Key Ideas
Discovery of the Asteroids: Astronomers first discovered the asteroids while searching for a “missing planet.”
- Thousands of asteroids with diameters ranging from a few kilometers up to about 1000 km orbit within the asteroid belt between the orbits of Mars and Jupiter.
Origin of the Asteroids: The asteroids are the relics of planetesimals that failed to accrete into a full-sized planet, thanks to the effects of Jupiter and other Mars-sized objects.
- Even today, gravitational perturbations by Jupiter deplete certain orbits within the asteroid belt. The resulting gaps, called Kirkwood gaps, occur at simple fractions of Jupiter’s orbital period.
- Jupiter’s gravity also captures asteroids in two locations, called Lagrangian points, along Jupiter’s orbit.
Asteroid Collisions: Asteroids undergo collisions with one another, causing them to break up into smaller fragments.
- Some asteroids, called near-Earth objects, move in elliptical orbits that cross the orbits of Mars and Earth. If such an asteroid strikes Earth, it forms an impact crater whose diameter depends on both the mass and the speed of the asteroid.
- An asteroid struck Earth 65 million years ago, probably causing the extinction of the dinosaurs and many other species.
Meteoroids, Meteors, and Meteorites: Small rocks in space less than 1 m in size are called meteoroids. If a meteoroid enters Earth’s atmosphere, it produces a bright trail called a meteor (or shooting star). A rock that does not burn up in the atmosphere and reaches Earth’s surface is called a meteorite; most of the meteoroids are too small to make it to Earth before being completely vaporized.
- Meteorites are grouped into three major classes, according to composition: iron, stony iron, and stony meteorites. Irons and stony irons are fragments of the core of an asteroid that was large enough and hot enough to have undergone chemical differentiation, just like a terrestrial planet. Some stony meteorites come from the crust of such differentiated meteorites, while others are fragments of small asteroids that never underwent differentiation.
- Rare stony meteorites called carbonaceous chondrites may be relatively unmodified material from the solar nebula. These meteorites often contain organic material and may have provided building blocks for the origin of life on Earth.
- Analysis of isotopes in certain meteorites suggests that a nearby supernova may have triggered the formation of the solar system 4.56 billion years ago.
Comets: A comet is a chunk of ice with imbedded rock fragments that generally moves in a highly elliptical orbit about the Sun.
- As a comet approaches the Sun, its icy nucleus develops a luminous coma, surrounded by a vast hydrogen envelope. An ion tail and a dust tail extend from the comet, pushed away from the Sun by the solar wind and radiation pressure, respectively.
Origin and Fate of Comets: Comets are thought to originate from two regions, the Kuiper belt and the Oort cloud.
- The Kuiper belt lies in the plane of the ecliptic at distances between 30 and 50 AU from the Sun. Its icy objects are thought to be the source of short-period comets, whose periods are less than 200 years.
- The Oort cloud is thought to contain billions of icy objects in a spherical distribution that extends out to 50,000 AU from the Sun. The long-period comets, with periods greater than 200 years, are thought to originate in the Oort cloud.
- Fragments of “burned out” comets produce meteoritic swarms. A meteor shower is seen when Earth passes through a meteoritic swarm.