Dozens of times in the past decade NASA Scientists have directed laser beams onto a reflector the size of a paperback novel about 385,000 kilometers from Earth. They announced today, along with their French colleagues, that they have received a signal back for the first time, an encouraging result that could improve laser experiments studying the physics of the universe.
The reflector sought by NASA scientists is mounted on the Lunar Reconnaissance Orbiter (LRO), a spacecraft that has been examining the moon from its orbit since 2009. One reason engineers placed the reflector on LRO was so that it could serve as a flawless target to help test the reflectivity of panels that were left on the lunar surface about 50 years ago. These older reflectors give back a weak signal, making them more difficult to use in science.
Scientists have been using reflectors on the moon since the Apollo era to learn more about our closest neighbor. It’s a pretty simple experiment: aim a beam of light at the reflector and measure the time it takes for the light to come back. Decades of measurements have led to significant discoveries.
One of the biggest revelations is that the Earth and Moon are slowly drifting apart at the rate at which fingernails grow, or 1.5 inches per year. This widening gap is the result of gravitational interactions between the two bodies.
“Now that we’ve been collecting data for 50 years, we can spot trends that we wouldn’t otherwise have seen,” said Erwan Mazarico, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who coordinated the data-coordinated LRO experiment described in Earth, Planets and Space magazine on Aug. 7.
“Laser ranging science is a long game,” said Mazarico.
However, if scientists want to continue using the surface plates in the future, they need to find out why some of them only return a tenth of the expected signal.
There are five reflective panels on the moon. Two were delivered by Apollo 11 and 14 crews in 1969 and 1971. They each consist of 100 mirrors, which scientists refer to as “corner cubes” because they are the corners of a glass cube. The advantage of these mirrors is that they can reflect light back in any direction it comes from. Another tablet with 300 corner cubes was set down by Apollo 15 astronauts in 1971. The Soviet robotic rovers Lunokhod 1 and 2, which landed in 1970 and 1973, carry two additional reflectors with 14 mirrors each. Together, these reflectors form the final scientific experiment from the Apollo era.
Some experts suggest that over time, dust settled on these reflectors, possibly after being kicked up by micrometeorite impacts on the lunar surface. As a result, the dust could prevent light from reaching the mirrors and also isolate the mirrors, causing them to overheat and become less efficient. The scientists hoped to use the LRO reflector to determine if this is the case. They found that they could use computer models to test if dust or something else was responsible if they found a discrepancy between the light returned by the LRO’s reflector and the surface reflector. Regardless of the cause, scientists could then take this into account in their data analysis.
Despite their first successful laser range finding experiments, Mazarico and his team have not yet resolved the dust issue. The researchers are refining their technique to collect more measurements.
The art of sending a beam of photons to the moon … and getting it back
In the meantime, despite the weaker signal, scientists continue to rely on the surface reflectors to learn new things.
By measuring the rebound time of laser light – about 2.5 seconds on average – researchers can calculate the distance between earth laser stations and moon reflectors to be less than a few millimeters. This is roughly the thickness of an orange peel.
In addition to the earth-moon drift, such measurements over a long period of time and over several reflectors have shown that the moon has a liquid core. Scientists can tell by monitoring the slightest wobble as the moon rotates. But they want to know if there is a solid core in this liquid, said Vishnu Viswanathan, a NASA Goddard scientist who studies the internal structure of the moon.
“Knowledge of the interior of the moon has greater implications for the evolution of the moon and the explanation of the timing of its magnetic field and its extinction,” Viswanathan said.
Magnetic measurements of lunar samples returned by Apollo astronauts revealed something that no one expected given how small the moon is: our satellite had a magnetic field billions of years ago. Scientists have tried to find out what could have created it inside the moon.
Laser experiments could help to find out whether there is solid material in the lunar core that would have supplied the now-extinguished magnetic field with electricity. To learn more, scientists first need to know the distance between earth stations and the lunar reflectors to a greater extent accuracy than the current few millimeters. “The precision 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 LRO’s reflector.
Getting more photons to the moon and back, and taking better account of those lost through dust, for example, are some ways to improve precision. But it is a Herculean task.
Look at the surface plates. Scientists must first determine the exact location of each one, which is constantly changing with the moon’s orbit. Then the laser photons have to move twice through the thick earth’s atmosphere, which tends to scatter them.
What starts as a beam of light that is about 10 feet 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 becomes much wider on bounce back. This corresponds to a one in 25 million chance that a photon fired from Earth will reach the Apollo 11 reflector. The few photons that make it to the moon have an even smaller chance, one in 250 million, of making it back, by some estimates.
When these opportunities seem daunting, reaching the LRO’s reflector is even more difficult. For one, it’s a 10th the size of the smaller Apollo 11 and 14 panels with only 12 corner cube mirrors. It also hangs on a fast moving target the size of a small car that is 70 times further from us than Miami is from Seattle. The weather at the laser station influences the light signal as well as the alignment of the sun, moon and earth.
This is why NASA’s Goddard scientists were only able to reach the LRO reflector in collaboration with French researchers, despite several attempts over the past decade.
Their success to date is based on the use of advanced technologies developed by the Géoazur team at the Université Côte d’Azur for a laser station in Grasse, France, which can pulsate an infrared wavelength of light at LRO. One benefit 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 only received about 200 photons from tens of thousands of pulses given off at LRO during some data in 2018 and 2019, as Mazarico and his team report in their paper.
It doesn’t seem like much, but even a few photons over time could help answer the question about the surface reflector dust. A successful laser beam return also shows the promise to use infrared lasers to precisely monitor the orbits of the earth and moon using many small reflectors that may be installed on NASA’s commercial lunar landers. Because of this, some scientists wish that new and improved reflectors would be sent to more regions of the moon, which NASA is planning. Others are calling for more facilities around the world equipped with infrared lasers that can pulse from different angles to the moon, which can further improve the precision of distance measurements. New approaches to laser removal like this one can ensure the legacy of these fundamental studies continues, say scientists.
Reference: “First two-way laser up to a moon orbiter: infrared observations from the Grasse station to the retro reflector array from LRO” by Erwan Mazarico, Xiaoli Sun, Jean-Marie Torre, Clément Courde, Julien Chabé, Mourad Aimar and 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, Gregor A. Neumann, Maria T. Zuber and David E. Smith, August 6, 2020 Earth, planets and space.
DOI: 10.1186 / s40623-020-01243-w