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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
Asteroid 253 Mathilde, a Nearasteroids have atmospheres.
Earth Asteroid photographed by
Asteroids are also known as planetoids or minor
NASA's NEAR (Near Earth
planets.
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). 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.
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
Asteroid 4 Vesta, the brightest
diameter. Some asteroids even have
asteroid and the fourth largest. Vesta
orbiting moons.
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
The asteroid 243 Ida and its tiny asteroid
moons of Mars, Phobos and Deimos,
moon, Dactyl. This is the first asteroid
are captured asteroids.
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.
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.
Near-Earth Asteroids
NEAR-EARTH ASTEROIDS
Asteroids whose orbits bring them within 1.3
AU (121 million miles/195 million kilometers) of
the Sun are called Near-Earth Asteroids
(NEA) or Earth-Approaching asteroids. These
asteroids probably came from the main
Eros, Asteroid
asteroid belt, but were jolted from the belt by
#433, is an
collisions or by interactions with other
elongated Nearobjects' gravitational fields (primarily
Earth Asteroid
Jupiter).
which is 21 by 8
by 8 miles (33 by
About 250 NEAs have been found so far, but
13 by 13
many, many more exist. The largest known NEA
kilometers). The
is 1036 Ganymede, with a diameter of 25.5
density of Eros is
miles (41 kilometers). According to
2.4 grams per
astronomers there are at least 1,000 NEA's
cubic centimeter,
whose diameter is greater than 0.6 miles (1
roughly the same
kilometer) and which could do catastrophic
as the density of
damage to the Earth. Even smaller NEA's could
Earth's crust.
cause substantial destruction if they were to
collide with the Earth.
There are three types of NEA's:
•
•
Amors (named for asteroid Amor, #1221): Asteroids which
orbit between the orbits of Earth and Mars. Eros (#433) is
an Amor.
Apollos (named for asteroid Apollo, #1862): Asteroids
which cross Earth's orbit and have a period longer than 1
year. They have semimajor axes greater than 1 astronomical
•
unit (AU), and perihelion distances less than 1.017 AU.
Geographos is an Apollo.
Atens (named for asteroid Aten, #2062): Atens are
asteroids that are always closer to the Sun than the Earth
is; they have a period shorter than 1 year (the semi-major
axis is smaller than Earth's). Ra-Shalom is an Aten.
Cruithne is an asteroid about 3 miles (5 kilometers) in diameter
that is co-orbital with the Earth, which means that it shares
roughly the same orbit as the Earth. It is a Near-Earth asteroid
(NEA 3753). From the Earth, it appears to have a horseshoeshaped orbit; it gets close to the Earth, then it moves away again.
Its orbit is highly inclined to Earth's orbit. At its closest
approach (which happens every 100,000 years), Cruithne comes to
within 10 million miles (15 million km) of Earth (40 times the
distance from the Earth to the Moon). Cruithne was named for
the first Celtic tribal group that settled in the British Isles.
Cruithne was discovered on October 10, 1986 by D. Waldron at
Siding Spring Observatory, Coonabarabran, Australia.
Composition of Asteroids
COMPOSITION OF ASTEROIDS
Percentage
Albedo
Type Composition
of
(reflectivity)
Asteroids
Carbon
(Ctype)
Carbon
over 75
percent
0.03-0.09
(Very dark)
Silicate
Metallic
17 percent
0.10 -0.22
The asteroids are
(Siron mixed
(Relatively
varied in their
type) with ironbright)
composition. Most
silicates
are made of rock,
and
but some are
magnesiumcomposed of
silicates
metals and other Metallic
0.10-0.18
Iron/
less than 7
materials.
(M(Relatively
nickel
percent
type)
bright)
Most of what we
Water
know about the
ice/frozen
0.05
composition of
Dark
carbon
less than 1 (Relatively
asteroids is from
(Dmonoxide
percent
dark and
studying the
type)
mixed with
reddish)
asteroids that
rock
have fallen to
Earth; thay are called meteorites at this point. Although some
meteorites come from the moon and from comets, most are
asteroids.
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 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.
WHERE COMETS ORIGINATE
Long-period Comets (comets with an orbital period over 200
years and up to 30 million years): The Oort Cloud is a cloud of
rocks and dust that may surround our solar system. This cloud
may be where long-period comets originate. The Oort Cloud was
named for Jan H. Oort, who proposed its existence in 1950. It
has been hypothesized that the Oort Cloud is responsible for the
periodic mass extinctions on Earth.
Short-period Comets (comets with an
orbital period under 200 years): The
Kuiper belt is a region beyond Neptune in
which at least 70,000 small objects orbit.
This belt is located from 30 to 50 (?)
A.U.'s and was discovered in 1992. It is a
region where the planet-building process
was stopped in before any large objects were formed; there are
only primitive remnants from the early accretion disk of the solar
system, 4.5 billion years ago. The Kuiper belt may be the source
of the short-period comets (like Halley's comet). The Kuiper belt
was named for the Dutch-American astronomer Gerard P. Kuiper,
who predicted its existence in 1951.
The Kuiper Belt
The Kuiper belt is a region
beyond the planet Neptune in
which at least 70,000 small,
icy, slow-moving objects
orbit. These relatively dark
objects, "trans-Neptunians,"
are perhaps from 6-30 miles
(10-50 km) in diameter. This
belt is located from roughly
30 to over 50 A.U.'s from
the Sun.
It is a region where the
planet-building process was stopped in before any large objects
were formed; there are only primitive remnants from the early
accretion disk of the solar system, 4.5 billion years ago.
Pluto and Charon (Pluto's moon) may be extremely large members
of the Kuiper belt. Another large KBO (Kuiper Belt Object) is
Quaoar.
Comets and the Kuiper Belt
The Kuiper belt may be the reservoir of the short-period
comets (like Halley's comet). Short-period comets orbit the sun
in less than 200 years.
The Kuiper belt objects may be
redirected into an eccentric orbit that
comes quite close to Sun. Occasionally,
one of the icy objects in the Kuiper belt
is disturbed by the gravitational forces
of one of the gas giant planets, causing it to approach Neptune,
which sometimes propels the small object into a new, very
elliptical solar orbit, where it may eventually approach the Sun
(and near the Sun, the characteristic comet tail is visible).
The Discovery of the Kuiper Belt
The Kuiper belt was discovered in 1992. The Kuiper belt was
named for the Dutch-American astronomer Gerard P. Kuiper, who
predicted its existence in 1951.
CROSSING A COMET'S ORBIT: METEOR SHOWERS
The Earth passes through the
orbit of some comets. When
this happens, the left-over
comet debris (rocks, etc.)
bombards the Earth and they
burn up in our atmosphere. This is called a
meteor shower. In it, many meteors fall
through the atmosphere in a relatively short
time and in approximately parallel trajectories.
Each meteor shower occurs at a predictable time each year.
Meteor showers are named after the constellation they seem to
originate from (their radiant). The next HUGE meteor shower (a
meteor storm) will be the Leonid shower of November, 1999,
which is expected to produce up to thousands of meteorite each
hour.
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).
METEOR SHOWERS
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.
What Causes a Meteor Shower?
Meteor showers (and meteor storms) occur
when the Earth passes through a comet's orbit,
and left-over comet debris (rocks, etc.)
bombards the Earth. Each meteor shower occurs at a predictable
time each year, as the Earth intersects that particular comet's
orbit.
How are Meteor Showers Named?
Meteor showers are named for the constellation they seem to
originate from (this is called their radiant, the point in the sky
from which the shower seems to be coming from). Meteor
showers have nothing to do with constellations or stars except
for their names. For example, the Orionids meteor shower is
named for the constellation Orion because the meteor shower
seems to originate in the constellation Orion (although the
meteors in this meteor shower are actually debris from Halley's
comet and have nothing to do with the constellation Orion).
METEORITES
METEORITES
FROM MARS
Many
meteorites
from Mars
have landed on
Earth, one
A meteorite is a meteor that
with a
has fallen to Earth. At least
suspected
100 meteorites hit the Earth
microbial
CHONDRITIC every year. Meteorites have
fossil.
survived a fiery fall through
METEOR
the Earth's atmosphere and
Chondritic
have lost a lot of mass in that
meteors are
stony meteors process. Most meteorites burn
with chondrules, up in the Earth's atmosphere;
all that is left is a bit of dust.
tiny glass
spheres. These Every day, about 3000 tons of
meteoroid dust falls to Earth.
meteors are
unchanged since
their formation, There are different types of meteorites:
shortly after the
• Stony (92.8%):
formation of the
o Chondrites stony meteorites - 4.55
sun. These
billion years old (from the formation
meteors consist
of the solar system)
of elements also
o Carbonaceous Chondrites
common in the
o Achondrites (7.1%): stony
Earth's core.
meteorites with chunks of
differentiated matter that has melted and
recrystallized
• Iron (5.7%) : crystalline iron-nickel alloy
o Pallasites: the mineral olivine enclosed in metal
o Mesosiderites
• Stony-iron: (1.5%) iron-nickel alloy and non-metallic mineral
matter.
When meteorites hit the Earth
they cause a crater to form. The bigger the meteorite, the
bigger the crater.
The Biggest Meteorites
The biggest meteorite ever found on Earth is
the Hoba meteorite. It weighed over 100 tons.
This iron meteorite was found in Namibia,
Barringer Crater
Africa. A lot of it has rusted away since it
in Arizona. This
landed on Earth; it is still in its impact crater
impact crater
in Namibia
formed roughly
49,000 years
The largest meteorite ever found in the United when a 300,000
States is Willamette meteorite (named for the ton meteorite hit
city in Oregon where it landed). This iron
the Earth and
meteorite weighed fifteen tons and was found
disintegrated.
in 1902.