Download Astronomy Chapter 11 – Meteors, Comets and Asteroids A. Main

Astronomy
Chapter 11 – Meteors, Comets and Asteroids
A. Main Ideas
Orbiting the Sun and scattered throughout the Solar System are numerous bodies much smaller than
planets--comets and asteroids. These bodies are of much interest to astronomers because they may
contain clues about the formation of the Solar System.
1. Meteors, Meteoroids, and Meteorites
• Heating of meteors
⇒ Meteors heat up on entering the atmosphere because it collides with atmospheric
molecules and atoms. These collisions convert some of the body’s energy of motion into
heat. The collisions are violent, tearing off atoms from the body and vaporizing them. The
hot gases emit light and result in “shooting stars”.
⇒ Meteoroids bombard the Earth continually, astronomers estimate that hundreds of tons
fall each day. Most meteors that we see last only a few seconds because they are very
small, about the size of a raisin or smaller. Meteors that survive the plunge through the
atmosphere are called meteorites
• Types of Meteorites
⇒ Astronomers classify meteorites into three broad categories based on their composition:
iron, stony (composed of silicate compounds), and stony-iron
⇒ Most stony meteorites are composed of smaller rounded chunks of rocky material stuck
together. These small grains are called chondrules. Chondrules contain traces of
radioactive material, which can be used to determine the age of the meteorite. Most have
been dated to be around 4.5 billion years old.
⇒ In some meteorites the chondrules are embedded in a black, carbon-rich, coal-like
substance. These meteorites contain organic compounds including amino acids, which
are the building blocks of proteins and genetic material.
2. Asteroids
Asteroids are small, rocky bodies that orbit the Sun. Most are found between the orbits of Mars
and Jupiter.
• Size and shape
⇒ The diameter of an asteroid is hard to measure because they all are very small. They
range in size from 1,000 kilometers to less than one kilometer.
⇒ Most asteroids are irregularly shaped because they have insufficient gravitational force to
crush their material into a sphere
• Composition
⇒ Asteroid belong to three main groups: carbonaceous bodies, silicate bodies, and metallic
iron-nickel bodies
• Origins
⇒ Asteroids are probably fragments of planetesimals that, because of disruptions by
Jupiter’s gravitational force, never formed a planet
• Unusual Asteroids
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⇒ Trojan asteroids, arranged in two groups, lie in Jupiter’s orbit and are located 60 ahead
of and behind that planet
⇒ Near Earth objects are small asteroids, diameters less than 1 kilometer, whose orbits
carry them into the inner Solar System across or close to the Earth’s orbit
3. Comets
• Structure of comets
⇒ Comets consist of two main parts, the tail which is a narrow column of dust and gas that
may stretch across the inner Solar System for as much as 100 million kilometers, and the
comet nucleus, a block of ice and gases that have frozen in the extreme cold of
interplanetary space into an irregular mass with a diameter of around 10 kilometers
→ The tail emerges from a cloud of gas called the coma, which can be some 100,000
kilometers in diameter. The coma and the tail contain very little mass.
• Composition of Comets
⇒ Comets are rich in water, carbon dioxide, carbon monoxide, and small amounts of other
gases
• Origin of Comets
⇒ Most comets cone from the Oort cloud, the swarm of trillions of icy bodies believed to lie
far beyond the orbit of Pluto
⇒ Some comets may come a region called the Kuiper belt which begins about the orbit of
Neptune and extends for an unknown distance beyond
⇒ In order for us to see a comet its orbit must be changed so that it travels through the
inner Solar System. A change in a comet’s orbit may be caused by the gravitational
disturbance of a passing star or the tidal forces caused by the Milky Way
• Formation of the Comet’s Tail
⇒ The comet’s tail is formed through the action of radiation pressure and the outflow of gas
the streams from the Sun into space, the Solar wind
• Short Period Comets and the Kuiper Belt
⇒ While most comets that we see swing by the Sun on orbits that will bring them back to
the inner Solar System after million of years, a small number of comets reappear at time
intervals less than 200 years. These comets are called short-period comets.
⇒ Short period comets will eventually be melted by repeated passes close to the Sun
leaving only a small amount of solid matter
• Meteor Showers
⇒ While on average you may see one meteor every 15 minutes, occasionally there are
events where the frequency of meteors may one every two or three minutes. These
events are called meteor showers.
⇒ Meteor showers occur whenever Earth passes through the path followed by the
meteoroids, which are pieces of dead comets or from the tails of active comets.
⇒ The meteor showers are named for the constellation from which they appear to diverge.
The most famous meteor shower is the Perseus meteor shower which occurs every year
in the middle of August.
4. Giant Impacts
• Every few thousand years, the Earth is hit by a huge meteoroid, a body tens of meters or
more in size. Such bodies will produce not only a spectacular glare as they pass through the
atmosphere but also an enormous blast on impact.
• Giant Meteor Craters
⇒ Because of the geological processes of Earth, there is little evidence of the impacts of
large meteors on Earth. There is an impact crater near Flagstaff, Arizona, and evidence
of impact craters in Canada and Europe.
• Mass Extinction and Asteroid/Comet Impacts
⇒ There is evidence that at least once in Earth’s history the impact of a large asteroid or
meteor is responsible for the extinction of most of the life on the planet. About 65 million
years ago this impact so disrupted Earth’s atmosphere causing major changes in climatic
conditions that the dinosaurs and many other life forms, both plant and animal, died.
B. Vocabulary
Achondrite
Coma
Meteorite
Radiant
Asteroid
Fluorescence
Meteoroid
Radiation pressure
Asteroid belt
Kirkwood gaps
Near Earth object
Short-period comets
Carbonaceous chondrite
Kuiper belt
Nucleus
Solar wind
Chondrite
Meteor
Oort cloud
Tail
Chondrule
Meteor shower
C. Discussion Questions
1. What is the difference between a meteor, a meteoroid, and a meteorite?
2. What kinds of meteorites are there?
3. What do asteroids tell us about the formation of the Solar System?
4. What parts make up a comet? What are they made of? How do we know?
5. What is the Oort cloud? What is the Kuiper belt?