How Long Does It Take For A Laser Beam To Reach The Moon?

On average, the Moon is around 238,855 miles from Earth. Depending on where it is in its orbit, it can be as close as 226,000 miles or as far away as 251,000 miles. With a radius of approximately 1,080 miles, it's the fifth-largest moon in our solar system, but it's only one-third as wide as the big blue marble we call home. For a better visual reference, think of Earth as the diameter of a nickel and the Moon as that of a small pea or coffee bean. Roughly 30 nickels, or Earth-sized planets, would fit between the two.

We're not delving into how a laser works, but know that a laser's tightly packed beam is very different from the scattered light emitted by a bulb or flashlight. The very word "laser" isn't a word at all, but an acronym for Light Amplification by Stimulated Emission of Radiation. A laser beam, which doesn't occur naturally, is made of individual particles of light energy called photons. This tightly focused stream of coherent light waves will travel in one direction without scattering. So, if the beam were fired into space, it would continue forever until it hit a solid object like the Moon.

Aside from the ever-expanding universe, nothing can go faster than the speed of light, at least as we currently understand it. That universal speed limit is 670,616,629.3844 miles per hour. Even though laser light differs from normal light, it's still light. Therefore, it would take a laser beam between 1.21 and 1.35 seconds to reach the Moon, again depending on where it is on its celestial trajectory.

Ah, but these numbers aren't just derived from a mathematical equation; they've been measured in real life. Unless you're a hardcore astrophile, you may not realize that five reflecting panels are currently sitting on the Moon. These panels have what scientists call "corner cube" mirrors attached to them because they're literally corners of a glass cube that can reflect light back to its source, no matter where it came from. See where we're going here?

The first was deposited by Apollo 11 astronauts in 1969, which had a total of 100 corner cubes. The Soviets dropped their first robotic rover (Lunokhod 1) in 1970, which had 14 mirrors. In 1971, the Apollo 14 crew placed one with 100 corner cubes, while the Apollo 15 crew placed another with 300. The Soviet Lunokhod 2 (with 14 mirrors) was left in 1973. The goal was to shoot a laser beam from Earth to the Moon to track the distance between them over time. On average, light takes about 2.5 seconds to make a round trip. As it turns out, these two celestial bodies are actually drifting about 1.5 inches apart every year.

But those old mirrors were starting to get dirty, likely caused by moon dust kicked up by tiny meteorite impacts. So, in June 2009, NASA launched the Lunar Reconnaissance Orbiter (LRO) to study our nearest celestial body from its own orbit. Attached was a new, improved, clean reflector that NASA said was "the size of a paperback novel." Interestingly, it took over a decade to get a beam to hit its mark, but it finally did so in August 2020.