A new space mission: How to tell time on the moon

Agreeing on an answer is key to ensuring safety in an increasingly crowded moonscape.

No matter where you are in the cosmos, navigation depends on precise timekeeping. Without it, spacecraft risk colliding. Lunar vehicles could get lost on the surface. Landers might touch down in the wrong place.

Yet you can’t just take the gold standard of timekeeping on Earth—known as Coordinated Universal Time, or UTC—and extend it to the moon, said Greg Heckler, program executive for commercialization at the National Aeronautics and Space Administration’s Space Communications and Navigation program.

The problem is time is affected by gravity. If you placed clocks on the Earth and moon and started them at exactly the same time, after 24 Earth hours, a clock on the lunar surface would be ahead by at least 56 millionths of a second, or 56 microseconds.

A difference of just one microsecond could result in an error of more than a mile in the estimated position of an object using Global Positioning System technology on Earth. Compound that error by at least 56 times for every day of a lunar mission’s lifespan, and suddenly the idea of a rocket hurtling toward the moon at 25,000 mph and then needing to slow down to slot into a congested lunar orbit sounds precarious.

To safeguard future missions, the White House has called upon NASA to establish a unified time standard on the moon that is traceable to Earth’s UTC. In a spring memo outlining the task, the Office of Science and Technology Policy called this standard Coordinated Lunar Time.

The European Space Agency is also cognizant of the need for establishing moon time. Achieving international consensus on a common definition will be a challenge but is essential for coordinating time-sensitive operations such as spacecraft docking, according to Javier Ventura-Traveset, a navigation science manager with the agency.

Once lunar time is established, it is likely that countries will use their own systems to track it, as they do with UTC, but the systems will be interoperable, NASA’s Heckler said.

In the past, when fewer people were trying to get to the moon, keeping accurate lunar time for precise navigation was less of an imperative. Managing fewer, shorter missions planetside, using Earth-based clocks that were synchronized with timekeepers onboard individual spacecraft, was sufficient.

But now, cislunar space—the vastness that includes the moon and everything between there and Earth—is getting crowded. Countries, private companies, universities and nonprofits are sending more missions up for longer and longer durations. Many aim to create permanent lunar settlements or use the celestial body as a launch point for missions to places such as Mars.

Within the next decade, NASA estimates that human activity in cislunar space will equal or exceed all past operations—more than 100 missions from a small number of countries that have occurred there since the Space Age began in 1957.

The stage for what amounts to today’s celestial gold rush was set about 25 years ago, when ice was discovered at the lunar south pole, according to Roger Launius, a former chief historian for NASA. Ice can be melted down to make drinking water. Its components can be broken down into breathable oxygen for future astronauts living on the moon or used to help make the rocket fuel that one day will be needed to get our species to Mars.

“There has been this realization that there are resources on the Moon that we could extract for a wealth of activities," Launius said. Yet while lunar missions have grown more sophisticated and frequent, the navigation and timing infrastructure undergirding those activities has stood still.

Every single thing operating at the moon is run by an operator on Earth, according to Philip Linden, a research fellow at Open Lunar, a California-based nonprofit focused on science and space exploration. Coordinated Lunar Time would allow missions to be quarterbacked from the moon, improving efficiency, he added.

Taking lessons from Earth-based timekeeping is at the heart of realizing this new policy objective.

Atomic clocks currently provide the most precise timekeeping on Earth, facilitating fast and accurate banking, computing and navigation. Based on the vibration of atoms, one second is defined as how long it takes a cesium-133 atom to oscillate 9,192,631,770 times. But in part because the moon is some 200,000 miles from the strongest part of Earth’s gravity, that one second passes more quickly on the moon than here.

A next step in defining a lunar time scale involves setting up atomic clocks there.

The European Space Agency has plans to include atomic clocks on board its lunar navigation satellites before 2030, Ventura-Traveset said, and intends to operate some on the moon’s surface as part of a lander mission scheduled for 2031.

In addition, there are opportunities for the U.S.’s future Artemis missions to bring atomic clocks to the moon. NASA could incorporate the devices into missions that will eventually take astronauts down to the surface, according to Heckler. The clocks could also be transported aboard private uncrewed missions contracted by the agency, though there are currently no plans in place.

The urgency surrounding NASA’s push for moon time is related to the U.S.’s efforts to get out ahead of its challengers in space, even as it needs competing space agencies and countries to agree on a common lunar time scale.

“Everyone has to be using the same reference," Linden said. “Or we haven’t solved the problem at all."

The recent federal memo said NASA will establish Coordinated Lunar Time as the international standard with the signatories of the United Nations’ Artemis Accords, a set of guidelines for exploring space and the moon. But some of the biggest spacefaring nations, like China and Russia, haven’t signed those accords.

Ultimately, an expensive, possibly decadeslong effort to help the world tell moon time is worth it, according to Heckler, because it lays the groundwork for timekeeping on other celestial bodies, too.

“NASA strategy is Moon to Mars," he said, adding, “We need to do things at the Moon to learn how to do them at Mars, and potentially other locations in the solar system. That may be for my grandchildren, or great-grandchildren. But the moon is that first step."

Write to Aylin Woodward at aylin.woodward@wsj.com