Module 14:
Beyond the Terrestrial
Planets
Activity 1:
The Asteroid Belt
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.
(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 Bode’s law*.
On new year’s 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 Bode’s law, click here
Ceres has diameter of about
30% of that of the Moon.
These figures show Ceres to
the same scale as the Moon
and the Earth.
Moon
Ceres
Earth
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.
By 1849, 10 such objects were known, and in
1868 the 100th asteroids or minor planets
was announced. By 1923, more than
1,000 asteroids were catalogued
Well over 20,000 asteroids orbiting
the Sun have since been discovered.
Asteroid
Gaspra from the
Galileo spacecraft which
passed within 1600 km
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!
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…
Orbital eccentricity
1.0
Asteroid distribution
0.5
asteroid belt
0.0
(b) Failed Planets?
Asteroid-like objects are believed to have filled the early
Solar System.
Computer simulations provide evidence that Jupiter’s 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.
It is now believed that Jupiter’s 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.
(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.
1.0
Orbital eccentricity
Kirkwood gaps, as these
features are known, exist
where asteroids’ orbital
periods would be 1/3, 2/5,
3/7 and 1/2 that of Jupiter.
asteroid belt
5:2
3:1
2:1
0.5
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 2:1 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
Jupiter’s gravitational field.
Click here to see an animation showing an asteroid
with a period half that of Jupiter.
General Orbits
So if the Kirkwood gaps are the places where we don’t find
asteroids, where do we find them??
Most of the Solar System’s 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 3:2 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 1:1
resonance, which means they have the same orbital period
as Jupiter.
Shown are the orbits of the first three
asteroids to be discovered: Ceres,
Pallas, and Juno.
Pallas
Ceres
Juno
Here we show the
asteroid distribution
again, but this time
with the various
asteroid groups
noted.
(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.
One group of
Trojan asteroids is
located at a stable
point that trails
Jupiter’s orbit by
60°.
Trojan asteroids
trailing group
Another group
leads Jupiter by
60°.
There are slightly
more leading that
trailing Trojans. Trojan asteroids
leading group
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.
There are also three unstable
points in such a system. The
two stable Lagrange points
are labelled L4 and L5.
L5
The first Trojan asteroid
was discovered in 1906.
L4
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.
(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 < 1 AU
(i.e. closer to the Sun than the Earth);
– Apollos, which are Earth-crossing; and
– Amors, with average orbital distances > 1 AU and
< 1.3 AU (i.e. between the Earth and Mars).
There are more than 2,800 known Near Earth Asteroids.
Since the Apollo
asteroids cross the
Earth’s orbit….
…it is just a matter
of time before these
asteroids collide with
the Earth.
Typical orbit of an
Apollo asteroid
Sun
Earth’s orbit
Within tens of millions of years many of the Apollo asteroids
will strike the Earth.
But the Solar system is billions of years old, far older
than tens of millions of years. So why haven’t 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.
(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, Pluto’s moon!), in 1977,
the most distant asteroid at the time. A surprise came in
1988, where Chiron’s 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)
(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. (They’re 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.
We’ll learn more about KBOs in the Activity Pluto, Charon
and the Plutons.
(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.
This series of images
from the Galileo spacecraft show the
belt 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”
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”
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.
“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 belt’s outer regions.
These asteroids have a relatively high content of carbon in the
form of organic compounds.
In this Activity it has become clear that Jupiter’s
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.
Image Credits
NASA:
Ida and Dactyl
http://nssdc.gsfc.nasa.gov/image/planetary/asteroid/idasmoon.jpg
Welcome to Planet Earth
http://antwrp.gsfc.nasa.gov/apod/ap971026.html
Full Moon
http://antwrp.gsfc.nasa.gov/apod/image/9809/fullmoonmosaic_gal_big.jpg
Gaspra
http://nssdc.gsfc.nasa.gov/image/planetary/asteroid/gaspra.jpg
Ida montage
http://nssdc.gsfc.nasa.gov/image/planetary/asteroid/ida_montage.jpg
Geographos
http://bang.lanl.gov/solarsys/raw/ast/geograph.gif
Asteroid distribution
http://ssd.jpl.nasa.gov/a_distrib.html
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
Bode’s “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 wasn’t 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
No-one paid much attention to Bode’s law until the discovery of
Uranus in 1781. Bode’s 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 Pizzini’s discovery of Ceres at
2.77 AU.
Neptune’s discovery in 1846, however, threw the law in doubt
as it didn’t fit with Bode’s law at all. Bode’s 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 Bode’s law - it
is purely a mathematical relation (and with 3 free parameters, it
is quite easy to fit the data). That said, Bode’s law works
remarkably well out to Uranus.
Return to activity!