For the first time, scientists have successfully bound a laser from a mirror attached to a spaceship orbiting the month.
Laser ejection from mirrors on the lunar surface is an old trick. Astronauts orbiting the moon first left reflectors behind in 1969. And since then, researchers have bounced lasers of these reflectors to make accurate measurements of the distance between the moon and Earth, using the speed of light and the time it takes for the laser beam to return to Earth.
But hitting a laser in orbit around the moon is a much more difficult task. The Lunar Reconnaissance Orbiter (LRO) has been roaming the moon with a mirror on its back since 2009. But nearly a decade passed without a single successful laser bounce. In an August 6 paper in the magazine Earth, planets and space, a team of researchers reports the first successful laser contact: Twice on 4 September 2018 and twice again between 23 August and 24 August 2019, technicians at the Lunar Laser Ranging (LLR) station in Grasse, France, fired laser bursts at the LRO and saw the light back 2.5 seconds later.
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To ensure that the light emanating from the LRO returns in the direction of where it came from, the mirror on the back is more intricate than the one in your bathroom. Like the older mirrors on the lunar surface, it is a “corner cube” – a series of three-dimensional mirrors, each literally shaped like the inside of one corner of a cube. When a laser hits it, the light bounces three times before the geometry of the mirror returns it in the exact direction it came from.
Following the movement of the LRO over time is an interesting scientific project in its own right. But, the researchers wrote, these four successful laser contacts do not provide enough data to track this movement. The LRO still moves too fast and too unpredictably to be reliably shot with a laser, and all four contacts were made under what the researchers described as ideal conditions. The moon, LRO and France were all perfectly stacked to improve the chance of laser contact.
In the long run, investigations of the LRO mirror could help solve a troublesome problem affecting the mirrors left on the lunar surface. All of these mirrors have become less reflective over time, and researchers are not sure why. But that lost quality makes accurate measurements more difficult. The problem could be that long-term exposure to solar radiation simply weakens the mirrors. In that case, the mirror of the LRO would have to weaken over time at the same rate. Alternatively, lunar dust like fluffy haze from the moon’s thin atmosphere could hide the mirrors, the researchers wrote. In that case, the reflectivity of the LRO would have to remain more or less unusual over time high in orbit, even if the surface levels degrade.
Originally published on Live Science.
