Comets - History, Stories and Science
From "cosmic visitors riding on their chariots" to just 'Hairy stars', Comets are one of the most fascinating celestial objects. Let's dive into their history, stories and science.
The Bayeux Tapestry depicts the story of the battle, conquest, the victory of one king over the other, on some beautiful embroidery on cloth almost 70 meters long! On a small section of it, if you look closely, you’ll see some lads on the left being amazed by what looks like a flying street lamp on a hair comb in the top centre. It is one of the earliest depictions of a Comet! This was Halley’s Comet seen over England and Normandy in April 1066. The phenomenon was perceived by people, to be a sign of a “cosmic visitor”, which lead to King Harold being overthrown by William the Conqueror at the Battle of Hastings. The same comet was seen again in 1456 and was considered to be a bad omen and an agent of the devil. The fear of the unknown vast sky was so much amongst people that the Comet was excommunicated by Pope Calixtus III, to never return to the heavenly skies again. We can’t blame the people in the 15th century. The knowledge of space was very primitive back then and was based mostly on speculations and theories rather than facts.
One of the most complete sets of observations of this periodic comet were made in China in the records of the Grand Historian, also called ‘Shiji’ in Chinese, which is an almost complete record of the history of China. The records are way more complete than western records as the earliest confirmed Chinese comet observation is from 613 BC and there is also a possible sighting of the Halley's Comet in 1059 BC.
When we humans first started looking at the night sky, we observed all sorts of celestial objects. To satisfy human curiosity, people came up with various stories and explanations for these events. Comets were no exception. There have been many stories about these beautiful objects, from the messengers of gods, flying through the sky on their chariots to the bringer of Luck or sometimes even bad omens, to just, ‘Hairy stars’. The name comet comes from the Greek word ‘komete’ meaning `the hairy one'. A hairy Star suddenly appears in the night sky, some shining brighter than others and then just as mysteriously as they appeared, they disappear. Different cultures have had different perspectives towards comets throughout the years.
We learned a lot more about comets because of the observations made by Tycho Brahe of the Great Comet of 1577. Through his observations, he realised that these were distant objects even further away from us than the moon, which means that these were not an atmospheric phenomenon as many believed at the time.
Edmond Halley was one of the first people to be fascinated by comets and wanted to study and understand how they moved, with the help of his friend Isaac Newton, he was the first to give a physical description of comets. Named in his honour, Halley’s Comet was marked the first time astronomers understood that comets could be periodic and revolve around the sun just like planets.
There have been many accounts like the one of Halley’s comet, of people observing bright Comets, thanks to the very low light pollution in those times. These accounts have helped researchers to now accurately calculate the periodicity of these comets and differentiate between periodic and one-time flyby comets.
But what are comets?
We now have a much better and scientifically accurate description of comets after many observations using both ground-based and space telescopes. Comets are basically giant lumps of rock, dust and Ice. The temperature of space is so low and close to absolute zero (-273 C) that even molecules that are in a gaseous state here on Earth, are frozen solid in the comets in the form of cometary ice.
Unlike asteroids which are mostly rocks and dust, comets are a well-balanced mixture of rocks, dust and ice, which is why some researchers also call them, “Dirty Snowballs”. The structure of a comet can be divided into two parts, the nucleus, which is the main body of the comet, and the tail. When a comet is at the path to its perihelion, which is a point at which the distance between the sun and its orbit is the shortest, the heat from the sun melts the top layers of ice and dust and molecules trapped in it which leak out forming an ice and dust atmosphere around the comet. Due to the weak gravity of a comet and inertia, the ice and dust mixture gets dragged behind the comet. The tails experience a force in the direction of the radiation from the sun, hence the tail bends outwards, forming the signature comet tail structure that first comes to mind when we think of comets.
Due to the difference in the densities of the dust and gas present in the tail, two separate tails are formed. The molecules in the tail get ionised from the photons coming from the Sun and emit light in different colours depending on the molecule groups present. This forms the gas/ion tail of the comet, separate from the dust tail. The light emitted from the tails and their structure is what gave them the name “Hairy Stars”.
Ever since Tycho Brahe and Edmond Halley started work on understanding Comets, the quest to uncover the secrets of these cosmic messengers is going strong. Comets are beautiful celestial objects that have caught the attention of human curiosity for centuries. These celestial objects can reveal a lot many mysteries of the universe. How the solar system was formed? How complex molecules are formed? And how are they transported around the universe?
Through consistent observations, astronomers can confirm if a comet has a closed orbit, that is, if it’s a periodic comet in an orbit around our sun, or if the comet has an open orbit, which means it gets affected by the sun’s gravitational pull just once and then shoots off into the Interstellar space.
Closed orbit comets can have a wide range of periods and eccentricities, with varying distances from the sun. In an elliptical orbit, the eccentricity of an orbit is the ratio of the distance from the centre to the focus to the distance from the centre to the vertex, it basically tells us how flat an orbit is if drawn on paper. Kepler’s third law states that “The square of a planet's orbital period is proportional to the cube of the length of the semi-major axis of its orbit”. Using this law, we can calculate that different eccentricities and distances from the sun, will give a comet a different periodicity. Comparing the observations made using modern technologies with the consistent observational accounts from early astronomers, we can tabulate the periodicity of comets with periodicities even as high as decades or centuries and figure out which comets are actually periodic and which ones were just onetime flyby comets.
The orbits of a comet are not static and hence, even if they were formed at medium or large distances from the Sun, their orbits can evolve into highly eccentric orbits, which means they fly by really close to the sun, allowing a close inspection by astronomers using spectroscopy to figure out the chemical composition of these comets.
But why does that matter? Well….
Elements heavier than hydrogen are synthesised in the core of stars. Our sun is what astronomers call a yellow dwarf star, which means the heaviest element that the sun will synthesis will be Oxygen, through a CNO (Carbon-Nitrogen-Oxygen) Cycle. We find the presence of heavier elements on Earth, such as Iron, which is forged in very massive stars and Uranium which is the final product of the most highly energetic supernovas. These elements can then keep getting deposited on space rocks and get trapped inside ice in the low temperatures of space, these rocks can then carry these elements to neighbouring star systems. This suggests that those elements must have been brought to Earth through various different means, and comets could very well be one of them.
NASA's Stardust mission was able to successfully obtain, particle samples from comet Wild-2, and returned those particles to Earth in a sample return capsule in the January of 2006. By analysis of those samples, researchers were able to figure out the composition of the comet. But, it’s not really as easy to just go to a comet and collect samples and get them back to Earth to test it in a laboratory. The technique through which researchers can figure out the composition of any celestial object is called spectroscopy, which is the study of how the light gets separated into its different components of wavelengths. Each molecule or element will cause light to have a specific spectroscopic signature. By comparing the spectrum obtained from the object with that of the standard molecular spectra, the presence of specific elements and molecules in the comet can be confirmed.
The results obtained from Spectroscopic analysis can give us a greater understanding of the formation of Planetary systems and their early stages. The analysed spectra can be used in the research of exotic matter as comets provide an ideal condition for researchers to investigate the matter in its many unstable forms which are very difficult to obtain in Earth-based laboratories. Comets are speculated to be one of the sources of water and prebiotic molecules during the early stages of the formation of Earth.
From "cosmic messengers on chariots" to the possible bringer of life to our planet, Comets are one of the most fascinating celestial objects. But there is still a lot more to learn and uncover about these celestial “Dirty snowballs”
To read more about the current research in Cometary spectroscopy, check out the alternate article here.