Lunar Time Standard Taking Shape

With traffic on and around the Moon expected to increase in the coming years, momentum is building behind the idea of creating a lunar timekeeping standard that accounts for relativistic effects to enable more precise navigation and communications.

The lower gravitational pull on the Moon and its motion relative to the Earth result in time moving faster there relative to an observer on Earth by approximately 58.7 microseconds per day. This difference has been manageable in the past but poses a challenge for future missions that need more precision and interoperability, such as that required by GPS-like navigation.

In August, the International Astronomical Union formally called on space agencies around the world to establish a lunar timekeeping standard. Work on such a standard has already officially begun in the U.S., with an April directive by the White House Office of Science and Technology Policy tasking NASA with developing an implementation strategy for lunar time standardization by the end of 2026. The standard is to be called Coordinated Lunar Time (LTC), an analogue to the primary time standard used on Earth, Coordinated Universal Time (UTC).

OSTP set four major requirements for the new standard: traceability to UTC, accuracy sufficient to support precision navigation and science missions, resilience in case of loss of contact with Earth, and scalability to environments beyond the Earth-Moon system.

The OSTP directive also makes clear that the U.S. intends for the LTC standard to be adopted by international participants in the Artemis program, NASA’s effort to return humans to the Moon.

Furthering the push for this standard, in June Rep. Jennifer McClellan (D-VA) introduced the Celestial Time Standardization Act, which would direct NASA to create a strategy for implementing LTC, essentially matching the requirements in the OSTP directive. This act was included in the NASA Reauthorization Act that passed the House Science Committee in July and now awaits a floor vote.

“We are entering a new era of United States space exploration and have an exciting opportunity to lead on standards that drive global competitiveness,” McClellan said in a press release. “As we pursue increasingly complex activities on the Moon and beyond, the need for a celestial time standard will only continue to grow.”

Other federal agency contributors to the standard include the departments of Commerce, Defense, State, and Transportation. Within the Commerce Department, researchers at the National Institute of Standards and Technology are helping develop mathematical models necessary to implement LTC.

NIST physicists Neil Ashby and Bijunath Patla published a paper in August that suggests LTC could be maintained by placing atomic clocks on the Moon’s surface and in orbit, similar to the network used to synchronize time on Earth. OSTP’s April directive also raises the possibility of such a system, stating, “an ensemble of clocks on the Moon might set Lunar Time.”

Patla emphasized in an interview that LTC would not be a time zone like Eastern Time or Mountain Time but rather a “reference coordinate” from which time zones could be derived.

“Even clocks, good clocks, when you synchronize them, they still wander off in some hours, and days, months,” Patla said. “You have to closely track, monitor, and keep it within a range for everything, for navigation to work, and also for communication, especially when people are thinking about returning humans back to the Moon and even possibly to Mars.”