Chapter 11: Meteors, Asteroids, Comets and Pluto


  • Survey orbitals and pp, of small bodies in the solar system, meteor, meteors and meteorites
  • Asteroids - minor planets
  • Comets
  • Pluto
  • Dwarf planets - plutoids
  • Meteoroids and Meteors
  • Meteoroids are objects orbiting the Sun that are too small to observe.
  • Sizes are from about 1 m down to less than 1 mm.
  • Speeds are >= 10 km/s and faster, much faster than a bullet.
  • When they hit Earth the friction with our atmosphere heats and vaporizes them, creating a meteor - “shooting star” alerting us to their existence.


  • If a meteoroid is large enough to reach the ground without being completely vaporized before it hits the ground.
  • Meteorites can be collected and taken into a lab for analysis.

Types of Meteorites

  • One type of meteorite is made of silicate compounds - “stony” meteorites.
  • Some, called “chondrites”, contain small grains indicating they have not been heated very strongly before hitting Earth.
  • Some are even more “primitive” and contain matter rich in carbon, indicating they have had almost no heating. Some even contain amino acids that are important to life.
  • Some stony meteorites have no chondrules, so they are named “achondrites” which indicated that they have been heated strongly enough to melt the chondrules.
  • These probably come from larger objects that got hot enough to melt the rock, and were broken apart by a collision.
  • Another type of meteorite is made of iron - “irony” meteorites.
  • These probably also came from the broken up large objects.
  • The iron in these meteorites has avery distinctive crystalline pattern that forms when the iron cools from molten to solid over millions of years.
  • The final type of meteorite is a mixture of irony and stony.
  • These probably also came from the broken up large objects, but are from the boundary between stone and iron.
  • Studying meteorites shows us what can happen inside larger objects, maybe even inside the Earth and other planets.


  • Aster (star) oid (body) - a rocky body orbiting the Sun that is so small that it looks like a star.
  • Another name for these is “minor planets”.
  • Discovered by observing that they shift their positions compared to the stars.
  • Before photography, they were really hard to find.

Where are Asteroids Located?

  • Most asteroids are located in the region between Mars and Jupiter = “asteroid belt”
  • A small number of asteroids are located in the orbit of Jupiter, 60 degrees ahead of and 60 degrees behind Jupiter - Trojan asteroids.
  • The Trojan asteroids orbit with Jupiter because of the combined gravity of the Sun and Jupiter.
  • The NEOs have probably had a gravitational interactions with Jupiter.
  • In the asteroid belt, Jupiter’s gravity creates gaps.

Are Asteroids Important?

  • Yes, because:
  • They are samples of the material that built the planets.
  • They can collide with planets, even Earth.
  • Their impact energy is so great that it can cause tremendous damage - even mass extinction of life on Earth such as happened 65 million years ago.
  • Such impacts can/will happen again.


  • Comets are also small bodies orbiting the Sun in very long, thin orbits.
  • They are mostly ice with a mixture of frozen gasses and rocky dust.
  • When they come into the inner solar system the sunlight causes the ice to turn to gas, freeing the other gasses and dust to produce a gas tail and a dust tail.

Where do Comets “Live”?

  • Because comets have very elliptical orbits, they spend most of their time far from the Sun where their ice doesn’t vaporize.
  • One location is called the “Kuiper belt”, which is beyond the orbit of Neptune.
  • Their other main location is called the “Oort cloud” at the edge of the solar system.


  • Recall (Ch 10) that Neptune was found because it altered the orbit of Uranus.
  • Astronomers thought (incorrectly) that Neptune’s orbit was also being altered and this led to a search that just so happened to discover Pluto in 1930. Pluto was discovered by luck.
  • Pluto is so far away (39.48 AU from the Sun) and so faint it couldn’t be studied.

Pluto “Rediscovered”

  • In 1978, it was discovered that Pluto has a moon that has been named Charon.
  • Charon made it possible to:
  • Measure Pluto’s mass == 0.0021 MEarth.
  • Measure Pluto’s radius == 0.181 REarth.
  • Without that moon, we would not know this.


  • Surveys have discovered other small bodies beyond Neptune that have physical properties and orbits similar to Pluto.
  • They all orbit far beyond Neptune.
  • They have very elliptical orbits.
  • Orbits tilted to the orbits of the major planets.

Dwarf Planets

  • Dwarf planets are a new family of objects in the solar system.
  • They probably formed closer to the Sun where the solar nebula was denser.

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