The Asteroid Belt

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Module 14 Beyond the Terrestrial Planets
Activity 1 The Asteroid Belt
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Summary
In this Activity, we will investigate (a) the
asteroid belt - vital statistics, (b) asteroids
as failed planets, (c) asteroid orbits, (d)
asteroids outside the Asteroid Belt, and (e)
properties of asteroids.
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(a) The Asteroid Belt - vital statistics
After the discovery of Uranus in 1781, the search
began for a planet between the orbits of Jupiter
and Mars as predicted by Bodes law.
On new years day in 1801, Italian astronomer
Guiseppe Piazzi discovered this missing planet at
about 2.8 AU and named it Ceres. However it soon
became apparent that this object was too small be
be a fully fledged planet its diameter is
only about 1000 km.
For more information on Bodes law, click here

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Ceres has diameter of about 30 of that of the
Moon.
These figures show Ceres to the same scale as the
Moonand the Earth.
Moon
Ceres
Earth
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So the search for the missing planet continued.
In March 1802, German astronomer Heinrich Olbers
found a second small body at a similar distance
to Ceres, called Pallas. Pallas is even smaller
and fainter than Ceres.
Well over 20,000 asteroids orbiting the Sun have
since been discovered.
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The Asteroid Belt
The overwhelming majority of asteroids are
located in the asteroid belt.

The asteroid belt is a region between the orbits
of Mars and Jupiter lying between about 2.1 to
4.1 AU from the Sun.
Not to Scale!
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Why are asteroids found so predominantly in this
region?
One theory is that the asteroids are the debris
of the disintegration of a planet that once
existed between Mars and Jupiter.
.
However, the theory lacks a plausible cause of
this disintegration
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(b) Failed Planets?
Asteroid-like objects are believed to have filled
the early Solar System.
Computer simulations provide evidence that
Jupiters strong gravity and tidal effects
disrupted the orbits of these planetesimals
within the asteroid belt.
As a result much of this material was ejected
from the Solar System.The total mass of all
asteroids in the asteroid belt is less than that
of our Moon.
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It is now believed that Jupiters gravitational
field cleared the asteroid belt before a planet
was able to form.
Simulations suggest that without this clearing
effect an additional planet would have formed
between Mars and Jupiter. In this context, the
asteroids could be considered to be a failed
planet.
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(c) Orbits
Kirkwood Gaps
Jupiter has an orbital period of 11.9 years. In
1867 Daniel Kirkwood observed that very few
asteroids have orbital periods which correspond
to simple fractions of 11.9 years.
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Why do Kirkwood gaps exist?
Consider an asteroid with an orbital period
exactly 1/2 that of Jupiter. The asteroid circles
the Sun twice in the time Jupiter circles the Sun
once. (This is also called a 21 resonance.)
Consequently the asteroid lines up between
Jupiter and the Sun at the same location every
second time it orbits the Sun. These repeated
alignments result in the asteroid being deflected
from its orbit and ejected from the Solar System
by Jupiters gravitational field.
Click here to see an animation showing an
asteroid with a period half that of Jupiter.
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General Orbits
So if the Kirkwood gaps are the places where we
dont find asteroids, where do we find them??
Most of the Solar Systems asteroids are found in
the Main Belt between about 2.1 and 4.1 AU. The
majority of main belt asteroids follow slightly
elliptical stable orbits, orbiting the Sun in
the same direction as the Earth. Typically the
orbital periods of these asteroids range from 3
to 8 years.
There are also a few special resonances where
asteroids like to group together, such as the 32
resonance at 3.97 AU (with periods 2/3 that of
Jupiter) where we find the Hilda group. And the
Trojan asteroids are found at the 11 resonance,
which means they have the same orbital period as
Jupiter.
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Shown are the orbits of the first three
asteroids to be discovered Ceres, Pallas, and
Juno.
Pallas
Ceres
Juno
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Here we show the asteroid distribution again, but
this time with the various asteroid groups noted.
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(d) Asteroids outside the asteroid belt
(i) Trojan asteroids
Two groups of asteroids orbit the Sun at
distances similar to Jupiter, outside the
asteroid belt. These are the Trojan
asteroids. Over 1,600 Trojan asteroids are
catalogued. Estimates of the their total number
go up to tens of thousands.
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Trojan asteroids trailing group
One group of Trojan asteroids is located at a
stable point that trails Jupiters orbit by 60.
Another group leads Jupiter by 60.
There are slightly more leading that trailing
Trojans.
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The combined gravitational forces of Jupiter and
the Sun in this 3-body rotating system produce
these two regions where small bodies can have
stable orbits.
French mathematician Louis Lagrange predicted the
existence of these stable orbits, called Lagrange
points, in 1772.
L5
The first Trojan asteroid was discovered in 1906.
L4
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Martian and Neptunian Trojan asteroids
Jupiter is not the only planet to host Trojan
asteroids. These two stable points predicted by
Lagrange actually apply to all planetary bodies.
To date Trojans have also been found around Mars
and Neptune, and there are currently 6 known
Martian Trojan asteroids and 1 Neptunian Trojan
asteroid.
It is more difficult to determine the orbits of
the Martian Trojans than the Jovian or
Neptunian Trojans, so there is some uncertainty
in this number.
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(ii) Near Earth Asteroids
As the name suggests, some asteroids are also
found near the orbit of the Earth.

• There are three subclasses
• Atens, with average orbital distances lt 1 AU
  (i.e. closer to the Sun than the Earth)
• Apollos, which are Earth-crossing and
• Amors, with average orbital distances gt 1 AU and
  lt 1.3 AU (i.e. between the Earth and Mars).

There are more than 2,800 known Near Earth
Asteroids.
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Typical orbit of an Apollo asteroid
Since the Apollo asteroids cross the Earths
orbit.
it is just a matter of time before these
asteroids collide with the Earth.
Sun
Earths orbit
Within tens of millions of years many of the
Apollo asteroids will strike the Earth.
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But the Solar system is billions of years old,
far older than tens of millions of years. So why
havent the Apollo asteroids all been destroyed
by colliding with the Earth already?
Apollo asteroids have not been in their current
orbits since the early stages of the Solar
System. The asteroid belt acts as a continual
source of Apollo asteroids. The influence of
Jupiter affects the orbits of certain asteroids
in the asteroid belt. If the orbit of an asteroid
subsequently passes close enough to Mars, it can
be pulled deeper into the Solar System.
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(iii) Centaurs
The Centaurs are asteroids that generally lie
between the orbit of Saturn and Neptune, with
average orbital distances between 10 AU and 30 AU.
The first object to be classified as a Centaur
was Chiron (not to be confused with Charon,
Plutos moon!), in 1977, the most distant
asteroid at the time. A surprise came in 1988,
where Chirons orbit carried it closer to the Sun
and its brightness nearly doubled. Chiron was
behaving like a comet! Being so far from the
Sun, the Centaurs generally contain some ices
such as CO2 which sublimate as they move closer
to the Sun.
There are now more than 140 catalogued Centaurs.
Sublimate means to go straight from a solid
(i.e. ice) to a vapour (i.e. gas)
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(iv) Kuiper Belt Objects
Kuiper Belt Objects (KBOs) are icy bodies found
in the outer reaches of the Solar System,
generally past the orbit of Neptune, between 30
to 50 AU and beyond. (Theyre also know as
trans-Neptunian objects or TNOs.)
The existence of the Kuiper Belt was suggested in
the late 1940s as a belt of icy bodies beyond
Neptune left over from the formation of the Solar
System. The first KBO was discovered in 1992 and
the orbits of about 800 have since been
catalogued. Well learn more about KBOs in the
Activity Pluto, Charon and the Plutons.
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(e) Properties - Collisions
A common image of the asteroid belt often
portrayed in science fiction is of a minefield
of asteroids through which spacecraft must
navigate. However the average distance between an
asteroid in the asteroid belt and its nearest
neighbour is thousands of times larger than the
distance between the Earth and the Moon.
In fact the spacecraft Galileo had to go out of
its way to approach close enough to asteroids to
provide us with the images shown throughout this
Activity. Nevertheless asteroids have
occasionally collided, as evidenced by their
cratered surfaces.
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This series of imagesfrom the Galileo spacecraft
show thebelt asteroid Ida pass through one full
rotation.Note its irregular shape and cratered
surface. Only Ceres, Pallas and Vesta are large
enough for differentiation to have taken place
and given them spherical shapes.
Differentiation is explained in the Activity on
Planetary Evolution
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Tidal Effects
The shape of asteroids is also affected by tidal
forces. Let us examine the Apollo asteroid
Geographos. As Geographos orbits close to the
Earth, the far side of the asteroid pulls out
while the side nearest the Earth pulls in. Thus
Geographos has gained its elongated shape aligned
roughly toward Earth.
Tidal Forces
Geographos
To review Tidal Forces refer to the Activity
Time and Tide
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Chemical composition

Asteroids are classified into 3 categories
depending on their surface colours and the
spectra of light which they reflect.
S type S type are the brightest asteroids,
are reddish in colour, and dominate the inner
belt region. These asteroids are composed of
silicates metals, largely iron. Their spectra
also indicates the presence of the mineral
olivine.
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M type M type asteroids are also bright but
are not red.They make up 10 of the total
population and are mostly in the inner belt
region. Their spectra indicates that they are
composed of iron nickel alloys.
C type About 75 of asteroids are C type which
are very dark. They inhabit the main belts outer
regions. These asteroids have a relatively high
content of carbon in the form of organic
compounds.
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In this Activity it has become clear that
Jupiters gravity plays an important role in the
continuing evolution of the Solar System, and in
particular in the distribution of asteroids. In
the next Module we will explore the Jovian Gas
Giants in general and Jupiter in particular.
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Image Credits
NASA Ida and Dactyl http//nssdc.gsfc.nasa.gov/im
age/planetary/asteroid/idasmoon.jpgWelcome to
Planet Earth http//antwrp.gsfc.nasa.gov/apod/ap97
1026.htmlFull Moon http//antwrp.gsfc.nasa.gov/ap
od/image/9809/fullmoonmosaic_gal_big.jpgGaspra ht
tp//nssdc.gsfc.nasa.gov/image/planetary/asteroid/
gaspra.jpgIda montage http//nssdc.gsfc.nasa.gov/
image/planetary/asteroid/ida_montage.jpgGeographo
s http//bang.lanl.gov/solarsys/raw/ast/geograph.g
if Asteroid distribution http//ssd.jpl.nasa.gov/
a_distrib.html
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• Now return to the Module 14 home page, and
  read more about the Asteroid Belt in the Textbook
  Readings.

Hit the Esc key (escape) to return to the Module
14 Home Page
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Bodes Law
Bodes law, more formally known as the
Titius-Bode law, is a mathematical formula that
describes the distances of the planets from the
Sun or at least it did in 1766 when only the
inner 6 planets were known.
The rule given by Bode was
for N 0, 3, 6, 12, 24, 48, 96
which worked ok, but seemed to indicate that
there should be a planet at 2.8 AU where the
clearly wasnt one
Mercury Venus Earth Mars ?? Jupiter Saturn
Predicted distance 0.4 AU 0.7 AU 1.0 AU 1.6 AU 2.8 AU 5.2 AU 10.0 AU
Actual distance 0.4 AU 0.7 AU 1.0 AU 1.5 AU ?? 5.2 AU 9.5 AU
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No-one paid much attention to Bodes law until
the discovery of Uranus in 1781. Bodes law
predicted a planet at 19.6 AU, just 2 further
than Uranus distance of 19.19 AU.
This encouraged Bode to urge people to search for
the missing 5th planet at 2.8 AU, which lead to
Pizzinis discovery of Ceres at 2.77 AU.
Neptunes discovery in 1846, however, threw the
law in doubt as it didnt fit with Bodes law at
all. Bodes 9th planet should be at 38.8 AU,
which fits Pluto (at 39.53 AU) better than it
does Neptune which is 30.07 AU from the
Sun. Further, there is no physical basis
whatsoever to Bodes law - it is purely a
mathematical relation (and with 3 free
parameters, it is quite easy to fit the data).
That said, Bodes law works remarkably well out
to Uranus.
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Return to activity!
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