1. Comets
A comet is a celestial body that revolves around the sun. Comets have a long tail. Early comets were described
as a phonomenon. The Danish astronomer, Tycho Brahe, proved that comets were celestial bodies. A comet is
made of a sharp nucleus, or center. It is also described as a "dirty snowball" because of the ice and dust on the
comet.
When a comet comes toward the sun, the solar heat evaporates so that the ice on the comet brightens. When
this happens, the comet has a tail that might sometimes lenghten to a point of being many millions of
kilometers in space. The tail of the comet is usually turned away from the sun. Comets also have elliptical
orbits.
Sometimes comets travel in a group that has nearly the same orbit. They are said to be members of a comet
group. The most famous group is Ikeya-Seki of 1956. Comets have been known to be important events. The
presence of a comet has also given fear of a collision between the comet and the earth. The earth has passed
through the tails of comets without much effect. Although a collision of a comet with a city would destroy the
city, it will probably never occur again.
Asteroids
Asteroids are one of the many minor planets of the solar system. The largest asteroids are Ceres, Pallas, and
Vesta. The mass of an asteroid of is much less than the mass of the Moon. Few scientists now believe that
asteroids are the remains of a former planet. They think that asteroids have a place in the solar system where
a planet could've formed, but the gravity of Jupiter prevented it from happening. Some unusual asteroids are
made of iron-nickel alloy.
Asteroids, like meteorites, can be classified into specific types. Most asteroids are related to the meteorites
known as "irons". A few asteroids, such as Vesta, are probably related to the rarest meteorite class of all. It is
the achondrites. These asteroids look like it has the surface composition of many lunar and terrestrial lava
flows.

2. Meteors
A meteor is a solid body that enters a planet's atmosphere from space. Brilliant meteors, consists of a radiant
head and then it is followed by a tail of light, like a comet. Other meteors, called bolides, have been seen to
explode. When bolides explode, they sound like thunder. Fainter meteors, called shooting stars, occur
occasionally. At some times, hundreds of meteors can occur at once. These swarms are called meteor showers.
Some appear annually on the same days of the year. They are called periodic showers. Others occur at
different times each year. A meteor that reaches the surface of the earth or another planet is called a
meteorite. A meteoroid is a solid body orbiting the sun. It will become a meteor if it enters earth's
atmosphere. The majority of meteoroids are the size of grains of dust, but they can be larger without any
limit.

3. Asteroids
ASTEROIDS
Asteroids are rocky or metallic objects, most of which orbit the Sun in
the asteroid belt between Mars and Jupiter. A few asteroids approach
the Sun more closely. None of the asteroids have atmospheres.
Asteroids are also known as planetoids or minor planets.
Asteroid 253 Mathilde, a Near-
Earth Asteroid photographed by
NASA's NEAR (Near Earth
Asteroid Rendezvous) mission in
June 1997. Mathilde is about 60
km in diameter and orbits in the
asteroid belt between Mars and
Jupiter.
THE ASTEROID BELT
The asteroid belt is a doughnut-shaped concentration of asteroids orbiting the Sun between the orbits of
Mars and Jupiter, closer to the orbit of Mars. Most asteroids orbit from between 186 million to 370
million miles (300 million to 600 million km or 2 to 4 AU) from the Sun. The asteroids in the asteroid
belt have a slightly elliptical orbit. The time for one revolution around the Sun varies from about three to
six Earth years.
The strong gravitational force of the planet Jupiter shepherds the asteroid belt, pulling the asteroids away
from the Sun, keeping them from careening into the inner planets.
THE KIRKWOOD GAPS
The asteroid belt is not smooth; there are concentric gaps in it (known as Kirkwood gaps). These gaps are
orbital radii where the gravitational forces from Jupiter do not let asteroids orbit (they would be pulled
towards Jupiter). For example, an orbit in which an asteroid orbited the Sun exactly three times for each
Jovian orbit would experience great gravitational forces each orbit, and would soon be pulled out of that
orbit. There is a gap at 3.28 AU (which corresponds to 1/2 of Jupiter's period), another at 2.50 AU (which
corresponds to 1/3 of Jupiter's period), etc. The Kirkwood gaps are named for Daniel Kirkwood who
discovered them in 1866.

4. HOW MANY ASTEROIDS ARE THERE?
There are about 40,000 known asteroids that are over 0.5 miles (1 km)
in diameter in the asteroid belt. About 3,000 asteroids have been
cataloged. There are many more smaller asteroids. The first one
discovered (and the biggest) is named Ceres; it was discovered in
1801.
Gaspra, Asteroid #951.
THE SIZES OF ASTEROIDS
Asteroids range in size from tiny pebbles to about 578
miles (930 kilometers) in diameter (Ceres). Sixteen of the
3,000 known asteroids are over 150 miles (240 km) in
diameter. Some asteroids even have orbiting moons.
Asteroid 4 Vesta, the brightest asteroid and
the fourth largest. Vesta is the only asteroid
that can be seen without a telescope (it is
sixth magnitude).
CERES: THE LARGEST ASTEROID
Ceres is the largest of the asteroids. It was the first asteroid ever discovered (by the
Italian astronomer Giuseppe Piazzi on January 1, 1801). Ceres is the size of the state of Texas! It is so
huge in comparison with the other asteroids that its mass is equal to over one-third of the 2.3 x 1021 kg
estimated total mass of all the 3,000 cataloged asteroids. Ceres is about 578 miles (930 kilometers) in
diameter. Ceres is now considered to be a dwarf planet.
ASTEROIDS BECOMING MOONS
Asteroids can be pulled out of their solar orbit by the
gravitational pull of a planet. They would then orbit
that planet instead of orbiting the Sun.
Astronomers theorize that the two moons of Mars,
Phobos and Deimos, are captured asteroids.
The asteroid 243 Ida and its tiny asteroid moon,
Dactyl. This is the first asteroid ever found with
an orbiting moon. Ida's dimensions are about 56 x
24 x 21 kilometers (35 x 15 x 13 miles). Dactyl is
only about 1.2 x 1.4 x 1.6 km (0.75 x 0.87 x 1
mile) across.

5. ORIGIN OF THE ASTEROID BELT
The asteroid belt may be material that never coalesced into a planet, perhaps because its mass was too
small; the total mass of all the asteroids is only a small fraction of that of our Moon. The total mass of all
the asteroids is about 2.3 x 1021 kg ); our moon's mass is 7.35 x 1022 kg; the asteroids' mass combined is
about 1/30 of the mass of the Moon. A less satisfactory explanation of the origin of the asteroid belt is
that it may have once been a planet that was fragmented by a collision with a huge comet.
TROJAN ASTEROIDS
Trojan asteroids are asteroids that orbit in gravitationally stable Lagrange points in a planet's orbit, either
trailing it or preceding it (these places are where the gravitational attraction of the Sun and of the planet
balance each other). Jupiter has the most Trojan asteroids; Mars also has some. Achilles was the first
Trojan asteroid found. The asteroids preceding Jupiter in its orbit were named for Greek heroes; those
following Jupiter in its orbit were named for Trojan heroes.
Introduction to Comets
A comet is a small, icy celestial body that orbits around the sun. It is
made up of a nucleus (solid, frozen ice, gas and dust), a gaseous
coma (water vapor, CO2, and other gases) and a long tail (made of
dust and ionized gases). The tail develops when the comet is near the
Sun. Its long ion tail of always points away from the sun, because of
the force of the solar wind. The tail can be up to 250 million km
long, and is most of what we see. Comets are only visible when
they're near the sun in their highly eccentric orbits.
PARTS OF A COMET
Nucleus: The nucleus is the frozen center of a comet's head. It is composed of ice, gas, and dust. The
nucleus contains most of the comet's mass but is very small (about 1 to 10 km across - or more).
Coma: The coma is the roughly spherical blob of gas that surrounds the nucleus of a comet; it is about a
million km across. The coma is comprised of water vapor, carbon dioxide gas, ammonia, dust, and neutral
gases that have sublimed from the solid nucleus. The coma and the nucleus form the head of a comet.
Ion Tail: A tail of charged gases (ions) always faces away from the sun because the solar wind (ions
streaming from the sun at high velocities) pushes it away (it is also called the plasma tail). When the
comet is approaching the Sun, the ion tail trails the comet: when the comet is leaving of the Sun, the ion

6. tail leads. The tail fades as the comet moves far from the Sun. The ion tail can be well over 100 million
km long.
Dust Tail: The dust tail is a long, wide tailcomposed of microscopic dust particles that are buffeted by
photons emitted from the Sun; this tail curves slightly due to the comet's motion. The tail fades as the
comet moves far from the Sun. Hydrogen Envelope: Hydrogen gas surrounds the coma of the comet and
trails along for millions of miles (it is usually between the ion tail and the dust tail). The hydrogen
envelope is about 10 million km across at the nucleus of the comet and about 100 million km long. It is
bigger when the comet is near the Sun.
A COMET'S ORBIT
Comets orbit the Sun in highly elliptical orbits. Their
velocity increases greatly when they are near the Sun and
slows down at the far reaches of the orbit. Since the comet is
light only when it is near the Sun (and is it vaporizing),
comets are dark (virtually invisible) throughout most of their
orbit. The solar wind pushes
the tail away from the Sun.
Some comets crash into the Sun or get so close that they burn up; these
comets are called sungrazers.
COMET EXPLORATION
NASA's Stardust Mission will visit the Comet Wild 2 in
2004. It will take a sample of comet particles and return
them to Earth. The small spacecraft (about 770 pounds = 350 kg) was launched
February 7, 1999 and rendezvoused with comet Wild 2 in January, 2004. It will return
to Earth on January 15, 2006, and land in western Utah, USA. Comet Wild 2 (aka Comet 81P) is a short-
period comet that was discovered by the Swiss astronomer Paul Wild on January 6, 1978. The comet's
nucleus is about 3 miles (5 km) across. Wild 2 orbits the Sun every 6.39 years; its elliptical orbit ranges
from about Mars' orbit to Jupiter's orbit.
COSMIC SNOWBALLS
There is a new and very controversial theory that comets (composed of frozen water) are constantly
bombarding the Earth. These "cosmic snowballs" have (perhaps) been seen by the visible imaging system
of the Polar Satellite. In theory, these frozen comets vaporize in the atmosphere, adding water vapor to the
environment.

7. METEOROIDS
Meteoroids are small bodies that travel through space. Meteoroids are smaller than asteroids; most
are smaller than the size of a pebble. Meteoroids have many sources. Most meteoroids come
from asteroids that are broken apart by impacts with other asteroids. Other meteoroids come from
the Moon, from comets, and from the planet Mars.
METEORS
A meteor is a meteoroid that has entered the Earth's atmosphere, usually making
a fiery trail as it falls. It is sometimes called a shooting star or a falling star.
Just before a meteor enter's the Earth's atmosphere, it is moving at roughly 70
kilometers per second. The friction between the fast-moving meteor and the gas
in the Earth's atmosphere causes intense heat; the meteor glows with heat and
then burns. Most meteors burn up before hitting the Earth. Only large meteors
can survive the trip through our atmosphere.
A fireball is any meteor that is brighter than Venus (magnitude -4).
A meteor shower is a phenomenon in which many meteors fall through the
atmosphere in a relatively short time and in approximately parallel trajectories.
A very intense meteor shower is called a meteor storm.
METEORITES
A meteorite is a meteor that has fallen to Earth. These rare objects have survived a fiery fall through the
Earth's atmosphere and have lost a lot of mass during that process. Meteorites are made up of rock and/or
metals
Sources:
http://www.enchantedlearning.com/subjects/astronomy/asteroids/index.shtml
http://library.thinkquest.org/J001665/cam.html#