History of Comets

Photo of Comet Lovejoy from the ISS taken by NASA astronaut Dan Burbank. Credit: NASA/Dan Burbank
Photo of Comet Lovejoy from the ISS taken by NASA astronaut Dan Burbank. Credit: NASA/Dan Burbank

Records of comet sightings date back as far as the 3rd millennium BCE (roughly 3000 BCE) which is typically referred to as the Bronze age. Ancient astronomers recorded comet sightings on clay tablets in ancient Babylon or on silk in ancient China. Throughout the world during these times comets were observed, but not understood. During these ancient times comets were thought of as bad omens or signs of something evil to come or some even believed that comets were just a phenomenon of our atmosphere and not actually in space. Either way comets were not truly understood during these times mostly because they did not fit in the astronomer’s model that the Earth is the center of the Universe. Astronomers of this time believed that everything you see in space is actually orbiting around our planet including the stars, planets, and the sun. This model was called the geocentric model (earth center) and was proposed by most astronomers including Aristotle and Ptolemy.

The first step in truly understanding what comets were was made when the Astronomer Tycho Brahe wanted to test if comets were part of the “heavens” or the earths atmosphere. Brahe decided the best way to test this is to measure the distance of the object by measuring its parallax. Parallax is the measurement of an objects distance by observing the object in two distance locations at the same time Using some geometry and trigonometry the objects distance can be calculated from these two observation points. Brahe observed the great comet of 1577 from Denmark and compared his observations with observations by other astronomers throughout Europe during that time and calculated that the comet was much further out in the “heavens” than our moon. With these calculations it was known from that point on that comets existed much further than the Earth’s atmosphere and inspired Brahe to design a hybrid heliocentric/geocentric model of the solar system.

Before Brahe did his observations in 1577 there was another astronomer that was making radical changes to the current models of the universe. In 1543 an astronomer by the name of Nicolaus Copernicus proposed that the geocentric model of the solar system that was the common model was actually wrong and that we actually exist in a heliocentric solar system (Sun center). Copernicus had a couple of great supporters in the realm of astronomy that helped push the heliocentric model. These supporters were the astronomers Galileo Galilei, who was the first astronomer to use a telescope to observe the sky, and Johannes Kepler who developed the laws of planetary motion. With this new model of the “heavens” the theory of a comet being part of the atmosphere was abandoned and also the negative beliefs of comets being a sign of something bad started to dwindle. Instead comets were starting to be observed more as a scientific inquiry.

Kepler struggled to explain the motion of comets because some had very elongated orbits and yet some did not. The true nature of comets orbits were understood after the publication from one man by name of Isaac Newton. Newton’s publication of Philosophiae Naturalis Principia Mathematica explained a detailed theory of universal gravity that would explain the orbits of planets, moon, and even comets. Newton’s work was supported and funded by an astronomer that would later be the pioneer of cometary orbits Edmond Halley. Halley used Newton’s mathematics to begin calculating the orbit of many different comets but began discovering that many of them have the exact same orbits. This led him to think that these comets that have the same orbit may actually just be one comet. He was the first to propose that comets may be periodic and appear in the sky in cycles based on their orbits. Halley used this theory to predict when the next time one particular cometComet Halley Credit: ESA/Max-Planck-Institute for Solar System Research

Comet Halley
Credit: ESA/Max-Planck-Institute for Solar System Research

would come back after observing it in 1682. This comet he claimed had roughly a 75 to 76 year orbit and would come back again between 1758 and 1759. Halley did not live to see the return of the comet but it did return December 25th 1758 and because of his precise prediction that comet is now known as Halley’s Comet.

With Halley’s discovery comets were now objects of science and no longer objects of superstition. Astronomers around the world studied comets and their orbits and led to a flood of comet discoveries. This naturally led to the next question about comets, where do they come from? The theory began with a publication by a German philosopher Immanuel Kant General History of Nature and Theory of the Heavens in 1755. His theory was that our solar system was created within a great cloud of debris called a nebula. This theory led to the belief that comets were objects that came from outside our solar system and came streaking through the neighborhood of our star. This theory was modified once the discovery was made that our star is actually travelling through the universe in this vast collection of stars called a galaxy. With the discovery of our stars motion it was proposed that this cloud of debris around our star that includes the planets, asteroids and comets all belong to the solar system itself. Therefore these comets that we observe mostly come from debris that orbits around our own star and not from outside of the solar system.

Once the theory of comets started to mold into shape a great invention was created known as photography. Once photography was invented astronomers naturally took photographs of comets and other celestial bodies. During this time the investigation of the light coming from distant objects formed a new way to investigate the composition of objects known as Spectroscopy. The first man to use this method to study the composition of a comet was Giovanni Donati who discovered that the comet C/1864 N1 had Carbon in its nucleus. This process was used further to discover that many comets have carbon, nitrogen, and oxygen based molecules in their nucleus.

Once humans began creating technology that can make it to space it was only natural that comets would be a destination for investigation up close. In 1985 NASA’s International Cometary Explorer (ICE) would be the first space mission to go to a comet. ICE passed through the tail of the comet 21P/Giacobini-Zinner.  One year later multiple space missions were developed the return of Halley’s Comet. Multiple countries launched missions to Comet Halley including the Russians (VEGA-1 and VEGA-2), Japanese (Sakigake and Suisei), European Space Agency (Giotto) and the United States (STS 51-L). After this fleet of spacecraft there were many more launched to investigate comets in different ways other than taking photographs. Deep Impact in 2005 launched

Photo taken after impactor from Deep Impact collided with Comet Tempel 1. Credit: NASA/JPL-Caltech/UMD
Photo taken after impactor from Deep Impact collided with Comet Tempel 1. Credit: NASA/JPL-Caltech/UMD

an impactor to collide with Comet 9P/Tempel and investigate what came from the collision and Stardust was launched in 2004 which collected samples from the coma of Comet 81P/Wild.

Currently comets are making the news again thanks to 2 recent events. Just this past weekend on October 19th, 2014 Comet Siding Spring passed by Mars within 87,000 miles which is about 1/3 the distance from earth to the moon. During this flyby multiple spacecraft were investigating the comet and we received the first photo of a comet ever taken from the surface of another planet. The other event is the European Space Agency mission known as Rosetta arriving at its destination Comet 67P/Churyumov-Gerasimenko. Rosetta has been orbiting around the nucleus of this comet since August 2014 and will be the first to spacecraft to ever send a lander (Philae) down to the surface of a comet which will happen in November 2014.

From ghostly curses to dirty snowballs comets have been the interest of humans for many years. Comets have helped drive scientific inquiry and discovery and may lead to further understanding of the origin of our planet and our solar system. We as humans have sacrificed lives and time understanding these celestial beings in the name of science and luckily have more questions to answer for the future.

Thank you to the ESA for most of the information and inspiration for this story.

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