What time is it on the moon?

Scientists and space agencies have not yet agreed on how to determine lunar time.Credit: NASA Goddard

The next decade will see a resurgence in lunar exploration – including dozens of missions and plans to establish permanent bases on the moon. Endeavors pose countless challenges. Among them is a hidden, but fundamental question that metrologists around the world are working to answer: What time is it on the moon?

“We’re just beginning to figure that out,” says Cheryl Gramling, an aeronautical engineer who leads the positioning, navigation and timing team at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

The moon does not currently have an independent time. Each lunar mission uses its own time scale linked, by its processors on Earth, to Coordinated Universal Time, or UTC OttC – The standard on which planetary clocks are set. But this method is relatively imprecise and spacecraft that explore the moon do not synchronize time with each other. This approach works when the moon is hosting a few independent missions, but it would be problematic when there are several ships 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 global lunar time will take. Clocks on Earth and the Moon naturally tick at different speeds, due to the different gravitational fields of both bodies. Official lunar time can be based on a clock system designed to sync with it UTCor it could be independent of Earth time.

Representatives of space agencies and academic organizations around the world met in November 2022 to begin formulating recommendations on how to determine lunar time at the European Space Agency’s (ESA) European Space Research and Technology Center in Noordwijk, Netherlands.

Decisions must be made soon, says Patricia Tavella, who leads the time department at the International Bureau of Weights and Measures in Sevres, France. She adds that if no official moon date has been set, space agencies and private companies will devise their own solutions. “That’s why we want to raise an alert now, to say let’s work together to make a joint decision.”

satellite tracking

The most urgent need for lunar time comes from plans to create a dedicated Global Navigation Satellite System (GNSS) for the Moon, similar to the way GPS and other satellite navigation networks enable accurate location tracking on Earth. Space agencies plan to install this global lunar navigation system from around 2030. The European Space Agency approved a lunar navigation project called Moonlight at its Ministerial Council meeting on November 22-23, 2022 in Paris, and NASA has created a similar project, called Lunar Communications Relay and Navigation Systems , the previous January.

Until now, lunar missions have determined their positions using radio signals sent to large antennas on Earth at the exact times. But with dozens of missions planned, “there aren’t enough resources to cover everyone,” says Joel Parker, an engineer who works on lunar navigation at Goddard.

SatNav on the Moon: Diagram showing the four satellites orbiting the Moon, a spacecraft, and a communications tower on the roof.

Image credit: NASA.

As a first step, starting in 2024, the European Space Agency and NASA will experiment with deriving locations on the moon using faint satellite navigation signals from a ground vehicle. Next, GNSS lunar projects plan to place dedicated satellites around the moon, each with its own atomic clock. A receiver, say on the surface of the moon, will triangulate its position using the time it takes the satellite signals to reach it. The European Space Agency (ESA) planned an initial constellation of four spacecraft that would cover navigation of the lunar south pole, which harbors much of the moon’s water and is an important target for exploration, says Joerg Hahn, an engineer working on the space agency’s Moonlight project. european.

Moon missions will also need formal lunar time for collaboration and networking, Hahn says. “All of this has to be traced back to one kind of time reference, or else it gets messy and things just don’t work together.”

Another open question, Hahn says, is whether astronauts will use Lunar Universal Time everywhere on the moon. Although lunar time will remain the official time scale, its users, as on Earth, may wish to substitute it in the time zones that correlate with the sun’s position in the sky. This is not a question for metrologists but rather a question of convention. “When someone really lives out there on the moon, I think that makes sense,” he says.

allocate time

Determining the lunar time is not easy. Although the definition of a second is the same everywhere, special theory of relativity dictates that clocks tick slower in stronger gravitational fields. The Moon’s gravity is weaker than Earth’s, which means that for an observer on Earth, a lunar clock will run faster than an Earth clock. Gramming estimates that the lunar clock will gain about 56 microseconds over a 24-hour period. Compared to a clock on Earth, the clock’s speed will also change subtly depending on its position on the moon’s surface, due to the moon’s rotation, Tavela says. “This is heaven for experts in relativity,” she adds, “because you have to take into account so many things.”

Determining the lunar benchmark, against which all clocks are compared, will involve fixing at least three master clocks ticking at the natural speed of the moon, the output of which is combined by an algorithm to generate a more accurate “virtual” clock.

What happens next depends on which option metrologists make. They may decide to rely on lunar time UTC. In this case, this hypothetical lunar time will be regularly synchronized with terrestrial time UTC. Between check-ins, the master lunar clocks will continue to tell the time until the next synchronization. This feature is simple for users back on Earth to interact with.

How to Build a Lunar Clock: Diagram showing three lunar atomic clocks that can be used to determine lunar time.

Image credit: NASA

An alternative would be to use the composite output of the lunar atomic clocks as the independent and continuous time of the moon, and trace its relation to UTC. That way, even if contact with Earth is lost, the clocks on the moon will still be in sync with each other and allow safe navigation and communications, Gramling says. Creating independent time is a model that will also work on distant planets that space agencies eventually target, like Mars. transmit UTC She adds that it will be more complicated than the moon.

In this scenario, the days on the Moon could be marked differently than those on Earth, to calculate the time from solar noon to solar noon which takes an average of 29.5 Earth days. Earth days will always be important to astronauts, given the human need for sleep on a roughly 24-hour cycle. But the definition is something metrologists will need to agree on.

Metrologists will also need to determine where on the moon to set the master clocks. As on the ground, the height of the devices will affect the speed of ticking. Hahn says the clocks could be in orbit around the moon or on the surface. “This is what we are discussing now with our colleagues at NASA.”

Space agencies are also considering other necessary parameters — such as lunar terrain maps and coordination systems for use in navigation — through the Interagency Operations Advisory Group, the Council of National Space Agencies, and the United Nations International Committee on Global Navigation Satellite Systems. To make different countries’ systems interoperable, Gramling says, reference systems must be agreed upon internationally.

With the help of the ESA, NASA is developing a framework called LunaNet, for which it hopes to gain international support. LunaNet consists of a set of rules that will enable all satellite navigation, communication and computing systems to form a single network similar to the Internet, no matter in which country you install it. Determining lunar time is part of a much larger picture.

“The idea is to create an internet for the solar system,” Gramling says. “And the first part will be on the surface of the moon.”

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