月は何時ですか？ 新しい月の時間帯の開発

2007 年 11 月に日本の月周回衛星「かぐや」が撮影した地球の高解像度画像。Credit: JAXA/NHK

月探査の新時代が到来し、今後 10 年間に数十の月探査ミッションが計画されています。 ヨーロッパはここで最前線にあり、ゲートウェイ月面ステーションとオリオン宇宙船の建設に貢献しています。これは、人間を自然の衛星に戻すように設定されているだけでなく、アルゴノートとして知られる大型の物流月面着陸船の開発にも貢献しています。 数十のミッションが月とその周辺で運用され、地球から独立して通信し、それらの位置を修正する必要があるため、この新しい時代には独自の時間が必要になります.

したがって、宇宙組織は月にどのように時間を刻むかを考え始めました。 昨年 11 月にオランダにある欧州宇宙機関の ESTEC テクノロジー センターで開催された会議から始まったこの議論は、共通点に合意するためのより大きな取り組みの一部です。ルナネット月面ナビゲーションと通信サービスをカバーするアーキテクチャ。

月探査シナリオのアーティストの印象。 クレジット: ESA-ATG

共同月面探査のためのアーキテクチャ

「LunaNet は、相互に合意された標準、プロトコル、およびインターフェイス要件のフレームワークであり、将来の月面ミッションが連携して機能することを可能にし、概念的には地球上で一般的な使用のために行ったことと同様です。」[{” attribute=””>GPS and Galileo,” explains Javier Ventura-Traveset, ESA’s Moonlight Navigation Manager, coordinating ESA contributions to LunaNet. “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.”

Timing is a crucial element, adds ESA navigation system engineer Pietro Giordano: “During this meeting at ESTEC, we agreed on the importance and urgency of defining a common lunar reference time, which is internationally accepted and towards which all lunar systems and users may refer to. A joint international effort is now being launched towards achieving this.”

On the 20th day of the Artemis I mission, Orion captures the Moon during its lunar flyby. The image was taken by a camera mounted on the European Service Module solar array wings, on December 5, 2022. Credit: NASA

Up until now, each new mission to the Moon is operated on its own timescale exported from Earth, with deep space antennas used to keep onboard chronometers synchronized with terrestrial time at the same time as they facilitate two-way communications. This way of working will not be sustainable however in the coming lunar environment.

Once complete, the Gateway station will be open to astronaut stays, resupplied through regular NASA Artemis launches, progressing to a human return to the lunar surface, culminating in a crewed base near the lunar south pole. Meanwhile, numerous uncrewed missions will also be in place – each Artemis mission alone will release numerous lunar CubeSats – and ESA will be putting down its Argonaut European Large Logistics Lander.

Artist’s impression of the lunar Gateway, a habitat, refueling, and research center for astronauts exploring our Moon as part of the Artemis program. Credit: NASA/Alberto Bertolin

These missions will not only be on or around the Moon at the same time, but they will often be interacting as well – potentially relaying communications for one another, performing joint observations or carrying out rendezvous operations.

Moonlight satellites on the way

“Looking ahead to lunar exploration of the future, ESA is developing through its Moonlight program a lunar communications and navigation service,” explains Wael-El Daly, system engineer for Moonlight. “This will allow missions to maintain links to and from Earth, and guide them on their way around the moon and on the surface, allowing them to focus on their core tasks. But also, Moonlight will need a shared common timescale in order to get missions linked up and to facilitate position fixes.”

ESA’s Moonlight initiative involves expanding satnav coverage and communication links to the Moon. The first stage involves demonstrating the use of current satnav signals around the Moon. This will be achieved with the Lunar Pathfinder satellite in 2024. The main challenge will be overcoming the limited geometry of satnav signals all coming from the same part of the sky, along with the low signal power. To overcome that limitation, the second stage, the core of the Moonlight system, will see dedicated lunar navigation satellites and lunar surface beacons providing additional ranging sources and extended coverage. Credit: ESA-K Oldenburg

And Moonlight will be joined in lunar orbit by an equivalent service sponsored by NASA – the Lunar Communications Relay and Navigation System. To maximize interoperability these two systems should employ the same timescale, along with the many other crewed and uncrewed missions they will support.

Fixing time to fix position

Jörg Hahn, ESA’s chief Galileo engineer and also advising on lunar time aspects comments: “Interoperability of time and geodetic reference frames has been successfully achieved here on Earth for Global Navigation Satellite Systems; all of today’s smartphones are able to make use of existing GNSS to compute a user position down to a meter or even decimeter level.

Picture of the far side of the Moon taken on flight day six of the Artemis I mission from the Orion spacecraft optical navigation camera. Credit: NASA

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

Setting global time

Accurate navigation demands rigorous timekeeping. This is because a satnav receiver determines its location by converting the times that multiple satellite signals take to reach it into measures of distance – multiplying time by the speed of light.

Your satnav receiver needs a minimum of four satellites in the sky, their onboard clocks synchronized and orbital positions monitored by global ground segments. It picks up signals from each satellite, which each incorporate a precise time stamp.
By calculating the length of time it takes for each signal to reach your receiver, the receiver builds up a three-dimensional picture of your position – longitude, latitude, and altitude – relative to the satellites. Future receivers will be able to track Galileo satellites in addition to US and Russian navigation satellites, providing meter-scale positioning accuracy almost anywhere on or even off Earth: satnav is also heavily used by satellites.
Credit: ESA

All the terrestrial satellite navigation systems, such as Europe’s Galileo or the United States’ GPS, run on their own distinct timing systems, but these possess fixed offsets relative to each other down to a few billionths of a second, and also to the UTC Universal Coordinated Time global standard.

The replacement for Greenwich Mean Time, UTC is part of all our daily lives: it is the timing used for Internet, banking, and aviation standards as well as precise scientific experiments, maintained by the Paris-based Bureau International de Poids et Mesures (BIPM).

Galileo is based on a worldwide time reference called Galileo System Time (GST), the standard for Europe’s satellite navigation system, kept close to UTC with an accuracy of 28 billionths of a second. Accurate timings enable accurate ranging for position and navigation services, and their dissemination is an important service in its own right. Credit: ESA

The BIPM computes UTC based on inputs from collections of atomic clocks maintained by institutions around the world, including ESA’s ESTEC technical center in Noordwijk, the Netherlands, and the ESOC mission control center in Darmstadt, Germany.

Designing lunar chronology

Among the current topics under debate is whether a single organization should similarly be responsible for setting and maintaining lunar time. And also, whether lunar time should be set on an independent basis on the Moon or kept synchronized with Earth.

A mosaic of the south pole of our Moon showing locations of major craters, with images taken by NASA’s Lunar Reconnaissance Orbiter. Credit: NASA/GSFC/Arizona State University

The international team working on the subject will face considerable technical issues. For example, clocks on the Moon run faster than their terrestrial equivalents – gaining around 56 microseconds or millionths of a second per day. Their exact rate depends on their position on the Moon, ticking differently on the lunar surface than from orbit.

“Of course, the agreed time system will also have to be practical for astronauts,” explains Bernhard Hufenbach, a member of the Moonlight Management Team from ESA’s Directorate of Human and Robotic Exploration. “This will be quite a challenge on a planetary surface where in the equatorial region each day is 29.5 days long, including freezing fortnight-long lunar nights, with the whole of Earth just a small blue circle in the dark sky. But having established a working time system for the Moon, we can go on to do the same for other planetary destinations.”

最後に、適切に協力するために、国際社会はまた、国際地上基準座標系が地球上で果たす役割と同様の共通の「中心基準座標系」を確立する必要があります。私たちの星。 適切に割り当てられた参照フレームは、今日の GNSS システムの不可欠なコンポーネントです。

「人類の歴史を通じて、探査は実際には、測地計時と参照モデルを改善する主な原動力でした」とハビエルは付け加えます。 「月のために今これを行うのは間違いなくエキサイティングな時期であり、国際的に合意されたタイムラインと共通の集中化基準の定義に向けて取り組むことで、異なる月航法システム間の相互運用性を確保するだけでなく、多数の研究を強化します。月空間でのアプリケーションの機会。」