Breaking Free from the Shackles of Terrestrial Time
Timekeeping is a core part of our existence - and has been since early human civilisations, but it is shackled to our terrestrial sphere. With the emergence of a globally growing space sector that fosters ambitions of exploration and settlement beyond Earth, we are now seeing the emergence of a new problem - how do we observe time, universally, across our Solar System? The answer might lie, in the most unexpected of astrophysical objects - pulsars.
A dimension that governs and controls nearly every aspect of our lives and environments. With each relentless second passing with the utmost regularity, blissfully apathetic towards our Earthly manners, we are carried forward with it - like a raft in a continuous river, moving in one direction. Our notion of time has come from observing the natural, rhythmic cycles of the celestial bodies. The Sun rising in the east, setting in the west, and restarting gives us the measure of our day. The phases of the Moon, give us an approximation for the month. The orbit of the Earth around our Sun gives us the year.
Our perception of time is somewhat disorientating. Memories we keep, for example, draw a distant line between our time as children, vs. adulthood. Yet, time seems to evaporate in front of our very eyes, when we clammer to meet the oncoming wall of a deadline. I often think about time, our perception of it, and its role in human experience.
Like how we are now at the halfway point to when Halley’s comet last visited the inner Solar System, and when it will next return. Or how 1990 is now further away from us than the year 2050. Both these things seem remarkable to me, as having a living memory of them, seemed only like it was yesterday. We all perceive time differently, but we all rely on it essentially in everything that we do.
So it should come as no surprise how important timekeeping has been in human history - starting early with megalithic observatories, such as Stonehenge to potentially observe the solstice, through to sundials, water clocks, and more. We’ve advanced in leaps and bounds since those days, and now accurately measure the oscillations and vibrations of atoms to determine our base unit of time - the second, using powerful atomic clocks.
Though, we didn’t stop there. We’ve synchronised a global network of timekeeping standards, and have accurate time delivered into the supercomputers we carry around in our pockets - our phones. But for all our advancement of timekeeping, time itself remains imprisoned in Earth’s gravitational well, and for the most part, enveloped under our precious atmosphere. That is, time is a terrestrial construct and is kept continually shackled to the Earth.
What time here on Earth has given us, however, is our curiosity. The exploratory fire in our bellies, in which we looked up over the millennia and wondered how and why. 50 years ago we took our first steps on another celestial body - the Moon. Since then, we’ve sent robotic explorers to visit the planets and far reaches of our Solar System. We’ve had a permanent human presence in space aboard the International Space Station, and are now taking the early steps of returning to deep space locations - firstly through the rapid growth of advanced robotic explorers, and eventually, proposed human settlements.
Which makes this an interesting time to be living in.
Exporting Time into Space
In every instance in which we have extended our hand, and reached out to in cosmos, we’ve been historically attached to the terrestrial sphere via an umbilical cord of Earth’s timekeeping that never breaks. For every mission, crewed or uncrewed, that we have sent out into the darkness, we have exported Earth time with it. We’ve achieved this by using radio signals, beamed back and forth between giant antennas at ground stations on Earth to synchronise with spacecraft. This method has, so far, proven well for mission objectives.
However, the approach is inconsistent. Different missions, have different science or exploration objectives, heading to different locations, launched by different agencies, from a range of different countries. We don’t have a global standard for exporting time into space, even though we have a global standard for measuring time in our terrestrial space.
So, how should humans and robots approach the concept of time in space, and in particular - away from Earth? Should we add atomic clocks to the Moon? Should we keep it simple, and continue to export time in the manner we currently do? And what about when we get to Mars - how should we address time then and there?
Einstein’s two theorems of relativity showed us that time is relative to the inertial observer. Move faster, and time slows down. Live in a stronger gravitational field and clocks tick slower. From this, we derive that time is no longer absolute nor the same at all locations.
Held in the safe and protective confines of our home spaceship, the Earth, we don’t experience time varying that much - instead, it remains constant, ever ticking forward beyond our control. But we don’t need to go far to see how quickly this can change. Our GPS satellites, for example, must take into consideration the altitude of the satellite in Earth’s gravitational field (general relativity) otherwise they will fall out of sync in a very short amount of time. A clock on the surface of our Moon, which has much less mass than the Earth, and thus, less gravity, ticks tens of microseconds faster than the clocks on our wrists (thank you, Einstein, once again).
Recently, the European Space Agency said that the agency and several space organisations are considering giving the Moon its own ‘timezone’, in particular, to meet the rapid growth of the recent commercial boom that is aiming for the Moon - mining activities, establishing a settlement either in orbit or on the Surface, manufacturing in space, and more. But is this truly a universal time solution that we can apply to our Solar System, or are we putting on a short-term band-aid solution?
Cutting Earth’s Umbilical Cord
Different locations in space will experience their own time independently. Away from the gravitational fields of the planets, clocks will tick faster. The varying mass of each of the planets, and moons, will result in different measured ticking rates. All of this causes chaos for a universal timekeeping system that relies on independent accurate measures of time - a factor that is absolutely needed if humans are ever going to go beyond landing and exploration activities.
Accurate timekeeping is core to providing position and navigation services - and in the vastness of space, we want to know exactly where our humans or multibillion-dollar spacecraft are, or even more importantly - ensure we land them in the correct spot with an accuracy of centimetres rather than tens of metres. This could mean the difference between dropping down onto the flat surface we would expect, vs. tumbling down the steep cliffs of a crater wall.
We apply this methodology here on Earth through our GPS satellite networks - where atomic clocks aboard the satellites triangulate your position to within metre-level accuracy by measuring how much time it takes for the signals from your device to reach them. But we can’t do the same in open space, or on other planets/moons that don’t have these types of satellite networks.
This problem has been recognised, and a number of demonstrator missions have now started to emerge (or are in planning) - sending atomic clocks into space aboard ships that can keep their own accurate timekeeping, effectively cutting the Earthly umbilical cord. NASA’s Deep Space Atomic Clock (DSAC) - in which a small atomic clock used the regular oscillations of mercury ions to count the time was one such program. Launched in 2019, the mission set a record for long-term atomic clock stability in space, achieving 10-fold stability over Earth’s GPS satellite network. Following the success of DSAC, a second DSAC is also planned to fly aboard the VERITAS mission that will head over to survey the planet, Venus. In both of these cases, the atomic clocks remain independent from Earth’s time and count time in situ aboard the spacecraft.
But this still doesn’t resolve the timekeeping problem of universality across our whole system. It simply defers it to requiring the launch of many atomic clocks for many locations and spacecraft. This introduces risks such as clocks failing or potentially becoming unstable in the long term.
There is another solution that could be considered, and it lies in the stars themselves.
A Galactic-Scale Time System
To be a truly universal system of time that can be applied across all locations of our Solar System, we need to set our inertial frame of reference at a larger scale. Terrestrial GPS and terrestrial-based timekeeping practices keep us constrained to the Earth. Atomic clocks shipped out into space present inherent risks. Trying to develop a Solar day on each body might get very confusing. But what if we had accurate and consistent clocks that were located at great distances all across the Galaxy, guaranteed to spin for billions of years and remain highly stable? What if we had an object analogous to the atomic clocks inside Earth's network of GPS satellites?
Well, nature has given us such an object. Millisecond Pulsars.
Pulsars are rapidly rotating, highly-magnetised neutron stars, that form from the collapse of massive stars (usually 8 - 25 times the mass of our Sun). Their rapid rotation and powerful magnetic fields generate the conditions in which beams of radio waves can emanate from their magnetic poles and blast out into space. They also emit across the whole electromagnetic spectrum (esp. in the higher energy bands), but they’re best known for their radio pulses that we observe with radio telescopes.
We know of about 3,300 pulsars which are mostly located in the Milky Way so far, but there are far more out there - many whose beams don’t shine in our direction. These objects are tiny and extreme - measuring roughly 20 kilometres across and packing in about 1.4 times the mass of the Sun into their tiny volumes. This makes them some of the densest objects in the Universe.
When a pulsar shines its beam in our direction, we see it (and ‘time’ it) as a pulse - and when we look at the population of pulsars, we note that some are spinning relatively fast (once every few seconds) and some are spinning extremely fast - rotating hundreds of times per second on their axis.
The fast spinners are the ones we call millisecond pulsars, as they have rotation periods within the millisecond range. They’re usually found in binaries and have gained this rapid angular velocity by stealing matter (and momentum) from their companions over billions of years (as per the above NASA video). Over that time, they’ve also settled down and become extremely stable.
As a result of this stability, and regularity in their ticking, we consider millisecond pulsars cosmic clocks, and given their location across the Galaxy - we can also consider using them like a giant, natural celestial GPS network. Just like Earth’s GPS uses satellites to triangulate our position, we can undertake a similar process to resolve our position in space by measuring the signals from a known set of pulsars (whose parameters are well-understood, and continually updated). If we know our position, we can apply this to our navigation, and our timekeeping.
We can observe and time our millisecond pulsars (in a practice known as pulsar timing) using both X-rays and radio waves, and this provides an advantage in terms of telescope sizes if we are to consider using pulsars as our clocks in space - because we somehow still need to measure and time them.
The good part is - the technology already exists today. For example, aboard the International Space Station resides the NICER (Neutron star Interior Composition Explorer) instrument - a 372 kg small observatory, which has been timing pulsars and observing other high-energy sources since its launch in 2017. Instruments of this size could easily be mounted aboard spaceships that travel through interplanetary space in our System.
Pulsar timing using radio waves does require larger dish antennas, especially if they are located on planets and moons - where they might need to gain stronger signal-to-noise from the pulsar. Whilst some radio telescope sizes are impressive, it would be harder to mount (and maintain) them on spaceships. These types of observatories could be built on the Moon, Mars, and the moons of the Gas Giants if we want them to be big. Though, here on Earth - even amateur astronomers have used their own 2-metre diameter dish antennas or yagi antennas to detect signals from some of the brightest pulsars.
If we consider millisecond pulsars as cosmic clocks for our time network, then we can derive that they will operate for billions of years without interruption, they are independent of any terrestrial timescales and they don’t rely on the quantum physics of atoms, rather offering macroscopic bodies that can be tuned into by any independent observatory, anywhere in our System - be that on a planet, moon or spacecraft - making the idea of building a pulsar timescale universal, scalable and effective for all users. One timekeeping system to rule them all, that only requires observatories to tap into it.
Pulsar timing for timekeeping is not a new concept, with many studies so far conducted on it. There’s even a pulsar clock in existence on Earth - the first installed in 2011 at St Catherine’s Church in Gdańsk Poland. It consists of 16 antennas receiving signals from six pulsars to measure the time. The time from this pulsar clock is also replicated and displayed in the European Parliament in Brussels, Belgium.
Navigation in space, using X-ray pulsar timing was also demonstrated when scientists used NICER, along with an enhancement known as the Station Explorer for X-ray Timing and Navigation Technology (SEXTANT) to independently determine the position of the station in space - achieving accuracy to within 7 kilometres in November 2018. These types of demonstrations, using the signals from millisecond pulsars, are the first steps in developing this Galactic-scale GPS system.
The SEXTANT experiment was not alone in achieving this. In 2016, China also launched a 270-kilogram satellite - on a mission known as XPNAV-1 to explore navigation using pulsar timing (X-rays in this case). The mission objectives were to detect photons from X-ray pulsars and measure pulsar parameters via its instruments with the vision of building out a constellation of future satellites that would use these X-rays from pulsars for Solar System navigation.
With a mountain of scientific papers now published about pulsar timing, and its potential applications for timekeeping, along with demonstrator technology that is currently available and scalable, it might be time to reconsider ideas like timezones for each individual body in the Solar System, and instead, work towards a universal Galactic-scale timekeeping system with what nature has already given us.
Time, and our pulsars, are ticking away …..
Video credit: NASA / SVG GSFC / Dana Berry