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’, 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.
Augsburger Wunderzeichenbuch, Folio 52 (Comet mit einem grosen Schwantz, 1401)
We learned a lot 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 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 as the first-time astronomers understood that comets could be periodic and revolve around the sun just like planets.
Tycho brahe: By Eduard Ender (1822-1883). The Comet of 1577, as viewed from Prague on November 12, 1577. This is from an engraving made by Jiri Daschitzky. Dr Halley: By Richard Phillips - National Portrait Gallery: NPG 4393.
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.
A close-up view of comet Hartley 2 was taken by NASA's EPOXI mission during its flyby of the comet. It was captured by the spacecraft's medium-resolution instrument. Image Credit: NASA/JPL-CalTech/UMD. Comet Tails. Some of a comet's dust and ice vaporize to form a tail. Credit: cessna152
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”.
Comet Image: NASA/Johns Hopkins APL/Naval Research Lab/Parker Solar Probe/Brendan Gallagher, Comet Orbit: Юкатан, CC BY-SA 4.0, via Wikimedia Commons.
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 Interstellar space.
The red orbit comets are short-period comets. The blue orbit comets are long-period comets. The green path comets are onetime flyby comets. Credit: Albert van der Sel.
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 closer inspection by astronomers using spectroscopy to figure out the chemical composition of these comets.
But why does that matter?
Elements heavier than hydrogen are synthesised in the core of stars. Our sun is what astronomers call a yellow dwarf star and the heaviest element that the sun will synthesis will be Oxygen. 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 these rocks can then carry these elements to neighbouring star systems. This suggests that those elements must have been brought to Earth through various means, and comets could very well be one of them.
Spectroscopy. Credit: ESO
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 as easy to just go to a comet and collect samples and get them back to Earth. 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.
Due to comets being one of the most untouched pristine objects in our solar system, Cometary Spectroscopy has a very wide range of applications in planetary sciences, like understanding the formation of the protostar, the formation and evolution of planets in the early days of the solar system and molecule formation and their transportation. Cometary spectroscopy gives us more insight into the physical and chemical properties of the coma that surrounds the nucleus of a comet. Molecule formations take place through the process of photo-dissociation when the comet is the closest to the sun i.e., at its perihelion. The molecules trapped in the cometary ice are released into the coma and can be identified through their unique spectral signatures. [1,2]
NASA’s ‘Deep Impact’ probe’s historic appointment with Comet Tempel 1 on July 4, 2005. Artist’s concept by Marco Nero in Sydney, Australia. NASA’s Spitzer Space Telescope has concluded after more than 16 years of exploring the universe in infrared light. Credits: NASA/JPL-Caltech.
One of the most detailed observations of Cometary Spectra was carried out on, 4 July 2005, by NASA’s Deep Impact Mission on comet 9P/Tempel 1. The main objective of the mission was to carry an impactor to comet 9P/Tempel 1 and collide it with the comet's nucleus to study the composition of the interior of the comet. The collision resulted in the ejecta of dust and cometary ice, which was analysed by an Infrared Spectrograph (IRS) and a Multiband Imaging Photometer for Spitzer (MIPS) mounted on the Spitzer space telescope, sent into orbit in 2003. The mission was able to provide highly detailed spectra of the ejecta in the range of 5 to 35-micrometer. The spectra were compared with the spectra of comet C/1995 O1 (Hale-Bopp) which has a highly active nucleus and it showed good agreements between the two. To identify the presence of specific molecule groups in the spectra researchers compared the spectra with emission signatures of finely grounded molecular dust obtained in a laboratory. These include amorphous and crystalline silicates, amorphous carbon, carbonates, phyllosilicates, polycyclic aromatic hydrocarbons, water gas and ice, and metal sulphides. The minerals found, can tell us a lot more about the formation and nature of the proto-stellar disk during the formation of our Solar-system. [3]
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 the main source 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 history and stories related to Comets, check out the alternate article here.
References
[1] 'Cometary Spectroscopy', N. Biver, Astronomical Spectrography for Amateurs J.-P. Rozelot and C. Neiner (eds) EAS Publications Series, 47 (2011) 165-18
[2] 'Recent results and future prospects for the spectroscopy of comets', Jacques Crovisier, Molecular Physics, Vol. 104, Nos. 16-17, 20 August 10 September 2006, 2737-2751
[3] Lisse, C. M., VanCleve, J., Adams, A. C., et al. 'Spitzer Spectral Observations of the Deep Impact Ejecta', Science 04 Aug 2006, Vol. 313, Issue 5787, pp. 635-640
Comments