Apollo astronauts left mirrors on the moon decades ago; scientists are still firing lasers at them, here's why

More than half a century after humans first walked on the Moon, some of the equipment left behind is still helping scientists make remarkably precise measurements. Among the footprints, experiments and hardware scattered across the lunar surface are special devices that can reflect laser light sent all the way from Earth. One of the most important is the Apollo 15 Lunar Laser Ranging Retroreflector, an experiment that continues to serve science decades after astronauts placed it on the Moon. According to NASA, scientists can still measure the distance between Earth and these lunar reflectors to centimetre-level precision or better.

What Did Apollo 15 Leave on the Moon?

Apollo 15 astronauts landed on the Moon in July 1971. Among the scientific equipment deployed during the mission was a Lunar Laser Ranging Retroreflector array. NASA describes the Apollo 15 array as the largest working retroreflector array on the lunar surface, measuring about 105 by 65 centimetres. Its size has helped make it a primary target for lunar laser-ranging measurements. Although these devices are sometimes casually described as "mirrors", they are more sophisticated. Retroreflectors are designed to return incoming light towards the direction it came from, making it possible for observatories on Earth to target them with lasers.

Scientists Still Fire Lasers Towards the Moon

The basic idea sounds almost like science fiction. Scientists direct short laser pulses from Earth towards the retroreflectors on the Moon. A tiny portion of that light makes the enormous round trip back to Earth. By measuring how long the light takes to travel there and back, researchers can calculate the distance to the reflector with extraordinary accuracy. NASA says the distance from Earth to these retroreflectors is still routinely measured to the centimetre level or better. Their positions relative to one another are also known to a similar degree of precision.

Why Do These Decades-Old Reflectors Still Matter?

Knowing the Earth-Moon distance precisely is only part of their usefulness. According to NASA, lunar laser-ranging measurements can help scientists determine the Moon's orbit and orientation with great precision. The observations can also be used in studies testing theories of gravitation and general relativity. The reflectors additionally provide reference points for establishing an extremely precise latitude-and-longitude coordinate system for the lunar surface information that becomes increasingly valuable as exploration of the Moon continues.

An Apollo Experiment That Outlived Its Mission

Apollo 15 returned to Earth in August 1971, but the retroreflector it left behind never needed to come home to remain useful. There are no batteries to replace or moving components that astronauts need to service. Instead, researchers on Earth can continue using the reflector as a fixed lunar reference point decades after it was installed. It is an unusual legacy of the Apollo era: astronauts travelled hundreds of thousands of kilometres to the Moon and left behind a relatively simple-looking device. More than 50 years later, scientists are still pointing lasers at it, turning a piece of Apollo history into an experiment that continues to help us understand the Moon, gravity, and our place in space.