Laser beam reflections between Earth and Moon 'a boost for science,' says NASA

12 Aug 2020

Notable discoveries include that the Earth and Moon are slowly drifting apart – at “fingernail growth” rate.

Over the past decade, scientists at NASA have regularly launched laser beams at a reflector on the surface of the Moon about the size of a paperback novel, 385,000 km from Earth. In a collaboration with French colleagues, NASA has now announced the team has received a reflected signal for the first time.

The reflector NASA scientists aimed for is mounted on the Lunar Reconnaissance Orbiter (LRO), a spacecraft that has been studying the Moon from its orbit since 2009. One reason engineers placed the reflector on LRO was so it could serve as a pristine target to help test the reflecting power of panels left on the Moon’s surface about 50 years ago. The older reflectors are returning a weak signal, which is making it harder to use them for science.

One of the biggest revelations of the laser reflection data is that the Earth and Moon are slowly drifting apart at approximately 38mm 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 see trends that we wouldn’t have seen otherwise,” commented Erwan Mazarico, a planetary scientist from NASA’s Goddard Space Flight Center in Greenbelt, Maryland, USA, who coordinated the LRO experiment that was described in the journal Earth, Planets and Space.

Reduced signals

But if scientists are to continue using the surface panels far into the future, they need to work out why some of them are returning only a 10th of the expected signal strength.

There are five reflecting panels on the Moon. Two were delivered by Apollo 11 and 14 crews in 1969 and 1971, respectively. They are each made of 100 mirrors that scientists call “corner cubes,” as they are corners of a glass cube; the benefit of these mirrors is that they can reflect light back to any direction it comes from.

Another panel with 300 corner cubes was placed by Apollo 15 astronauts in 1973. Soviet robotic rovers called Lunokhod 1 and 2, which landed in 1970 and 1973, carry two additional reflectors, with 14 mirrors each. Collectively, these reflectors comprise the last working science experiment from the Apollo era.

Some experts suspect that dust may have settled on these reflectors over time, possibly after getting kicked up by micrometeorite impacts to the Moon’s surface. As a result, the dust could be blocking light from reaching the mirrors and also insulating the mirrors and causing them to overheat and become less efficient. Scientists hoped to use LRO’s reflector to determine if that’s true.

Scientists continue to rely on the surface reflectors to learn new things, despite the weaker signal. By measuring how long it takes laser light to bounce back — about 2.5 seconds on average — researchers can calculate the distance between Earth laser stations and Moon reflectors to less than a few millimeters. Over time, such measurements have revealed that the Moon has a fluid core.

Solid core

But they want to know whether there’s a solid core inside of that fluid, said Vishnu Viswanathan, a NASA Goddard scientist who studies the internal structure of the Moon. “Knowing about the Moon’s interior has bigger implications that involve the evolution of the Moon and explaining the timing of its magnetic field and how it died out,” Viswanathan said.

Laser experiments could help reveal if there’s solid material in the Moon’s core. To learn more, scientists first need to know the distance between Earth stations and the Moon reflectors to a higher degree of 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 better accounting for ones that are lost because of dust, for instance, are a couple of ways to help improve precision. Consider the surface panels. Scientists must first pinpoint the precise location of each one, which is constantly changing with the Moon’s orbit. Then, the laser photons must travel twice through Earth’s atmosphere.

Thus, what begins as a light beam a few meters wide on the ground can spread out to more than 2 kilometers across, by the time it reaches the Moon’s surface, and then much wider when it bounces back. That translates to a one-in-25-million chance that a photon launched from Earth will reach the Apollo 11 reflector. For the few photons that manage to reach the Moon, there’s an even lower chance, one in 250 million, that they will make it back, according to some estimates.

If those odds seem daunting, reaching LRO’s reflector is even more challenging. For one, it is a tenth the size of the smaller Apollo 11 and 14 panels, with only 12 corner cube mirrors. Weather at the laser station impacts the light signal, too, as does the alignment of the Sun, Moon and Earth. That explains why NASA Goddard scientists had been unable to reach LRO’s reflector until their collaboration with French researchers.

Their success thus far is based on using advanced technology developed by the Géoazur Team at the Université Côte d’Azur, Nice, for a laser station in Grasse, both in France, that can pulse an infrared wavelength of light at LRO. One benefit of using infrared light is that it penetrates Earth’s atmosphere better than the visible green wavelength of light that scientists have traditionally used.