nucleus: relatively solid and stable, mostly ice and gas with a small amount of dust and other solids;
coma: dense cloud of water, carbon dioxide and other neutral gases sublimed from the nucleus;
hydrogen cloud: huge (millions of km in diameter) but very sparse envelope of neutral hydrogen;
Components of a comet
        lumps of ice and dust
        periodically come into the center of the solar system
          close enough to the Sun,
            heat makes them start to evaporate.
                Jets of gas and dust form long tails
                 These tails can sometimes be millions of miles long.

Components of a comet
        dust tail: up to 10 million km long   smoke-sized dust
        most prominent part of a comet to the unaided eye;
        ion tail: as much as several hundred million km long
        composed of plasma interactions with the solar wind.

Comet West
        shows two distinct tails.
        thin blue plasma tail made up of gases
        broad white tail made up of microscopic dust particles.

What’s next
    Eventually- becomes just another rocky mass in the solar system.
        comets are said to be short-lived, on a cosmological time scale.
    Many scientists believe that some asteroids are extinct comet nuclei
 

http://www.solarviews.com/eng/comet.htm

An explanation
        Dutch astronomer Jan Oort
                In 1950
                hypothesized that comets came from a vast shell of icy bodies about 50,000 times farther from the Sun than Earth is.

        astronomer Gerard Kuiper
                A year later
                some comet-like debris from the formation of the solar system beyond Neptune.
 

The Oort cloud is an immense spherical cloud
surrounding the planetary system
extending approx 3 light years,
about 30 trillion kilometers from the Sun.
considered the edge of the Sun's orb of physical, gravitational, or dynamical influence.

Oort Cloud
    source of long-period comets
         Halleys and Swift-Tuttle.
    come from an average distance of 44,000 astronomical units.
            appear at any time and come from any direction.
    Bright comets can usually be seen every 5-10 years.
    Two recent Oort cloud comets were Hyakutake and Hale-Bopp.
            Hyakutake was average in size, but came to 0.10 AU (15,000,000 km) from Earth
            Hale-Bopp,  at comparable distances from the sun, making it appear quite bright, a distance of  1.32 AU (197,000,000 km) to the Earth.

Asteroids
       rocky and metallic object orbit the Sun
        too small to be considered planets.
            known as minor planets.
        range in size from Ceres
            1000 km, down to the size of pebbles.
        found inside Earth's orbit to beyond Saturn's orbit.
    Most contained within a main belt
         between the orbits of Mars and Jupiter.
    Some have orbits that cross Earth's path
        some have even hit the Earth in times past.

theory
    material left over from the formation of the solar system.
        never coalesced into a planet.
another theory suggests
    remains of a planet
        destroyed in a massive collision long ago.

Asteroids
    asteroids are material from the very early solar system,
    scientists interested in composition.
    asteroid belt empty space
    asteroids are separated by very large distances.
         visited by the Galileo spacecraft
            Gaspra
            243 Ida.

253 Mathilde
    June 27, 1997
    the spacecraft NEAR - high-speed close encounter
    first close-up look of a carbon rich C-type asteroid.
Ida
    http://www.solarviews.com/cap/ast/vida1.htm
        This animation shows the shape and surface of Ida. North is located at the top and the camera view is situated at the equatorial plane of Ida. The animation starts at -90 degrees longitude.
        S-type asteroids composed of metal-rich silicates
Gaspra
        S-type asteroids composed of metal-rich silicates.

Ceres
    the largest known rock in the solar system.
            Texas-sized asteroid
                    about 930 kilometers (580 miles) across,
     first asteroid ever detected.
         identified in 1801 by astronomer Giuseppe Piazzi,
                    noted over a few nights a shifting point in the sky
                    `wasn't one of the planets, their moons or a star.

Meteoroid
    http://www.solarviews.com/eng/meteor.htm
    What is a Meteroids
        Asteroids that are on a collision course with Earth are called meteoroids.
        When a meteoroid strikes our atmosphere at high velocity, friction causes this chunk of space matter to incinerate in a streak of light known as a meteor.
           If the meteoroid does not burn up completely, what's left strikes Earth's surface and is called a meteorite.

Meterorites
    92.8 percent are composed of silicate (stone), and
    5.7 percent are composed of iron/ nickel
     the rest are a mixture of the three materials.
    Stony meteorites - hardest to identify
            they look very much like terrestrial rocks.
 

One of the primary goals of studying meteorites is to determine the history and origin of their parent bodies.