How can we keep track of time on the moon? •

With dozens of lunar missions planned for the coming decade, a new era of lunar exploration is on the rise. But since all of these missions will soon be operating on and around the Moon, different teams will need to be able to communicate and fix their positions independently of Earth, thus requiring a commonly agreed upon method of keeping time on the Moon. This is part of a larger effort to agree on a common ‘LunaNet’ architecture that will cover lunar communications and navigation services.

“LunaNet is a framework of mutually agreed standards, protocols and interface requirements that allow future lunar missions to work together, conceptually similar to what we did on Earth for the joint use of GPS and Galileo,” explained Javier Ventura-Traveset, the European Space Agency’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.”

According to ESA’s navigation system manager Pietro Giordano, it is of the utmost importance and urgency to define a common lunar reference time that is internationally accepted and to which all lunar systems and users can refer in order to ensure the correct functioning and communication of future lunar missions.

“Interoperability of time and geodetic reference frames has been achieved here on Earth for global navigation satellite systems; all today’s smartphones are able to use existing GNSS to calculate a user position down to the meter or even decimeter level,” says Jörg Hahn, Chief Engineer at ESA .

“The experience of this success can be reused for the technical long-term lunar systems to come, although stable timekeeping on the Moon will present its own unique challenges – such as taking into account the fact that time passes at a different rate there due to the Moon’s gravity and speed effects.”

Although all the terrestrial satellite navigation systems, such as the US’s GPS or Europe’s Galileo, operate on their own distinct time systems, they nevertheless have fixed offsets relative to each other down to a few billionths of a second, as well as to the UTC Universal Coordinated Time global standard ( the time used for internet, banking and aviation operations).

Scientists are now debating whether a single organization should be responsible for setting and maintaining lunar time, and whether this time should be kept synchronized with Earth time or set on an independent basis on the Moon. Creating such a system currently faces significant challenges, since clocks on the Moon run faster than their equivalents on Earth and their exact speed depends on their position on the Moon.

Moreover, the agreed time system must also be practical for astronauts, who will require a common ‘selenocentric reference frame’, similar to the International Terrestrial Reference Frame on Earth, which allows consistent measurements of precise distances between all locations on Earth.

“Throughout human history, exploration has indeed been a key driver of improved timekeeping and geodetic reference models. It is certainly an exciting time to do so for the Moon, working towards defining an internationally agreed time scale and a common selenocentric reference, which will not only ensure interoperability between the different lunar navigation systems, but which will also promote a large number of research opportunities and applications in cislunar space,” concluded Ventura-Traveset.

Of Andrei Ionescu, Staff writer

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