Although the examples in our History section are few, humans in many cultures have been observing comets for a very long time. Oriental astronomers recorded comet sightings in ancient China, Arabic astronomers, Greeks, and Mayans were all fascinated by them. Your students may be interested to know that a Japanese boy named Matasaburou observed a comet in the 17th century, which he named the "tiger-tailed star."
Comets in Science
The development of western scientific thinking coincides rather closely with history and knowledge of comets. First there was wonder, followed by fear and apprehension. Some leaders no doubt used the appearance of a comet as a propaganda opportunity for their own ends, but unlike planetary movements, or eclipses, no one learned to predict when a comet would appear until the late 17th century. They were attributed to the whim of God or gods, and comets were often depicted as supernatural or god-sent omens of tragedies or miracles (1300, 1402).
But people went on observing, recording, and sharing information about comets. By the end of the Renaissance in Europe observers knew that not all comets looked the same, and people tried to define a morphology, a classification system based on the apparent differences in comet forms (1587). Classification by appearance, although a step toward systematizing knowledge, doesn't lead directly to physical understanding. With comets, differences in shape and color were eventually discovered to be due mostly to our viewing angle, and the comet's proximity to the Sun. If the earth and comet are situated differently, the same comet can look quite different from one periodic appearance to another, or even from one observation to the next.
Newton and Halley
The next real progress was to show that comets obeyed the same natural laws as other bodies in the heavens, and this had to wait until someone--Sir Isaac Newton--determined what those laws were. Sir Edmund Halley, a close friend of Newton, tamed the mystery of comets and broke the prediction barrier. The popular recent theory of Kepler was that comets travel in straight lines. This seemed to allow for the way comets' tails changed orientation to point anti-sunward as they moved. But Newton's finding that heavenly bodies moved on closed, gravitationally bound orbits appealed to Halley. He began to wonder in 1703 whether a comet he had observed in 1682 had ever been seen before.
Halley noted other comet sightings had occurred about every 76 years. Newton had demonstrated his laws of gravity and motion by using them to calculate the motions of familiar objects like the earth and moon, which have simple, nearly circular orbits. Both men had observed the comet of 1682* and had data on its position at various times. Using Newton's laws, Halley did some very difficult
calculations, and discovered that a comet with an elliptical orbit and period of 76 years could account in detail for the sightings of 1607, 1531, and 1456.
The practice of actually checking a theory by comparing it with nature was still fairly new at the time! Halley predicted the next sighting of the comet, in 1758-1759. Although he did not live to see it, Comet Halley was named for him after it was first sighted on Christmas night of 1758. It is now easily the best known comet. There are still people living in 2000 C.E. who witnessed its spectacular appearance in 1910. The latest flyby in 1986 was less visible, but 2062 is coming...
From the 1600's forward, western science was developing rapidly, and researchers created powerful tools of observation and calculation to describe the workings of nature. The investigation of comets went forward inexorably, using telescopes, photography, and today, multi-wavelength space instruments. The main highlights described in the student pages are:
Today, superstition about comets is much less common in our society, but the idea of "space rocks" as dangerous missiles has recently resurfaced. Are comets or meteors (they can have similar orbits) really a hazard after all? See our small investigation of one well-known possibility in Killer Comets?. The earnest scientific search for evidence of a pre-historic impact and its result are now changing our vision of the Earth's history and future.
Understanding the origin of comets means going back to the formation of the solar system. Comets probably formed soon after the Sun, during the early stages of collapse and contraction in a cloud of gas and interstellar dust called a solar nebula. Formation of comets and asteroids was soon followed by the formation of the planets. Please take a few minutes to read this nice narrative on the formation of the solar system planets:
Planets' Origin of the Solar System page: seds.lpl.arizona.edu/nineplanets/nineplanets/
Read between the lines of paragraphs 5, 6, & 7; metals are cooling and crystallizing, dust is accumulating into rocky material, and gases are freezing into ices, all depending on their distance from the Sun and their mass. Comets are distinguished by being a mixture of ices and mixtures of carbon dust, silicate rock, and metals.
The Kuiper Belt and short period comets
Comets are mostly ice, with requires the lowest temperatures to freeze into a solid state. Thus, the oldest comets were probably those that formed at the outer edge of the proto-planetary disk. This region would be first area cool enough to freeze after the original ignition of the young star. The collection of objects outside the orbit of Neptune is known today as the Kuiper Belt. Kuiper Belt objects, thought to be the source of most short-period comets, are also thought to be some of the most primitive remaining objects in the solar system.
Forming the Oort Cloud: source of long period comets
Small objects (planetesimals) also formed in the inner solar system during the phase of "runaway growth" (paragraph 7). Many objects from the outer regions and a few of these inner objects were ejected from the system instead of becoming part of a larger planet. This happened by gravitational "scattering," whenever a planetesimal passed close to one of the giant planets without colliding or being captured in an orbit. The effect of gravity caused the two objects to "bounce" off each other. The result was that the tiny planetesimal would completely change direction, while the enormous planet was hardly affected-- something like bouncing a ping pong ball off a bowling ball.
Shape and size
A great many of these events occurred in the early solar system. Some of the ejected objects stayed in
new, more distant orbits around the Sun, forming the Oort Cloud, more than 1000 times farther out than Pluto. It has been established that most of the comets in the Oort cloud come from about 50,000 AU away, and from many directions. This means the cloud is not an evenly filled sphere, but a spherical shell with some small thickness.
It makes sense that the cloud is a sphere centered on the Sun. The first small objects would have orbits of many angles and shapes and would approach large planets like Jupiter or Neptune from many different angles. So they would get scattered in every direction. If you could stand 50,000 AU from the Sun, not even Jupiter could be distinguished from the Sun, and as one scientist put it, "it would look like the Sun was spitting out planetesimals."
Scientists have also observed that about half of the comets from the Oort Cloud have orbits that are what is called "retrograde." This means that while nearly everything in the solar system goes round in the same direction, because of the original solar nebula's rotation, half of the comets circulate the other way. This is taken as further proof that the Oort Cloud objects represent a set of random scattering events, which would result in an even spherical distribution.
Comet Composition: What's in a Comet, Anyway?
The content of comets can be investigated just like that of stars. Astronomers use spectroscopes, which separate different wavelengths of light from a single source. This allows them to look for the
"signatures" of various elements and molecules in the resulting light spectrum. Of course meteorites also provide samples of the rocky and metallic parts of comets as well. The substances found in comets and their proportions are one things that helped scientists determine that comets were created early in the solar system.
The "dirty snowball" model of comets is easy to grasp, and fairly accurate. The main difference student need to understand is that the "snow" is not always composed of water. Besides water and carbon dioxide, other organic substances like formaldehyde and methane are frequently detected in comets by their spectral signatures in the light from the plasma tail and coma. These substances freeze at temperatures much lower than water. Talking about how cold it is in most of space (-270 degrees Celsius) may help students visualize this.
For more variations on the compositions of comets, see the Space Telescope Science Institute's "Amazing Space" module on comets:
The game and comet-making activity are good opportunities to introduce the general idea of organic materials. This term differs from the "organic" in "organic produce," and doesn't
necessarily mean "about an organ" or even a living organism, as some students might think. When it comes up later in life sciences (if it hasn't already), they will have it from another context and hopefully some of the possible confusions will be avoided. An exercise to help students with the idea of organic substances is to have them find as many examples as they can in the picture that illustrates the glossary entry. It is not necessary to know the exact names of all molecules:
Comets and life
The comet making activity is also a good time to reinforce the ideas from "Older Than Dirt." If you make comets, they will be mostly water and carbon dioxide. The reason the organics dominate is simple: the elements hydrogen, carbon, nitrogen and oxygen are some of the most common elements in the universe (aside from helium, which is a noble
gas, and doesn't participate in chemical reactions). This is also what life is made of, so life is made of the most common stuff--also not surprising, when you think about it. Simple compounds like ammonia and CO2 (and even a few sugar-like molecules) were all present in the original solar nebula; these were all nature started with to make life.
Amazing, but this is part of what makes comets truly interesting today. The fact that the relative amounts of these various compounds can be observed in comets means we might one day have the list of ingredients and the relative amounts needed for life. Of course, some scientists are already working out the cooking instructions: the chemical transformations that took place along the way. These might have to do with the various changes in climate conditions over the earth's history. In a way, each time a comet goes through perihelion inside the orbit of Jupiter, it undergoes a little bit of this aging and evolution. This means some comets are more or less "evolved" than others, depending on how many times (if ever) they have entered the inner solar system.
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