The coming decade will see a resurgence in lunar exploration including dozens of missions and plans to establish permanent lunar bases. The endeavors present myriad challenges. Among them is a subtle but fundamental question that metrologists around the world are trying to answer: what time is it on the moon?
The moon currently has no independent time. Each lunar mission uses its own time schedule, which is linked through its operators on Earth to Coordinated Universal Time, or UTc—the standard by which the planet’s clocks are set. But this method is quite imprecise, and spacecraft exploring the Moon do not synchronize time with each other.
This approach works when the Moon hosts several independent missions, but will be a problem when multiple craft are working together. Space agencies will also want to track them using satellite navigation, which relies on precise time signals.
It is not clear what form universal lunar time would take. Clocks on Earth and the Moon naturally tick at different speeds due to the different gravitational fields of the two bodies. The official lunar time could be based on a clock system designed to synchronize with UTC, or it could be independent of terrestrial time.
Representatives from space agencies and academic organizations around the world met in November 2022 to begin developing recommendations on how to define lunar time at the European Space Agency’s (ESA) European Center for Space Research and Technology in Noordwijk, the Netherlands.
Decisions must be made soon, says Patrizia Tavella, who heads the time department at the International Bureau of Weights and Measures in Sèvres, France. If no official lunar time is established, space agencies and private companies will come up with their own solutions, he says. “That’s why we now want to raise awareness and say let’s take a joint decision together.”
Satellite tracking
The most pressing need for lunar time comes from plans to create a dedicated Global Navigation Satellite System (GNSS) for the Moon, much as GPS and other satellite navigation networks enable precise location tracking on Earth. The space agencies plan to install this lunar GNSS starting around 2030. ESA approved a lunar satellite navigation project called Moonlight at a ministerial council meeting on November 22-23, 2022 in Paris, and NASA established a similar project called Lunar Communications Relay and Navigation Systems last year January.
Until now, lunar missions have determined their positions using radio signals sent to large antennas on Earth at scheduled times. But with dozens of missions planned, “there just aren’t enough resources to cover them all,” says Joel Parker, an engineer who works on lunar navigation at Goddard’s center.
as a first step, from 2024, ESA and NASA will try deriving positions on the Moon using weak satellite navigation signals from Earth-based craft. Further, lunar GNSS projects plan to place dedicated satellites around the Moon, each containing its own atomic clock (see “Satnav on the Moon”). A receiver, for example on the surface of the moon, then triangulates its position using the time it takes for satellite signals to reach it. ESA planned an initial constellation of four spacecraft to cover navigation at the lunar south pole, which harbors much of the moon’s water and is an important target for exploration, says Jörg Hahn, an engineer working on ESA’s Moonlight project.
Characterizing time
Defining lunar time is not easy. Although the definition of the latter is the same everywhere, special relativity dictates that clocks tick more slowly in stronger gravitational fields. The Moon’s pull is weaker than Earth’s gravity, meaning that for observers on Earth, the lunar clock would run faster than the Earth’s. Gramling estimates that a lunar clock would gain about 56 microseconds per 24 hours. Compared to a clock on Earth, the clock’s speed would also vary slightly depending on its position on the lunar surface due to the moon’s rotation, Tavella says. “This is a relativityist’s paradise because you have to take so many things into account,” he adds.
Defining a lunar standard against which all clocks are compared will involve installing at least three master clocks that tick at the Moon’s natural pace and whose output is combined by an algorithm to create a more accurate virtual clock (see ‘How to Create a Moon Clock’).
What happens next depends on which option metrologists choose
They may choose to base lunar time on UTC. In such a case, this virtual lunar time would periodically synchronize with terrestrial UTC. Between check-ins, the lunar master clock would mark the time until the next synchronization. This has the advantage of being easy for Earth users to interact with.
Metrologists will also have to decide where to place the master clock on the moon. Just like on Earth, the tick speed will be affected by the height of the device. The clock could be in lunar orbit or on the surface, Hahn says. “That’s what we’re discussing right now with our colleagues at NASA.”
Space agencies are also considering other necessary standards—such as what lunar terrain maps and coordinate systems to use for navigation—through the Interagency Operations Advisory Group, the National Space Agency Council, and the UN International Committee on GNSS. To make different countries’ systems interoperable, reference systems will need to be agreed internationally, Gramling says.
With ESA’s help, NASA is developing a framework called LunaNet for which it hopes to gain international buy-in. LunaNet consists of a set of rules that would allow all lunar satellite navigation, communication and computing systems to form a single Internet-like network, regardless of which country installs them. Setting the lunar time is part of a much bigger picture.
Reference : https://www.nature.com/articles/d41586-023-00185-z
Written By: Vaishali Verma
