Comet
* All comets are part of the suns family, just as the Earth and the other planets
  - consist of a nucleus, a coma, and a tail
  - have a coma which never changes b
* Any comet has the possibility of impacting Earth, while very few asteroids ever can.
* Many comets also have a tail made of gas.
* Many comets have extremely eccentric orbits
  - two tails
* Most comets are and remain extremely faint.
* Most comets are only a few kilometers across
  - observable through a telescope
  - too faint to be visible without the aid of a telescope
* Most comets have orbits that take thousands of years to complete one circuit
  - very eccentric orbits
  - remain undetected frozen balls of ice in the outer reaches of our solar system
* Most comets travel around the Sun in elliptical paths
  - in a three shape
  - typically go through the inner solar system with no major changes to their orbits
* Some comets also develop separate, curved tails or dust clouds
  - are bright, others have long tails, still others have interesting orbits
  - come into the inner solar system every few hundred years and travel around the Sun
  - go so near the sun that they completely evaporate and burn up
  - have short periods of less than seven years
  - never come back, with orbital periods of tens of thousands of years
  - seem to act differently in nearly the same conditions
* Some comets travel a fifth of the way to the next nearest star
  - is such long orbit that they are near the sun once in thousands of years
* affect Earth.
* also carry the noble gases argon, krypton, and xenon
  - develop dust and gas tails
  - follow different kinds of paths
  - grow tails
  - have frozen carbon dioxide, methane, ammonia, and a few other chemicals
  - leave waste products behind in their orbits, in the form of millions of meteoroids
  - vary in the different types of trace elements and hydrocarbons are present
* always orbit the sun in the same direction as the planets orbit.
* appear as little blurs in the sky when far away
  - fuzzy and nebulous, the comets are far off the sun and return to distant regions
  - in our skies from time to time
* appear to be bright balls with fat tails
  - come primarily from the direction of the constellation Hercules
  - hang in the sky, changing position slowly
* appear, brighten and fade away.
* approach the sun briefly - closest approach to the sun of all objects is called parahelion.
* are a bit like giant dirty ice-balls with diameters between five and fifty kilometres
  - mixture of ice, water and frozen gases
  - real problem, and can do nearly as much damage as asteroids
  - vehicle that brings organic materials to the Earth
  - about the same size as the smaller asteroids
  - almost always invisible to astronomers until they become active
* are also cause many of earth's well known meteor showers
  - chunks of rock, but they have ice mixed in as well
  - icy balls
  - an amazing part of our sky
  - astronomical objects
* are balls of dirty ice that measure from a few hundred yards to several miles in diameter
  - mineral-laden ice that orbit the sun
* are balls of ice and rock, and they travel far out beyond the planets
  - that orbit around the sun
  - believed to represent the most unaltered material in the solar system
  - blocks of frozen ice mixed with cosmic dust
  - bodies of ice, rock, and organic compounds that can be several miles in diameter
  - candidates, for they appear sporadically, move, and even seem to point down to the earth
* are chunks of ice, gravel, dust and rock stuck together in a lumpy ball
  - rock and ice that orbit the Sun in elliptical Paths
  - clumps of dust and ice a few miles across and they are very loosely held together
  - cold bodies consisting of frozen gases and boulders
  - composed of significant fractions of water ice, dust, and carbon-based compounds
  - conglomerates of ice and rock
  - cosmic icebergs
  - delicate, gossamer objects
  - different in nature to asteroids
* are dusty 'iceburgs' in space and can contain large rocks held together in a matrix of ices
  - early-formed 'ice balls', with eccentric orbits
  - easy to find because they are bright, being surrounded by vast clouds of water vapour
  - frozen relics from the formation of our solar system
  - fuzzy, luminous objects that orbit the sun
  - generally smaller than asteroids and travel in the broadest oval paths about the sun
  - giant dirty snowballs in space, made of ice and dust
  - huge dirty snowballs that are found in the farthest parts of the solar system
* are hunks of ice and dirt a few miles wide
  - mixed with dust and rock
* are icy bodies that reside far away in the solar system where it is cold
  - which grow long tails of gas and dust when they come near the Sun
  - independent masses of ice and debris that orbit around the sun
  - interplanetary bodies that contain abundant ices and dark, carbon-rich soil
* are invisible except when they are near the Sun
  - to people on Earth, except when they are near the Sun
  - irregular in shape, with their longest dimension often twice the shortest
* are large chunks of ice and rock that are floating in space
  - leftover material
  - like large dirty snowballs traveling through space
* are located in orbits
  - outer space
  - universes
* are lumps of ice made of water , carbon dioxide , methane and ammonia
  - ice, rock and dust
* are made up of a mixture of frozen water, gases and dust like dirty muddy ice balls
  - the most primitive material in the solar system
  - mostly icy material that spend most of their time far from the sun
  - mountains of ice and rock that orbit the sun
  - much larger than asteroids
  - objects composed mostly of ice and dust that grow tails when they approach the sun
* are only visible when they are near the Sun in their very eccentric orbits
  - they're near the sun in their highly eccentric orbits
  - patches of light that appear in the night sky from time to time
  - present at large distances from the sun
  - probably similar to the planetesimals from which the planets formed
  - shy creatures and disguise themselves often as nebulae
  - similar to asteroids, but they are composed of various ices as well as rocky material
  - single-tailed goldfish and are considered the most hardy of all pond fish
* are small bodies of ice and dust which travel around the Sun
  - members of the solar system, usually a few miles or kilometers in diameter
* are small, icy bodies that orbit the sun
  - irregularly shaped bodies composed mainly of ice and dust
  - something like dirty snowballs that populate the very outer parts of the solar system
  - sources of water
  - the best-preserved remnants of the material from which the solar system formed
* are the icy left-overs
  - leftovers from when our solar system formed
  - most numerous sizeable bodies in the solar system
  - remainders of material formed in the coldest part of our solar system
  - solar system's long-distance travelers
  - warm fuzzies of the firmament
* are thought to consist primarily of ammonia, methane, carbon dioxide, and water
  - originate from a region beyond the orbits of the outermost planets
  - time capsules that hold clues about the formation and evolution of the solar system
  - too small to be seen from large distances
  - typically a few miles in diameter
* are usually dust and ice
  - in highly elliptical or hyperbolic orbits
* are very old bodies of ice mixed with rock and organic material
  - old, mostly made of icy materials, and they move on the same plane as the planets
  - small in size relative to planets
  - visible for two reasons
  - white and creamy, like milk
* arrive nearby after falling in as far as a light-year to approach the sun.
* become fountains of gas and dust as they get near the Sun.
* begin to form a coma as a result of sublimated gasses as the nucleus nears the sun.
* break apart under small stresses.
* bring in new dust.
* can and do break up into pieces
  - be spectacular objects seen in the night-time sky
  - change from dull to brilliant over the course of days or hours
  - create a meteor shower when they run into the atmosphere
  - exhibit highly variable behaviour as the nucleus heats up
  - suddenly brighten or fade from view in a matter of hours
  - take hundreds of years to go around the sun
  - vary in brightness on short time scales
* can, and have, struck the Earth.
* change at short intervals
  - throughout their orbit around the Sun
* circle the sun and are assumed to be as old as our solar system.
* collide with planets.
* come and go, and some only appear once
  - from two regions in the outer solar system
* comes from a Greek word that means long hairy star.
* consist mainly of ice and dust
* consist mostly of dirty ice
* consist of primarily volatile molecules making up various ices
  - two parts, a tail and a coma
* contain a lot of ice
  - dust, ice, carbon dioxide, ammonia, methane and more
  - lots of water
  - many of the organic materials thought to be essential for the origin of life
  - matter left over from the formation of the solar system
  - methane and ethane
* disintegrate too quickly
  - rapidly and have no mechanism to reform
* do it easily whenever they pass close enough to the sun to heat their frozen nucleus.
* dominate the celestial imagery in Hawaiian myths.
* don t streak or run.
* expel dust and gas, usually from localized regions, on the sunward side of the nucleus.
* explain photography of the solar system and optical systems for magnification.
* generally follow parabolic orbits.
* generally have two tails, a dust and a gas tail
* generate their own light.
* graze the heavens from time to time.
* have a nucleus
  - extremely exaggerated orbits
  - highly elliptical orbits
  - interesting side effects
  - no light of their own, but reflect sunlight
  - tails and because of that people used to think that a comet was the hair of a woman
  - the most eccentric orbits of any solar system object
  - three distinct parts, the nuculeus, coma, and the ion tail
* includes sections.
* lack the solid substance.
* lose a little mass every time they go by the sun and blow off gas and dust
  - material and thus brightness with successive passages near the sun
* mop up and vastly amplify a minute surviving fraction of bacteria from a presolar cloud.
* move against a field of background stars
  - around the sun like planets, but in an oval course
  - but nebulae are stationary
  - very slowly across the sky
* natural object
* obey the laws of gravity, as do the planets.
* often have two tails, a sweeping dust tail and a thinner, straighter plasma tail
  - move quickly against the starry background
* originate from sources outside the solar system
  - the outer regions of our solar system
* passes close to earth.
* pose a much smaller risk than asteroids.
* really have two visible tails.
* release particles along a cometary orbit via the tails.
* represent an intermediate stage in planet-building, and they contain much water ice
  - some of the oldest, basically untouched objects in the solar system
* reveal themselves as local sources of hydrogen.
* seem to appear suddenly because they brighten when they come close to the sun
  - be little more than chunks of dirty ice
* spend most of their time a. in the outer solar system
  - in one of two areas
  - well outside the inner solar system
* talks about comets in general.
* traditionally have negative connotations.
* travel in elliptical orbits and their tail usually points away from the sun
  - something called an elliptical orbit
* usually mean the end of things, like dinosaurs
  - peak several weeks after being closest to the sun