Meteors, Asteroids & Comets


Meteor - the bright trail of light created by small, solid particles entering the Earth's atmosphere and burning up.  Technically, these are small asteroids that reach Earth.
Meteoroid - the small solid object that forms a meteor when it impacts the atmosphere.
Meteorite - the solid remains of a meteor that hits the ground.

Three types of meteorites:

  1. Iron
  2. Stony (silicate compounds)
    • Chondritic - composed of smaller chunks of rocky material (chondrules) stuck together by rapid heating and cooling.
      • Carbonaceous chondrites have chondrules embedded in a black, carbon-rich substance that contains organic compounds such as amino acids.
  3. Stony-Iron

Chondrules often contain radioactive material, which allow us to estimate their age.  Some contain particles from before the birth of our solar system.  Carbonaceous chondrites indicate that some of the building blocks of life were present on Earth since it formed.

Asteroids are small, rocky bodies that orbit the Sun.  Most are in the Asteroid Belt between Mars and Jupiter (about where Bode's Law predicts a planet).  A few form the Trojan Asteroids - two clumps located on Jupiter's orbit.  There is also a group called the ApolloAsteroids, whose orbits cross Earth's orbit.

The largest found is Ceres, but the mass of all asteroids combined is probably less than 0.001x the mass of Earth.

A few asteroids are large enough to be round (due to their gravity), but most are irregular.

Three types of asteroids:

  1. Carbonaceous Bodies
    • tend to be more towards the outer Asteroid Belt
  2. Silicate Bodies
    • tend to be more towards the inner Asteroid Belt
  3. Metallic Iron-Nickel Bodies

Origins of Asteroids
  • Planetesimals form, differentiate into iron cores & silicate crusts.
  • Planetesimals collide and break into pieces to form asteroids:
    • cores form iron asteroids
    • crusts form silicate asteroids
  • Jupiter's gravity keeps the asteroids stirred up, so they never form into a planet.

If an asteroid 1km in diameter hits the Earth, the impact will release as much energy as a 40,000 megaton nuclear explosion.  As a result, monitoring programs have been put in place to provide advanced warning of large asteroids headed our way. 



Comet - a small, icy body that orbits the Sun and has a small icy core and a tail of gas and dust.  (The tail appears as the comet nears the Sun.)

An interesting online comet interactive can be found here.

Parts of a Comet

  1. Nucleus - block of ice and frozen gases, usually about 10km in diameter.
  2. Coma - cloud of gas and dust that surrounds the nucleus.  It is formed as material is boiled off the nucleus by the Sun's heat.  The coma may be up to 100,000km across.
  3. Tail - narrow column of gas and dust that stretches as much as 100 million km, coming out of the coma.

Comets are studied by looking at the spectra of light from their coma and tail, and by direct sampling of the material in the coma and tail.  They contain a lot of water, carbon dioxide, and carbon monoxide.  They also contain traces of other gases that condensed from the solar nebula.

The Oort Cloud is a shell of icy bodies that surrounds the solar system, extending as far as 150,000 AU from the Sun.  These bodies are comets - even though they have no tail until they reach Jupiter's orbit. Occasionally, the orbits of some of these bodies are distrubed by the gravity of passing stars or the Milky Way, or even by interactions with other bodies.  At that point, they are kicked in towards the Sun.  comets from the Oort Cloud may take millions of years to complete an orbit.

The Kuiper Belt is a much smaller zone of comets that begins near Neptune's orbit. 

As comets approach the Sun, the sunlight strikes dust grains in the comet nucleus and pushes them off the comet - this creates the tail.  The pressure of sunlight on these particles is called solar pressure, and is much too small for us to feel.

The solar wind, a flow of gas particles that streams off of the Sun, pushes on the gas of the coma and forms a second tail (this one made of gas) coming off of the comet.

Because the dust tail is heavier, it gets tugged by the Sun's gravity slightly.  This causes it to bend slightly.  Both tails always point away from the Sun because the Sun's energy and particles are pushing on them.

 - The dust tail (yellow) is lit up when the Sun's light is reflected off of the

    dust particles in the tail.

 - The gas tail (blue) is light up by fluorescence - the process in which

   atoms absorb light energy and re-emit it at a different wavelength

   (making them glow).

Short-Period Comets are comets who complete an orbit in less than 200 years.  These are believed to come from the Kuiper Belt (30-55AU from the Sun).  As they orbit, they become smaller and smaller until only a condensed nucleus of rocky dust remains.  This forms an asteroid.


Meteor Showers

On a clear night, it is possible to see an average of 1 meteor every 15-20 minutes.  A meteor shower occurs when this rate increases to one meteor every few minutes.  Meteor showers occur when the Earth crosses the path of a comet - the tail of dust and particles of solid matter that get cast off as the comet's tail burn up in the atmosphere.  Meteor showers seem to radiate from a point in the sky (the radiant), and the shower is named after the constellation this point appears to be in.


Giant Impacts

Impacts of meteoroids tens of meters in diameter occur every few thousand years.  The size and speed of these meteoroids give them a huge amount of kinetic energy - some of which is absorbed as they fall through the atmosphere and are slowed; the rest of which is released when they either break apart or hit the ground.

According to the text, a meteoroid with a mass of 100kg (about 220 lbs)and moving at a rate of 30km/s will hit the ground with a force of 100 tons of dynamite.

A meteroid 10m across (just smaller than our classroom) would have the energy on impact of a thermonuclear bomb and make a crater about 1km in diameter.



Homework from the Text

  • Read Chapter 10 (pg. 299-319)
  • Do Review Questions #1-10, 12-17 on pg. 319.
Subpages (1): Files for Meteors