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For the first time, scientists have successfully bounced a laser off a mirror attached to a spaceship revolving around the Moon.
Bouncing lasers off mirrors on the lunar surface is an old trick. Astronauts walking on the moon first left the reflectors behind in 1969. And since then, researchers have bounced lasers off these reflectors to make precise measurements of the distance between the moon and the moon. Earth, using the speed of light and the time it takes for the laser beam to return to Earth.
But hitting a laser orbiting the moon is a much trickier task. The Lunar Reconnaissance Orbiter (LRO) has been circling the Moon with a mirror on its back since 2009. But nearly a decade has passed without a single successful laser bounce. In an August 6 article in the newspaper Earth, planets and space, a team of researchers reports the first successful laser contact: twice on September 4, 2018 and twice again between August 23 and August 24, 2019, technicians from the Lunar Laser Ranging (LLR) station in Grasse, France, shooting at laser bursts at the LRO and sees the light come back 2.5 seconds later.
Related: Why is the moon shining?
To ensure that the light bouncing off the LRO returns in the direction it came from, the mirror on its back is more complicated 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 having the literal shape of the inside of a corner of a cube. When a laser hits it, the light bounces three times before the mirror geometry bounces it back in the exact direction it came from.
Tracking 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 don’t offer enough data to track this movement. The LRO is still moving too fast and too unpredictably to reliably strike with a laser, and all four contacts were made in what the researchers described as ideal conditions. The moon, LRO, and France were all perfectly aligned to improve the chances of laser contact.
In the long term, studies of the LRO mirror could help solve a thorny problem impacting mirrors left on the lunar surface. All of these mirrors have become less reflective over time, and researchers aren’t sure why. But this lost quality makes precise measurements more difficult. The problem could be that long-term exposure to solar radiation simply weakens the mirrors. In this case, the mirror of the LRO should weaken over time at the same rate. Alternatively, moon dust or light haze from the moon’s thin atmosphere could obscure the mirrors, the researchers wrote. In this case, the reflectivity of the LRO should remain more or less unchanged over time in high orbit, even as the surface mirrors degrade.
Originally posted on Live Science.
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