In 1969, Apollo 11 astronauts left lunar lasers on the moon. Decades later, the experiment remains active, with India's Chandrayaan-2 carrying reflectors to its mission to the moon.

The original laser reflector experiment onboard the Apollo 11 contained 100 small prisms that scientists on Earth would shoot with laser beams. The array of cubes were placed in an 18-inch-square aluminum panel.

Lunar missions that followed suit, Apollo 14 and 15 left more of those prisms. The main objective of the reflectors is to measure the distance between the Earth and the moon.

The special laser retroreflector that NASA supplied to ISRO is positioned on the Chandrayaan's Vikram Lander. The new microretroreflector device weighs only 22 grams and can be seen from the lunar orbit. It is similar to the one deployed on the InSight Mars lander.

NASA's Laser Experiment

The laser ranging retroreflector is a special type of mirror that can always reflect an incoming light beam to the same direction that it came from. The reflector requires no power and is still capable of operating.

The first precise measurement of the distance from Earth to the moon was achieved on Aug. 1, 1969, in a landmark experiment involving Lick Observatory astronomers and the Apollo 11 astronauts.

At the time, Lick Observatory's Shane Telescope was the second largest in the world. The 120-inch telescope fired a powerful laser beam at the moon and detected the light that bounced back from retroreflector array placed by Apollo 11 astronauts on the lunar surface.

When the laser pulse reached the moon, its light spanned over an area about 2 miles in diameter. The delay between the short pulses of light from the laser and the return signals from the moon was used to calculate the distance between the two bodies with unprecedented accuracy.

"It was a big technological feat to detect such a small signal," said Elinor Gates, staff astronomer at Lick Observatory.

The amount of light that came back to the telescope from the reflector array was just a few photons. When a laser beam hits a reflector, scientists at the observatories use sensitive filtering and amplification equipment to detect any return signal. NASA attributes the improvement of measurement of the moon's distance to better lasers and computing equipment.

Chandrayaan's Mission To The Moon

NASA congratulated ISRO on the successful launch of the Chandrayaan-2 on July 22.

"We're proud to support your mission comms using our Deep Space Network and look forward to what you learn about the lunar South pole where we will send astronauts on our #Artemis mission in a few years," NASA tweeted.

The mission experienced a setback when the launch on July 15 was aborted an hour before liftoff due to technical issues. Chandrayaan's soft landing on the south side of the moon is scheduled on Sept. 7. It will journey for 48 days to reach the moon.

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