(ORDO NEWS) — When NASA’s Lunar Orbiter (LRO) arrived in orbit on the moon in 2009, scientists immediately began firing lasers at it.
Specifically, they fired lasers at a small array of reflectors about the size of a book in an attempt to reflect light back to Earth. And after almost 10 years it succeeded.
This is the first time that photons have been successfully reflected back to Earth from a lunar orbiter.
As part of the Apollo program, astronauts visited the moon from 1969 to 1972. They left behind (among other things) equipment for continuous monitoring, such as seismometers and three laser reflectors. The Soviet space program also installed reflectors on robotic lunar rovers.
Why laser reflectors? Well, if you aim a really powerful laser beam at the moon and determine how long it will take for it to travel backwards, you can really accurately measure the distance between two points based on the speed of light. Thus, we can determine the distance to the moon with millimeter precision.
Over time, these measurements can paint a picture of how the moon is moving. This is how we know the Moon has a liquid core based on how it rotates; and if there is solid material in that liquid core, that, in turn, could tell us how the Moon once launched its magnetic field.
Thanks to these precise measurements, we also know that the Moon is slowly receding from the Earth at a rate of about 3.8 centimeters per year. Distance measurements can tell us a lot if we are patient.
“Now that we collect data from the past 50 years, we may see trends that would otherwise go unnoticed,” said planetary scientist Erwan Mazarico of NASA’s Goddard Space Flight Center. “The science of laser ranging is a long game.”
But there is a problem. Over time, the amount of light returned by these lunar reflectors has decreased to 10 percent of what it should have. And it’s not clear why.
So this is where the LRO comes in. If we can receive signals reflected from its reflector, scientists can compare results from surface reflectors.
Using simulations, this could help determine the reason for the decrease in the effectiveness of surface reflectors – and, possibly, reveal how much the Moon is bombarded by micrometeorites and how much dust is raised by this bombardment.
However, this is much easier said than done. It is quite difficult to hit the reflectors on the surface of the Moon, largely due to the atmospheric effects of the Earth and the attenuation of electromagnetic radiation. The LRO reflector is even more complicated. It is a small, fast-moving target, only 15 by 18 by 5 centimeters, and is, on average, 384,400 kilometers from Earth.
Initial attempts by the research team to reach the reflector using green visible light were unsuccessful. But then they teamed up with scientists at the University of the Côte d’Azur in France to develop an infrared laser – light that is much more efficient at penetrating gas and clouds.
On September 4, 2018, a laser locating station in Grasse, France, for the first time detected an infrared laser beam reflected from an LRO.
Then, in two sessions, August 23 and 24, 2019, the result was repeated – except this time the team also turned the spacecraft to orient the reflector towards Earth.
The amount of light returned was minimal – just a few photons. However, this is not enough to find out what is blocking reflectors on the lunar surface. But over time, even a few photons can make up a fairly accurate picture.
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