When contemplated as one grand whole, on the nights we lift our gaze to the sky, there is a wide, all-encompassing black infinity, regardless of the ‘blue sky’ we see during the day, it is there. In every conceivable direction, the extension is endless; and the sensation of depth can often be overwhelming. The darkness is eternal. Where light does exist, it is pure, intense, and barbarous; but light exists almost nowhere, and the blackness itself is equally as pure, intense, and barbarous.

When we as a species do look up, given we are looking into this wonderous dark abyss, there is a limit to how much of the universe we can see. The observable universe is finite in that it hasn’t existed forever. It extends 46 billion light years in every direction from us. While our universe is 13.8 billion years old, in that we know it had a beginning with the Big Bang, the observable universe reaches further since the universe is expanding. Leftover radiation from the Big Bang, which we call the “cosmic microwave background”, represents the earliest picture of the universe, back when it was smaller, hotter and denser. As the theory goes, when the universe was born it underwent rapid inflation, expansion and cooling, (the universe is still expanding today). This all-sky image of the cosmic microwave background, created from data collected by the European Space Agency’s Planck satellite’s first all-sky survey, shows echoes of the Big Bang left over from the dawn of the universe.

Image credit: ESA/ LFI & HFI Consortia

When discussing the size Universe, we need to talk about time in order to talk about distance. The starlight now reaching our eyes is ancient. Some of it originated when dinosaurs stood where we are now standing; some of it before even the Earth itself existed. The laws of physics are the same throughout the universe in the fact that instantaneous motion between two different places is not possible.

For example, here’s how I talk about space exploration with my children (while optimistically informing them that when they are my age, it’s likely they could visit Mars). I digress, let’s say we drove to Duxford Imperial War Museum in Cambridgeshire, UK, one of their favourite places to visit, and it took one hour to drive there. If we could travel twice as fast, we would get there twice as quickly, but it would still take time to get there. If however, we were able to travel at the speed of light – at 186,000 miles per second – it would take around 0.0002 seconds to get there. No matter how fast you go, or how short a distance you’re covering, some time must always elapse. In the one hour that it takes us to drive to Duxford, light setting off from the Sun has made the majority of its way to Saturn. But it will take over five hours before it reaches Pluto – and four years until it reaches our nearest celestial neighbours, the triple star system in the southern constellation of Centaurus, Alpha Centauri. The same thing cannot be in two places at the same time: not even light. So, the Universe is big, and it takes time to travel, anywhere, at any speed.

As our ever-growing understanding of the Universe increases, we often hide our inability to comprehend it’s size by taking it for granted, even more so when we consider that we may be alone in it too. When we look up at the night sky, we see only a tiny fraction of what is out there. Despite our overwhelming blindness, the predominant reaction on how vast the universe is, is how insignificant a place we hold in it. Our “insignificance” becomes even more acute when we think that Earth, and even the whole of the solar system, is nothing but a very small, lonely stage in this vast, great enveloping, dark cosmic arena. However, sometimes, rarely, the Universe sends us something out of the dark.

Comets have inspired awe and curiosity, and even some anxiety, on Earth for as long as they have been observed. We hunger to understand.

The first systematic observations of the night sky were carried out by the Chaldeans, who lived in Ancient Babylon in the third millennium BC. They recorded their measurements in cuneiform script on clay tablets. The Roman philosopher Seneca reported that the Chaldean astronomers were very interested in comets – the phenomena that appear unpredictably in the sky.

The Bayeux tapestry portrays 58 scenes that document the conquest of England by the Normans in 1066. It is likely the oldest depiction of Halley’s Comet, which passed its perihelion at the time of the Battle of Hastings on 14 October. In the following scene, King Harold, vanquished in battle, asks whether he should consider the comet a harbinger of doom.

image credit: ESA / D. Pazos

In more modern times, Danish astronomer Tycho Brahe decided to try and estimate the distance to “the great comet of 1577” by measuring its parallax, the effect whereby the position or direction of an object appears to differ when viewed from different positions. He proposed that comets (like planets) return to their respective positions in the sky, meaning that comets too follow an elliptic path around the sun. On the other-hand, astronomers like Johannes Kepler believed that these celestial bodies proceed on a linear course throughout the cosmos.

Sketches found in one of Brahe’s notebooks seem to indicate that the comet may have travelled close to Venus. Not only that, Tycho observed the comet travelled by Mercury, Mars, and the sun as well. After this discovery, Tycho Brahe created a new model of the Universe – a hybrid between the classical geocentric model and the heliocentric one that had been proposed in 1543 by Polish astronomer Nicolaus Copernicus – to add comets. Brahe made thousands of very precise measurements of the comet’s path, and these findings contributed to Johannes Kepler’s theorizing of the laws of planetary motion and realization that the planets moved in elliptical orbits.

image credit: jstor.org

Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of his inverse-square law of universal gravitation must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet’s path through the sky to a parabolic orbit, using the comet of 1680 as an example.

image credit: Principia, 1687

By 1900 comets were categorized as “periodic”, with elliptical orbits, or “non-periodic”, one-time with parabolic or hyperbolic orbits. Astronomers believed that planets captured non-periodic comets into elliptical orbits; each planet had a “family” of comets that it captured, with Jupiter’s the largest. In 1907 A. O. Leuschner proposed that many non-periodic comets would have elliptical orbits if studied longer, making most comets permanent parts of the solar system, even those with orbital periods of thousands of years. This implied a large group of comets outside the orbit of Neptune, the Oort cloud.

The Oort cloud, first described in 1950 by the Dutch astronomer Jan Oort, is a theoretical concept of a cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from 0.03 to 3.2 light-years. The Oort cloud is thought to have developed after the formation of planets from the primordial protoplanetary disc approximately 4.6 billion years ago. The most widely accepted hypothesis is that the Oort cloud’s objects initially coalesced much closer to the Sun as part of the same process that formed the planets and minor planets, and is thought to be many times farther from the Sun than the outer reaches of the Kuiper Belt.

Comets are thought to have two separate points of origin in the Solar System. Short-period comets (those with orbits of up to 200 years) are generally accepted to have emerged from either the Kuiper belt or the scattered disc, which are two linked flat discs of icy debris beyond Neptune’s orbit. Very long-period comets, such as C/1999 F1 (Catalina), whose orbits last for millions of years, are thought to originate directly from the outer Oort cloud. Other comets modelled to have come directly from the outer Oort cloud include C/2006 P1 (McNaught) with a 92,663 year orbital period, C/2013 A1 Siding Spring with a 740,000 year orbital period, and C/2017 T2 (PANSTARRS) with a 106,000 year orbital period.

image credit: Wikipedia

Comet C/2022 E3 (ZTF) is a long-period comet—originally thought to be an asteroid – was discovered on March 2, 2022 in the constellation of Aquila by astronomers using the 48-inch telescope at the Zwicky Transient Facility at Mt. Palomar near San Diego, California. It’s a telescope that’s often used to discover new asteroids and comets. The “E3” refers to it being the third comet discovered in the fifth fortnight of 2022.

On the 31st January 2023, Comet C/2022 ES (ZTF) came closest to the Earth. It hasn’t been seen for 50,000 years, since the last ice age and prior to the extinction of the Neanderthals. Approaching within a distance of around 26 million miles (42 million kilometers) from our planet, the equivalent to about 28% of the distance between Earth and the Sun.

On this night, I craned my neck to the clear night sky, my camera busily counting as it undertakes its series of long-exposures, my eyes adjust and a green smudge against the inky black focuses my attention, a galactic traveller on its journey.

Image credit: Don Machholz / EarthSky

I am nothing more than a bystander on the seashore looking out at the horizon, and Comet C/2022 E3 is a passing ship. Although seemingly stationary, she has in fact spread her sails and is moving across the ocean expanse. I stand and watch until, at length, the Comet is gone from my sight.

Gone where? Her diminished illumination is in me – not her, as she continues on to her destined port. So to speak.

image credit: The Rogue Astronaut/pcdphotography

image credit: Antoine & Dalia Grelin/Galactic Hunter

The Green Comet, as the media calls it, gets its colour due to the sunlight hitting gasses such as diatomic carbon and cyanogen.

As is so often the case when we look to the night sky, while some of our brightest celestial bodies are actually extinct now, their energy long since cooled, miraculously because they float in the heavens so far away from us, that beautiful light will continue to shine on us all seemingly forever, and their glow will be so bright it’ll warm your heart, it’ll make your eyes glisten. With comets, they exist in the here and now, they come from out there, and appear on our proverbial doorstep.

To be honest, part of our fascination with comets comes from the fact that we can do nothing about them. Comets are really problematic and are often described as icy snowballs, and they come from much further out, the Kuiper Belt or the Oort Cloud as previously discussed. With this, from their natural habitat, because of collisions and interactions in the Kuiper Belt or Oort Cloud, or even passing stars can disturb these comets, they can come and ‘fall’ in towards the Sun. Recently, Shoemaker-Levy 9, well relatively recently, in 1994, hit Jupiter and we didn’t see it coming until a few Months before it hit. It made a bit of a mark in Jupiter, the hole it left in Jupiter’s gaseous atmosphere was bigger than the Earth.

So comets, they are big, and they are unexpected, and they are unpredictable. We as a species, as a planet would have a lot of difficulty in dealing with one. We would have less trouble with asteroids at the moment, we’d have an awful lot of trouble with the comets. There’s nothing we could do actually. Comets have inspired awe and curiosity, and even some anxiety, on Earth for as long as they have been observed. Comets such as Comet C/2022 ES (ZTF), are messengers from the outermost reaches of our solar system, taking tens of thousands of years to make their way into our vicinity.

Comets change very little over time. Thus, they are the most primitive objects in the Solar System. Many scientists think that they have kept a record of the physical and chemical processes that occurred during the early stages of the evolution of our Sun and Solar System.

The abundance of volatile material in comets makes them particularly important and extraordinary objects. This characteristic demonstrates that comets were formed at large distances from the Sun and have been preserved at low temperatures since their formation. Cometary material therefore represents the closest we can get to the conditions that occurred when the Sun and our Solar System were born.

A comet does not know why it moves in just such a direction and at such a speed, it receives an impulsion from a push or pull from a star, asteroid, or collision… this is the place to go now. Seen from Earth, a comet is a prodigy, coming out of the void for no reason, returning to the void for no reason. We call it unpredictable because we cannot predict it. From the comet’s own point of view, nothing could be simpler.

Here’s to our next celestial visitor, whenever it may be.