Moon are slowly drifting apart at the rate that fingernails are growing, or 1.5 inches (3.8 centimeters) per year. This widening gap is the result of gravity interactions between the two bodies.
“Now that we’ve been collecting data for 50 years, we can see trends we would not otherwise have seen,” said Erwan Mazarico, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. t the LRO experiment that was described on August 7 in the journal Earth, Planets and Space.
“Science with laser varying is a long game,” Mazarico said.
But if scientists need to use the surface panels far into the future, they need to figure out why some of them only return a 10th of the expected signal.
A close-up photo of the laser-reflecting panel deployed by Apollo 14 astronauts a month in 1971. Credit: NASA
There are five reflective panels on the moon. Two were handed down by Apollo 11 and 14 crews in 1969 and 1971. They are each made of 100 mirrors that scientists call ‘corner cubes’ because they are corners of a glass cube; the advantage of these mirrors is that they can reflect light back in any direction where it comes from. Another panel with 300 corner cubes was launched in 1971. by Apollo 15 astronauts. Soviet robot controversies called Lunokhod 1 and 2, which landed in 1970 and 1973, carry two additional reflectors, each with 14 mirrors. Collectively, these reflectors comprise the latest working science experiment from the Apollo era.
Some experts think that dust may have settled on these reflectors over time, possibly after the onset of micrometeorite impact on the moon’s surface. As a result, the fabric could block light to reach the mirrors and also insulate and overheat the mirrors and become less efficient. Scientists hope to use the LRO reflector to determine if this is true. She pointed out that if they found a deviation in the light returning from the reflector of LRO versus the surfaces, they could use computer models to test whether dust, if anything, is responsible. Whatever the cause, scientists could then take this into account in their data analysis.
Despite their first successful experiments on laser, Mazarico and his team have not yet completed the dustbin. The researchers are refining their technique so that they can collect more measurements.
The art of sending a photon bar to the moon … and getting it back
In the meantime, scientists are relying on surface fluctuators to learn new things, despite the weaker signal.
By measuring how long it takes for laser light to bounce back – about 2.5 seconds on average – researchers can calculate the distance between Earth’s laser stations and Moon reflectors to less than a few millimeters. This is about the thickness of an orange peel.
In addition to the Earth-Moon drift, such measurements over a long period of time and across various reflectors have revealed that the moon has a liquid core. Scientists can tell by taking care of the smallest wibbles as the moon rotates. But they want to know if there is a solid core of that liquid, said Vishnu Viswanathan, a NASA Goddard scientist studying the moon’s internal structure.
“Knowledge of the moon’s interior has far-reaching implications that include the moon’s evolution and the timing of its magnetic field and how it died,” Viswanathan said.
This photo shows the laser-guided facility at the Goddard Geophysical and Astronomical Observatory in Greenbelt, Md. The facility helps NASA track satellites. Both shown trees, derived from two different lasers, are pointed at NASA’s Lunar Reconnaissance Orbiter, which runs around the moon. Here, scientists use the visible, green wavelength of light. The laser facility at the Université Côte d’Azur in Grasse, France, developed a new technique that uses infrared light, which is invisible to the human eye, to beam laser light to the moon. Credit: NASA
Magnetic measurements of lunar samples returned by Apollo astronauts reveal what no one 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 within the month it could have generated.
Laser experiments could help discover if there is any material in the core of the moon that would have helped the now-extinct magnetic field. But to learn more, scientists must first know the distance between Earth stations and the Mountain Reflectors to a greater degree. accuracy then the current few millimeters. “The accuracy of this one measurement has the potential to refine our understanding of gravity and the evolution of the solar system,” said Xiaoli Sun, a Goddard planetary scientist who helped design the LRO reflector.
Getting more photons after the moon and back and doing better accounting for, for example, lost by dust, are a few ways to improve the precision. But it is a Herculean task.
Think of the 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 thick atmosphere of the earth, which tends to scatter them.
Astronaut Edwin E. Aldrin Jr., lone module pilot, exposes two components of the early Apollo scientific experiment package to the surface of the moon during Apollo 11’s extravehicular activity in 1969. A seismic experiment is in his left hand, and to its right is a laser-reflecting panel. Astronaut Neil A. Armstrong, mission commander, took this photo. Credit: NASA’s Johnson Space Flight Center
That, which begins as a ray of light that is about 10 feet, if a few meters wide on the ground, can spread to more than 1 mile, or 2 kilometers, until the time the moon’s surface arrives, and much wider as it jumps backwards. That translates to a chance at one million in 25 million that a photon launched from Earth will reach the Apollo 11 reflector. For the few photons that manage to reach the Moon, there is an even lower chance, one in 250 million, that they will make it back, according to some estimates.
If these odds seem daunting, LRO’s reflector is even more challenging to achieve. For one, it’s a 10th the size of the smaller Apollo 11 and 14 panels, with only 12 corner cube mirrors. It is 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. Water at the laser station also affects the light signal, as does the alignment of the sun, moon and earth.
That’s the reason that despite several attempts in recent decades NASA Goddard scientists have not been able to reach the reflector of LRO until its collaboration with French researchers.
Their success to date is based on the use of advanced technology developed by the Géoazur team at the Université Côte d’Azur for a laser station in Grasse, France, which can pulse an infrared wavelength of light at LRO. One advantage of using infrared light is that it penetrates the Earth’s atmosphere better than the visible green wavelength of light that scientists have traditionally used.
But even with infrared light, the Grasse telescope recovered only about 200 photons from tens of thousands of pulses cast at a few dates in 2018 and 2019 at LRO, Mazarico and his team reported in their paper.
It may not seem like much, but even a few photons over time can help to respond to the surface reflector’s dust demand. A successful laser beam result also shows the promise of using infrared laser for precise surveillance on Earth’s orbits, and for the use of many small reflectors – perhaps installed on NASA’s commercial landers – to do this. to do. This is why some scientists want to see new and improved reflectors sent to more regions of the moon, which NASA plans to do. Others advocate for more facilities to get the world equipped with infrared lasers that can pulsate from different angles to the Moon, which can further improve the accuracy of remote measurements. New approaches to laser like this could ensure that the legacy of these fundamental studies will continue, scientists say.
Reference: “First two-way laser, varying to a lunar orbiter: infrared observations from the Grasse station to the LRO’s retro-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
