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Data rates during two-way laser link between Earth and the moon were well behind US experiment over a decade ago

China has pulled off two-way laser communication between Earth and the moon, making it only the second nation after the United States to do so. But the data rates it achieved were well behind those recorded by Nasa more than a decade ago, and that has left space experts scratching their heads.

Using telescopes in Yunnan province and an experimental satellite that is now orbiting the moon after a launch mishap in 2024, the team recorded a downlink speed of 100 megabits per second and an uplink speed of 1.25Mbps, according to the Chinese Academy of Sciences, which led the project.

By contrast, Nasa’s 2013 lunar laser communications demonstration achieved download speeds of 622Mbps and upload speeds of 20Mbps – roughly six times faster for downloads and 16 times faster for uploads.

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Jonathan McDowell, a London-based space historian and former Harvard astronomer, said the Chinese and US experiments were comparable.

“It sounds like China has not yet reached the data rates the US can achieve,” he said, adding that he did not know what accounted for the gap.

Yang Lei, a deep-space laser communications expert and head of the research team, noted the speed advantage of laser compared to other methods in an interview with state news agency Xinhua last week.

“An 8K high-definition image of the lunar surface would take about four to five minutes to transmit using a conventional 5Mbps microwave link,” he said. “With a 100Mbps laser link, it would take only about 12 seconds.”

The technology is taking on new importance as China and the US prepare to send astronauts to the moon and establish a long-term presence near the lunar south pole, with Nasa targeting its first crewed landing in more than half a century in 2028.

On Thursday, Nasa administrator Jared Isaacman said China had delayed its latest Chang’e-7 moon mission, but “at Nasa, we are not slowing down”.

“We are not counting them out. They are a very capable rival in the space race and we are not taking the foot off the gas,” he told US cable news network NewsNation.

Conventional communications – from everyday 5G networks to China’s Queqiao-2 satellite relaying signals between Earth and spacecraft on the moon’s far side – rely largely on radio waves, which spread like ripples and weaken dramatically over vast distances.

Lasers operate at much higher frequencies and can be focused into far narrower beams, concentrating energy and carrying more data faster with smaller, lighter equipment. But that narrow beam also makes long-distance laser communication extremely difficult.

“Earth-moon communication is like threading a needle from thousands of kilometres away,” Yang said.

At a distance of about 400,000km (248,550 miles), even tiny movements of the satellite or disturbances in Earth’s atmosphere can throw the beam off course, with a minute angular error resulting in a miss of several kilometres near the moon.

Yang said the team developed a tracking system that corrected for the satellite’s orbit, atmospheric distortion and the time it took light to travel between Earth and the moon, keeping the ground and space terminals locked onto each other while in motion.

The returning signal poses another challenge. After travelling 400,000km, it can be so faint that ground telescopes receive only a few photons at a time, amid interference from moonlight, starlight and city lights – like trying to hear a needle drop on a busy street.

The team used ultra-sensitive single-photon detectors and algorithms to extract the signal from the background noise, while new processing and coding technologies helped boost transmission speeds.

“We have now opened an ‘information highway‘ between Earth and the moon,” Yang said, adding that the technology would provide high-speed data links for China’s crewed lunar missions, planned research base and deep-space exploration.

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This article originally appeared on the South China Morning Post (www.scmp.com), the leading news media reporting on China and Asia.

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