Lunar time, this mysterious object of spatial desire

The United States wants to put things right. But not on Earth. Washington commissioned NASA to find a specific time frame for the moon. The US space agency must “present a specific strategy to the White House by the end of 2026 to establish a standard for lunar time.”The press release issued by the US Presidency on Tuesday, April 2 indicates.

US President Joe Biden is not the first to care about this. Already in 2023,The European Space Agency (ESA) stressed the importance of finding the lunar time as quickly as possible to suit everyone.

The more the moon, the merrier.

At the moment, we are far from this lunar time company. Each country conducting a mission on or around Earth's natural satellite can, if desired, use its own time zone or UTC for in-situ observations. In other words, if a Chinese astronaut, an American astronaut, and a European astronaut meet at five o'clock on the moon's surface, they will not necessarily all arrive at the same time. This is confirmed by the German daily newspaper Süddeutsche Zeitung.

The absence of a frame of reference has not yet been a problem in the conquest of space. So why this sudden interest on both sides of the Atlantic in determining lunar time? Since there was only one small step a man could take, the need to synchronize his clocks was not necessarily obvious. But more and more people now want to take this big step for humanity.

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“It is very likely that one day there will be a rich ecosystem of thousands of spacecraft orbiting around the Moon or on the Moon's surface that will need to interact with each other. It will then be necessary to have a common repository of a fundamental resource such as time,” he emphasizes. Philip Linden, expert For the Open Moon FoundationIt is an American association working for “peaceful and sustainable development on the moon”, and Chris Rabotin, engineer of Blue Ghost, which is designing the lander.

Everyone can agree to use Coordinated Universal Time (UTC) on the Moon. “It is possible to carry out short-term missions, but the goal is to develop a long-term, even permanent, presence on the Moon, and in this case, it is better to have a certain autonomy with respect to the Earth,” objects Javier. Ventura Travest, Head of ESA's Navigational Sciences Office. In fact, to stay on Universal Reference Time, a permanent connection to Earth is required. What happens if the connection is lost? Will on-site employees find themselves lost in time?

About the importance of 58.7 microseconds

It is also impossible to consider the Moon simply as a new time zone according to Coordinated Universal Time (UTC). In fact, “to define time zones, we add or subtract hours from the global reference time, but this time passes just as quickly in Tokyo or in Paris. This is not the case on the Moon,” as Philip Linden and Chris Rabotin emphasize.

According to the theory of general relativity, the stronger gravity, the slower time passes. Since it is weaker on the Moon, the clock hands move faster there, by 58.7 microseconds per day.

These may be small details to you, but they mean a lot to the exact location of things and people. In GPS or Galileo (the European variant), time plays an essential role in the accuracy of geolocation. In every GPS satellite, for example, there is an atomic clock – extremely accurate – and a “transmitter-to-transmitter distance.” [comme un smartphone, NDLR] Rodolphe Le Targat, a physicist at the Paris Observatory, explained to France 24 in 2013, “The satellite is inferred according to the data transmission time, in atomic clock time.” It is this distance that then allows the location to be determined.

For GPS, this microsecond difference in time flow between the Moon and Earth “can lead to a geolocation error of 17 kilometers,” astrophysicist Robert Lamontaigne asserts. He gave an interview to Radio Canada.

A mistake can have consequences “If it is, for example, a question of locating astronauts or giving them directions, it is a matter of not making a mistake,” emphasizes Agnès Vinga, an astronomer at the Observatory of the Sahel d'Azur, which is involved in international activities. Working groups to determine time on the moon.

Atomic clocks to avoid lunar chaos

Again, as robots and astronauts move by the hundreds or more on the lunar surface, a lunar time frame will be necessary to avoid lunar chaos and potential accidents associated with geolocation errors.

“The European Space Agency is in the process of developing a complete Galileo-type in-space communications and navigation system Moonlight program. The first step is to find a definition of lunar time acceptable to everyone,” points out Javier Ventura Traveste.

This first hurdle can be overcome very soon. “A definition of Coordinated Lunar Time (TLC) will be submitted to the International Astronomical Union in August 2024,” confirms Agnes Finja.

This astronomer adds: “The second step will then consist of creating a system to link lunar time with terrestrial Coordinated Universal Time (UTC).” We should be able to find equations between time on Earth and time on its natural satellite. This could be very important, for example, for scientists “on board the International Space Station who are in UTC,” explains Javier Ventura Travest.

Finally, it will be time to move on to practical work. “We must then place the atomic clocks in orbit around the Moon and also attach them to the surface in order to have a frame of reference,” explains Javier Ventura Travest. NASA estimates that three atomic clocks on the Moon are needed to “set the lunar standard to which all other clocks will be compared.” The scientific journal Nature explains.

A huge program that will take time. “Goal 2026 [fixé par le communiqué de la Maison Blanche] “Seems a bit short given all there is to do and discuss,” estimated Philip Linden and Chris Rabotin. The European Space Agency, for its part, is looking further ahead, perhaps not before 2030.

Time standard for the next Mars?

The formulation of the goals set by Washington for NASA is very complex, according to experts interviewed by FRANCE 24. It is not about ending everything, but about “providing a specific strategy” that contains “coordinated considerations of lunar timing.” It's enough to get lost in interpretations.

If the US presidency has thus set the target of 2026, it may also be to give the impression that it is ahead of the issue. While the White House is actually “trying to play catch-up a little bit with Europe and the European Space Agency here,” says Katherine Heymans, the astronomer royal of Scotland. A desire to assert American leadership, which will not be without ulterior geopolitical motives… especially with regard to Chinese space ambitions, according to specialists interviewed by France 24.

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This race for lunar time may be just the first step towards the moon and beyond. “It may then be a matter of a Martian clock on Jupiter and elsewhere. Moreover The Planetary Branch of the Internet Society is already working on this question. “Therefore, we can well envision a solar system in which multiple ‘spacetimes’ coexist and where each region could have its own local time standard,” emphasize Philipp Linden and Chris Rabotin.