The next decade will see a resurgence in lunar exploration. This includes dozens of missions and plans to build permanent bases on the moon. There are myriad challenges in the effort. Among them are subtle yet fundamental questions that metrologists around the world are working to answer. What time is it in the month?
Aerospace engineer Cheryl Gramling, who leads the Position, Navigation, and Timing team at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, said:
The Moon currently has no independent time. Each lunar mission will use its own timescale linked through its handlers on Earth to Coordinated Universal Time (UTc), the reference by which planetary clocks are set. However, this method is relatively inaccurate, and spacecraft exploring the Moon are not synchronized with each other. This approach works when the moon hosts a small number of independent missions, but becomes problematic when multiple spacecraft are working together. Space agencies also hope to track them using satellite navigation, which relies on precise timing signals.
It is not clear what the universal lunar time will look like. Clocks on the Earth and the Moon naturally tick at different rates due to the different gravitational fields of the two bodies. Official lunar times may be based on clock systems designed to be synchronized with UTC, or they may be independent of Earth time.
In November 2022, representatives of space agencies and academic institutions from around the world will meet at the European Space Research and Technology Center of the European Space Agency (ESA) in Noordwijk, the Netherlands, to draft recommendations on how to define lunar time. have started.
Patrizia Tavera, head of the time department at the International Bureau of Weights and Measures in Sèvres, France, says a decision must be made soon. Space agencies and private companies will come up with their own solutions if no official lunar time is established, she says. That’s why I want to leave.”
tracking satellite
The most pressing need for lunar time arises from plans to create a dedicated global navigation satellite system (GNSS) for the moon. This is similar to how GPS and other satellite navigation networks enable precise position tracking on Earth. The space agency plans to deploy this lunar GNSS from around 2030. ESA approved a lunar satellite navigation project called Moonlight at a ministerial meeting in Paris on November 22 and 23, 2022, and NASA established a similar project called Lunar Communications Relay and Navigation Systems. Did. January of last year.
Until now, lunar missions have determined their positions using radio signals that are transmitted to large antennas on Earth at scheduled times. But with dozens of missions planned, “we don’t have enough resources to cover everyone,” says Joel Parker, an engineer in charge of lunar navigation at Goddard’s center.
As a first step, starting in 2024, ESA and NASA will attempt to derive the position of the Moon using faint satellite navigation signals from Earth-based spacecraft. Next, the lunar GNSS project plans to place dedicated satellites around the moon, each with its own atomic clock. For example, a receiver on the moon’s surface uses the time it takes for satellite signals to triangulate its position. ESA planned the first constellation of four of her spacecraft to cover navigation in the lunar south pole. The lunar south pole holds much of the lunar water and is an important target for exploration, says Jörg Hahn, an engineer working on his Moonlight project at ESA.
Lunar missions will require official lunar hours to cooperate and communicate, Hahn says. “All of this has to follow one kind of time reference, otherwise there will be confusion and things will not work together.”
Another open question is whether astronauts will use universal time anywhere on the moon, Hahn says. The lunar time remains the official timescale, but its users may wish to offset it with a time zone linked to the position of the sun in the sky, much like it does on Earth. is not much of a problem, it’s a matter of convention. “If someone really lives on the moon, I think it makes sense,” he says.
characterization of time
Defining lunar hours is not easy. The definition of a second is the same everywhere, but according to special relativity, a clock ticks slower in a stronger gravitational field. Because the moon’s gravitational pull is weaker than the gravitational pull of the earth, the moon’s clock runs faster than the earth’s clock for an observer on earth. Gramling estimates that the lunar clock advances about 56 microseconds in his 24-hour period. Compared to their terrestrial counterparts, clock speeds also vary subtly depending on their position on the moon. This is because of the moon’s rotation, says Tabera. “This is paradise for relativity experts because there are so many things to consider,” she adds.
To define the lunar standard against which all clocks are compared, at least three master clocks ticking at the natural pace of the moon are installed, and their outputs are combined algorithmically to form a more accurate “virtual” clock. must be generated.
What happens next depends on which option the metrologist chooses. They may decide to base their time on UTC. This virtual lunar time is then periodically synchronized with Earth’s UTC. During check-in, the monthly master clock will continue to mark the time until the next synchronization. This has the advantage of being easy to operate for the user back on Earth.
An alternative is to use the composite output of the Moon’s atomic clocks as an independent continuous time on the Moon itself, and track it relative to UTC. That way, even if the connection with Earth is lost, the moon’s clocks will match each other, allowing for safe navigation and communication, Gramling says. Establishing independent time is a model that works even for more distant planets, such as Mars, which are ultimately targeted by space agencies. Transmitting UTC there would be more complicated than the moon, she added.
In this scenario, we can even define the number of lunar days differently than the number of Earth days, considering that the time from solar noon to solar noon takes on average 29.5 Earth days. A human needs sleep approximately every 24 hours, so every day on Earth is important for an astronaut. But the definition is something metrologists need to agree on.
Metrologists also have to decide where on the moon to place the master clock. As on Earth, the altitude of the device affects the ticking speed. Hahn says the clock could be in lunar orbit or on the surface. “This is what I’m currently discussing with my colleagues at NASA.”
The space agency is also considering other necessary standards. For example, which lunar topographic map and coordinate system to use for navigation are being discussed through the Inter-Agency Operations Advisory Group, the Council of National Space Agencies, and the United Nations International Commission on GNSS. Gramling says a reference system needs to be agreed internationally to ensure interoperability of systems in different countries.
With ESA support, NASA is developing a framework called LunaNet and hopes to gain international buy-in. LunaNet consists of a set of rules that allow all lunar satellite navigation, communications, and computing systems to form a single Internet-like network, regardless of which country they are located in. . Setting the time of the month is part of a larger picture.
“The idea is to create an internet of solar systems,” Gramling says. “And the first part is on the moon.”
This article is reproduced with permission and was first published on January 24, 2022.
