* A meteor occurs when a meteoroid enters the Earth's atmosphere and vaporizes, heating itself and atmospheric gases so that they glow. Most meteoroids are no more than 1 cm in diameter.
* High meteor rates occur during meteor showers, when the Earth
runs into a swarm of meteoroids. Showers take place on or close
to the same date each year, when the Earth crosses the common
orbit of the meteoroids.
* The number of recovered meteorites has risen dramatically with
the discovery that Antarctic ice fields collect and preserve meteorites
for millions of years.
* Meteorites are classed as stones, irons, and stony-irons. Stones
resemble Earth rocks and are the most common meteorites. Carbonaceous
chondrites are a type of stony meteorite and may represent unaltered
material from early in the history of the Solar System. Irons
are alloys of iron and nickel and stony-irons are mixtures of
stone and metal.
* The radioactive elements in meteorites show that most of them
solidified at almost the same time as the oldest Moon rocks, about
4.6 billion years ago.
* Many meteorites appear to have been kept at high temperatures
for a long period of time or to have cooled very slowly. To cool
slowly, they must have been part of a body at least 100 km in
diameter. The orbits of meteorites show that the bodies from which
they came had orbits that carried them into the region between
Mars and Jupiter.
* Most of the known asteroids orbit in a belt located between
the orbits of Mars and Jupiter. These asteroids are very widely
spread out.
* Many asteroids are not found in the asteroid belt. The Trojan
asteroids, for example, either trail or precede Jupiter in its
orbit around the Sun. In addition, the asteroids Hidalgo and Chiron
have orbits larger than Jupiter's.
* Many asteroids have orbits that carry them inside the orbit
of the Earth. Within tens of millions of years, these asteroids
are likely to be destroyed by striking the Earth.
* Many asteroids are fragments of larger asteroids that were shattered
by impacts. Most of the small asteroids are the eroded cores of
much larger bodies.
* The reflectance spectra of many asteroids resemble those of
various kinds of meteorites. However, asteroids that resemble
ordinary chondrites, the most common kind of meteorite, are either
absent or very scarce.
* The nucleus of a comet is a low-density chunk of ice and dust.
Upon coming within 3 AU of the Sun, however, the nucleus is warmed
enough by sunlight to release gas and dust. These flow away from
the nucleus to produce the coma and the dust and plasma tails.
* Comets with orbital periods shorter than 200 years are called
short-period comets. Comets with orbital periods longer than 200
years are called long-period comets.
* The Sun is surrounded by the Oort cloud, a swarm of comets extending
as far as 100,000 AU from the Sun. There may be as many as 1 trillion
comets in the Oort cloud. Passing stars alter the orbits of Oort
cloud comets, causing some of them to enter the planetary system
and become visible as new comets. Short-period comets are thought
to come from the Kuiper belt, a disk of comets just beyond the
orbit of Neptune.
* A comet loses icy material each time it passes the Sun. Eventually,
the ice is entirely eroded. The dust particles left behind form
meteoroid swarms that produce meteor showers. Some comet nuclei
may have rock cores that become asteroids once the surrounding
ice is gone.
* Five space probes intercepted Comet Halley during its 1986 appearance.
The nucleus of Comet Halley proved to be larger than anticipated,
irregularly shaped, and darker than coal. Jets spewed gas and
dust outward from the nucleus.
* Comets may have formed in a region near the orbit of Neptune.
A number of bodies recently found beyond the orbit of Neptune
may be among the largest comets that still remain in that region.
* If a large meteoroid or comet struck the Earth, there would
be serious local and global consequences. The global consequences
might include darkness for weeks or months, very acidic rain,
and temporary heating of the atmosphere.
* An excess of the element iridium, discovered in rocks formed
at the end of the Cretaceous period 65 million years ago, suggests
that an asteroid struck the Earth at that time. The consequences
of the impact may have played a role in the Cretaceous extinctions.
However, the way that the impact affected life on the Earth and
the relationship between extinctions and impacts is not yet understood.
achondrite, Amor asteroid, Apollo asteroid,
asteroid, asteroid belt, Aten asteroid, C-type asteroid
carbonaceous chondrite, chondrite, chondrule, coma, comet, cosmic
ray exposure age, dust tail, fireball
iron meteorite, Kuiper belt, long-period comet, M-type asteroid,
meteor, meteor shower, meteorite
meteoroid, micrometeorite, minor planet, new comet, nucleus, Oort
cloud, plasma tail,radiant,reflectance spectrum,S-type asteroid,
short-period comet, solidification age, stony meteorite, stony-iron
meteorite
terminal velocity,Trojan asteroid, V-type asteroid, zodiacal light
1. How do we know that the "new" comets are members of the solar system and not just interstellar objects passing near the sun? they follow elliptical orbits
2. What is the Oort cloud? a swarm of comets far from the sun
3. When does a comet have the longest tail and largest coma? when
it is nearest the sun
4. Which of the following best describes the material which makes
up the nucleus of a comet? dirty ice
5. Which part of a comet is the only one which exists when the
comet is far from the sun? the nucleus
6. What motivated 18th century astronomers to search for a planet
with an orbit about 2.8 AU from the sun? such a planet was "predicted"
by Bode's law
7. Most of the asteroids are located between the orbits of Mars
and Jupiter
8. Why would global temperature drop is the Earth were struck
by an asteroid several km in diameter or larger? the resulting
dust cloud would block out sunlight
9. Approximately how large must a meteoroid be in order to survive
passage through the atmosphere and reach the ground as a meteorite?
as big as a fist
10. The particles which produce meteor showers come from dead
comets