Knowing what time it is on the Moon isn’t as easy as it may seem

The coming decade is set to witness a surge in lunar exploration, with numerous missions planned to explore and establish a presence on our natural satellite. This new era will require a unified system for keeping time in order to ensure seamless communication and navigation among the various missions operating on and around the Moon.

The discussion, which began at ESA’s ESTEC technology center in the Netherlands last November, is part of a broader effort to establish a common ‘LunaNet’ architecture for lunar communication and navigation services.

Importance of a common moon time reference point

“LunaNet is a framework of mutually agreed-upon standards, protocols and interface requirements allowing future lunar missions to work together, conceptually similar to what we did on Earth for joint use of GPS and Galileo,” explains Javier Ventura-Traveset, ESA’s Moonlight Navigation Manager.

“Now, in the lunar context, we have the opportunity to agree on our interoperability approach from the very beginning, before the systems are actually implemented,” he continued.

ESA navigation system engineer Pietro Giordano adds, “During this meeting at ESTEC, we agreed on the importance and urgency of defining a common lunar reference time, which is internationally accepted and towards which all lunar systems and users may refer to. A joint international effort is now being launched towards achieving this.”

Challenges of keeping time on the Moon

Currently, each new mission to the Moon operates on its own timescale exported from Earth. Deep space antennas are used to keep onboard chronometers synchronized with terrestrial time while facilitating two-way communications. However, this approach will not be sustainable in the upcoming lunar environment.

The Gateway station, once complete, will be open to astronaut stays and resupplied through regular NASA Artemis launches.

Meanwhile, numerous uncrewed missions will also be in place, with each Artemis mission releasing numerous lunar CubeSats. ESA will also be deploying its Argonaut European Large Logistics Lander.

These missions will not only be operating simultaneously on or around the Moon but will also be interacting with each other, potentially relaying communications, performing joint observations, or carrying out rendezvous operations.

Moonlight, LunaNet, and lunar timekeeping

ESA is developing the Moonlight program, a lunar communications and navigation service that will allow missions to maintain links to and from Earth and guide them around the Moon and on its surface.

Wael-El Daly, system engineer for Moonlight, explains, “Moonlight will need a shared common timescale in order to get missions linked up and to facilitate position fixes.”

NASA is also sponsoring an equivalent service called the Lunar Communications Relay and Navigation System (LCRNS). To maximize interoperability, these two systems should employ the same timescale, along with the many other crewed and uncrewed missions they will support.

Lunar time lessons from Earth’s GPS satellites

Interoperability of time and geodetic reference frames has been successfully achieved here on Earth for Global Navigation Satellite Systems. All of today’s smartphones are able to make use of existing GNSS to compute a user position down to meter or even decimeter level.

“The experience of this success can be re-used for the technical long-term lunar systems to come, even though stable timekeeping on the Moon will throw up its own unique challenges – such as taking into account the fact that time passes at a different rate there due to the Moon’s specific gravity and velocity effects,” explaines Jörg Hahn, ESA’s chief Galileo engineer.

Debate on setting and maintaining moon time

Among the current topics under debate is whether a single organization should be responsible for setting and maintaining lunar time and whether lunar time should be set independently on the Moon or kept synchronized with Earth.

The international team working on this subject will face considerable technical issues. For example, clocks on the Moon run faster than their terrestrial equivalents, gaining around 56 microseconds per day. Their exact rate depends on their position on the Moon, ticking differently on the lunar surface than from orbit.

“Of course, the agreed time system will also have to be practical for astronauts. This will be quite a challenge on a planetary surface where in the equatorial region each day is 29.5 days long, including freezing fortnight-long lunar nights, with the whole of Earth just a small blue circle in the dark sky,” says Bernhard Hufenbach, a member of the Moonlight Management Team from ESA’s Directorate of Human and Robotic Exploration.

But having established a working time system for the Moon, we can go on to do the same for other planetary destinations.

Need for a common frame of reference

To work together effectively, the international community will also need to agree on a common ‘selenocentric reference frame’, similar to the role played on Earth by the International Terrestrial Reference Frame (ITRF).

This allows for the consistent measurement of precise distances between points across our planet and is an essential ingredient of today’s GNSS systems.

“Throughout human history, exploration has actually been a key driver of improved timekeeping and geodetic reference models,” Javier Ventura-Traveset adds.

“It is certainly an exciting time to do that now for the Moon, working towards defining an internationally agreed timescale and a common selenocentric reference, which will not only ensure interoperability between the different lunar navigation systems, but which will also foster a large number of research opportunities and applications in cislunar space,” he concluded.

Challenges and rewards of keeping time on the moon

In summary, as we stand on the cusp of a new era in lunar exploration, the international space community must come together to establish a unified timekeeping system and a common selenocentric reference frame.

These essential components will ensure seamless communication, navigation, and collaboration among the numerous missions set to operate on and around the Moon in the coming years.

By working towards an internationally agreed-upon lunar timescale and a standardized reference frame, we will pave the way for successful lunar exploration while advancing a wealth of research opportunities and applications in cislunar space.

The time has come to synchronize our efforts and create a robust foundation for the future of lunar exploration and discovery.

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