Cosmic Clocks: Timekeeping in Space

By Aika Lanes

For decades, space exploration has captured the interests of individuals all over the globe. The pursuit to explore and understand the vast expanse of the cosmos has led to the advancement of many technologies and breakthroughs in scientific knowledge that have enhanced society’s desire to explore even further. Humanity’s interest in space has brought people together both locally and internationally (Montana Aerospace Scholars, n.d.). Despite the benefits, space exploration comes with its challenges. The question of how to communicate, navigate, and keep track of time in space presents itself as our reach into the cosmos expands. Timekeeping allows for the tracking of objects and the navigation of spacecraft to precise locations. Currently, missions to space operate on the time used on the ground. However, as humanity reaches further into space, keeping track of time gets more and more difficult. Many different methods of timekeeping on missions into the cosmos are proposed, and it is important to ensure that all space agencies, both public and private, work together to create a common, universal system of timekeeping in space before a variety of different solutions are developed (Gibney, 2023).

If society is to expand its reach into the cosmos, agencies will need to find effective and efficient ways to keep track of time, both locally and universally. There are various systems of timekeeping and natural “clocks” that exist around the world, and humanity is continuing to discover new ways to keep track of passing time from the creation of hourglasses to atomic clocks. Society’s system of timekeeping is not only a functional necessity. It is a constructive guide that helps shape the way humanity perceives the world and organizes their lives. Therefore, it is critical to carefully consider the practical development of a system of time tracking and measurement in order to ensure the success and prosperity of future explorers.

The Biology

The human body has an internal clock that is well synchronized with the rotation of the Earth. Circadian rhythms are changes that occur physically, mentally, and behaviorally over a cycling period of around 24 hours (ESA, 2017). This cycle is regulated by an internal biological clock that determines what time of day it is when managing processes such as the sleep cycle and metabolism. Maintaining a healthy circadian rhythm is essential for the wellbeing of a crew in space. A disruption in sleep can lead to a decrease in efficiency and concentration. This may lead to major effects on missions as the probability of fatigue-induced errors would increase in an environment of high risk. To mitigate this problem, the International Space Station has set up a special lighting system that adjusts its color and intensity in order to follow a 24-hour cycle that mimics the patterns on Earth (Harbaugh, 2017).

Due to circadian rhythms in organisms, it is critical to establish a healthy and practical method of timekeeping. With the absence of a “day” in spacecraft such as the International Space Station, it is difficult to simulate a terrestrial light cycle that supports a human sleep cycle. Additionally, it becomes challenging to create a system of timekeeping that supports a circadian rhythm on extraterrestrial bodies such as the Moon or Venus where different orbits and rotations exist. For instance, it would be arduous to establish a system of timekeeping that correlates to the 708.7-hour lunar day or the 5,832-hour rotation of Venus (NASA, 2017). Even so, space agencies will need to address this problem and find ways to solve or mitigate the issues.

Light travels at approximately 300,000 kilometers per second in a vacuum, and it does not simply arrive at its destination instantaneously. Greater distances take longer for light to travel. It would take 4.25 years for a radio signal to reach the closest star outside our solar system (NASA, 2020). Due to this, it would be nearly impossible to synchronize clocks to an appropriate degree at vast distances in space unless clocks are adjusted between them. Even then, it would be difficult to determine which clock to rely on for the “correct” time.

Time Dilation

The phenomenon of time dilation may also become a problem with cosmic timekeeping. Albert Einstein’s theory of relativity explains how time moves slower for observers that are in motion relative to stationary observers (Paris, 2019). This is why clocks on the International Space Station move slightly slower compared to clocks on Earth, so astronauts on the station age more slowly compared to those on Earth due to being 0.0035 seconds behind for every year. At current achievable velocities, time dilation is not an issue with communication and navigation. Theoretically, however, as society expands its reach into the cosmos and attains incredibly high velocities, there may be problems with navigation from the ground as clocks necessary for determining distances would run at different rates in between the two points. Additionally, communications would be impacted as the two messengers would need to adjust the time either forward or behind in order to account for the dilation. This would make universal timekeeping difficult when spacecraft or celestial bodies are moving at rapidly significant speeds.

Before determining how to establish a timekeeping system in space, it is necessary to understand how society keeps track of time on Earth. The majority of modern clocks in common use such as watches use quartz crystal oscillators to keep track of time. These clocks are used on satellites as well. With an applied voltage, the clocks measure the precise vibration frequency of the quartz crystals. This works like a pendulum, keeping track of how much time has passed as it vibrates.

Atomic Clocks

Globally, however, time is precisely kept by a network of atomic clocks that observe the shift in energy levels of cesium atoms. Every atom on the periodic table requires different microwave frequencies for electrons to shift levels. This stays consistent throughout the cosmos, allowing for atomic clocks to be universally reliable. Quartz crystal clocks are not the most stable, especially for space navigation standards. They generally can go off by approximately plus or minus twenty seconds a month, or four minutes a year, heavily impacting the position measurements of rapidly traveling spacecraft (Japan Watch and Clock Association, n.d.). Stable clocks are necessary for accurate measurements. Reliable clocks need to have consistent and high-precision ticks over long durations of time. For this reason, the International Bureau for Weights and Measures (BIPM) in collaboration with the International Earth Reference and Rotation Service (IERS) maintains Coordinated Universal Time (UTC) using atomic clocks that observe the consistent frequencies in shifts of atomic energy levels.

UTC, the main global standard that regulates time, relies on two scales for its accuracy. The first scale, Universal Time (UT), keeps track of time based on Earth’s rotation (International Telecommunications Union, 2015). International Atomic Time (TAI), the second scale, is based on the definition of a second. TAI is maintained by BIPM based on data collections from the atomic clocks of cooperating organizations in the form of a continuous scale consisting of measurements such as days, minutes, and seconds. UTC aligns with TAI, but is adjusted with the deletion or insertion of seconds based on the position and rotation of the Earth to account for the UT scale. Clocks add or subtract time from UTC in order to localize the time. For example, Japan Standard Time is nine hours ahead of UTC while Mountain Standard Time is six to seven hours behind depending on whether the area is in daylight saving.

Creating a Universal System for Timekeeping

As future missions to space have the potential for interacting with each other through activities such as the relay of communications and the performance of joint operations, it is essential to create a universal system of timekeeping. For the past years, missions to the moon have used the time from the country of origin. Regardless, as multiple international lunar missions are in the process of preparation, this method will become more problematic. A lunar time zone may help improve chances for mission success and determine times of events on the Moon. Establishing a time zone for the Moon can offer strengthened communication and contact between space agencies who are planning missions to its surface (Fahy, 2023). Coordinated time is also required for successful lunar navigation.

Looking further into separate time zones for extraterrestrial objects, discussions are ongoing over whether time should be synchronized with Earth or whether it should be kept separate. If times are synchronized with Earth, it may be easier for mission control to plan events or activities. However, a couple roadblocks arise with time synchronization. The special theory of relativity explains how in different gravitational fields, clocks would run at different rates. The Moon’s gravity causes atomic clocks to be fast by around 56 microseconds each day (ESA, 2023). This makes it difficult to maintain synchronization with the Earth. Additionally, the system of time on extraterrestrial objects would need to be practical for human visitations and settlements. UTC is designed specifically for Earth, and practicality may not be supported by synchronized time considering extraterrestrial differences such as the Moon’s 29.5-hour day.

It would be difficult to synchronize the time of an extraterrestrial settlement to UTC. However, the method for establishing UTC could be applied to extraterrestrial objects such as the Moon in order to create a local and personalized time. This can help improve communication between concurrent missions on the same surface. Similar to the maintenance of UTC, a time system on an extraterrestrial body must find a way to divide up the time of rotations and orbits in a manner that is practical for use. Once this is determined, settlers may better plan and keep track of time on the surface of their extraterrestrial body.

Time must also be kept with spacecraft missions. Clocks play a critical role in navigation through space (Nelson, 2019). Spacecraft Event Time (SCET) is the time in UTC aboard the spacecraft for events that occur (NASA, 2023). One-Way Light Time (OWLT) is the time that it takes a signal from a spacecraft to reach the Earth. Ground UTC is the sum of SCET and OWLT, representing the time it takes for a signal to reach Earth from a spacecraft. As the electromagnetic waves have a constant speed relative to the observer, clocks can help calculate the distance to a spacecraft by sending a signal to the object and measuring the time it takes for a signal sent from Earth to be returned back. Navigation on Earth can find the trajectory of a spacecraft by sending several signals over time to the spacecraft and taking measurements.

As it is crucial for the data of time to be precise in order to determine an accurate position of a spacecraft, high-precision clocks are necessary. NASA launched the Deep Space Atomic Clock (DSAC) in June of 2019, which was a mercury-ion atomic clock created to improve timekeeping technology in space. The DSAC was incredibly precise, only being off by a nanosecond for every four days, or one second for every ten million years. When compared to the atomic clocks of GPS satellites, the DSAC had the potential to be 50 times more stable in keeping track of time. This space clock can be useful for individual robotic missions into the cosmos such as an orbiter mission to Europa. The DSAC is a crucial technological step allowing spacecraft in deep space to more effectively navigate on their own instead of needing to rely on time-consuming two-way communications to Earth. Being more efficient, practical, and reliable, the usage of the DSAC could create a shift that greatly impacts the future of navigation in space (Skelly, 2021).

Natural Clocks

In addition to man-made timepieces, natural clocks exist in the cosmos. One example is the pulsar which is a neutron star that rapidly rotates while regularly emitting pulses of radio waves (Sutter, 2022). Signals from pulsars are so precise that when they were first detected, astronomers sincerely considered the idea that they were signs of extraterrestrial intelligent life. Considering this, it has become a widespread notion that the precision and accuracy of pulsars leads them to be the most reliable clocks in the universe. A pulsar-based system of timekeeping called “PulChron” located in a European Space Agency establishment measures time by keeping track of radio pulses of millisecond-frequencies from several of these rapidly rotating neutron stars (ESA, 2019). Its accuracy comes within a couple billionths of a second, and it can use atomic hydrogen maser clocks as well as cesium clocks to create high-stability signals of time to contribute to the maintenance of UTC. Though keeping time based on pulsar measurements is generally less stable in the short term than atomic clocks, it comes to great use considering the long-term benefits. After several decades, atomic clocks will individually fail while pulsar measurements remain reliable for thousands of millennia. PulChron offers the opportunity to eliminate the necessity for agencies to track time dilation. Any type of clock can become synchronized based on the pulses emitted by a pulsar whether or not it is affected by a gravitational field or relativity as it can be observed the same from anywhere in its observable range. Additionally, measuring time using pulsars offers the possibility of a reliable and independent timekeeping method from UTC as it does not rely on observing electrons between energy states but rather it watches the pulses of rapidly rotating neutron stars.

The method that numerous scientists from all over the Earth believe is the most promising for problems that come up with timekeeping across the cosmos is to develop a separate universal system of timekeeping that is independent from UTC. This time system can be used for communications and navigation through space. For example, the Earth and the Moon may have their own local times specified for their celestial body. Earth could continue to use UTC while the Moon would develop a timekeeping system that works best for its human settlements. However, when communicating between each other, interplanetary and extraterrestrial settlements may use a universal system of pulsar-determined time. This is similar to how there are localized scales of measurement such as the imperial system but the metric system is universal.

Final Thoughts

To many cosmic admirers around the world, the thought of sending humans to another world evokes a feeling of excitement. In July of 1969, it was estimated that around 650 million people all over the world watched the live footage of the first steps on the Moon (Loff, 2022). People were fascinated with the feat, and now there are humans living in a space station that orbits Earth at nearly five miles per second. Space agencies are currently planning ways to settle the Moon and Mars. Since time plays a significant role in navigating distances and organizing human lives, it is necessary for society to begin preparing new ways to keep track of it in a way that is practical and reliable for everyone. Settlements on extraterrestrial bodies could develop their own system of time to effectively plan their lives around, each with their own unique definition of the second. Atomic clocks such as the DSAC have shown themselves useful for timekeeping in the depths of space through an efficient and reasonably dependable way while improving navigation technology. These can be used on missions such as robotic orbiters to a nearby planet. However, as the reach of human presence spreads throughout the universe, humanity will most likely find itself turning to widely accessible and universally accurate clocks such as pulsars to keep track of time between various settlements. These cosmic clocks offer society the opportunity to enhance communications between locations and improve navigation methods. As more understanding of the universe is acquired, the thirst for more knowledge will grow. Organized space exploration offers humanity the opportunity to connect with each other while learning more about the world they live in. Even without a clock, humanity seems to constantly acknowledge that it is always the right time to learn something new.

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