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Chapter 4

Chapter Fourteen Review: Solar System Debris

Course Code
Brian Wilson

of 8
Nov. 16th, 2010
Intro to Astronomy and Astrology Part I
Chapter 14: Solar System Debris
Asteroids are relatively small, predominantly rocky objects that revolve around the
Few asteroids are larger than 300 km in diameter
Generally move on somewhat eccentric trajectories
Orbital Properties
Over 250,000 asteroids and their precise orbits catalogued asteroids
The total number of known asteroids now exceeds 500,000
The vast majority of these bodies are found in the asteroid belt, located
between 2.1 and 3.3 AU from the Sun
Such a compact concentration of asteroids in a well-defined belt suggests
that they are either the fragments of a planet broken up a long time ago or
primal rocks that never managed to accumulate into a genuine planet
Probably not the remnants of a planet, as the total mass isnt sufficient to
suggest this origin
Physical Properties
Asteroids have been found to differ amongst themselves in composition, not
only against the planets and moons
oThe darkest/least reflective ones contain large fractions of carbon in
their makeup
These asteroids are known as C-type asteroids
Are believed to consist of very primitive material
oThe more reflective S-type asteroids predominate in the inner portions
of the asteroid belt, and contain silicate, or rocky, material
Vesta, despite its small size (540 km in diameter), appears to have undergone
volcanism in the distant past
Asteroid Observations from Space
Collisions between asteroids may be quite common, providing a source of
both interplanetary dust and smaller asteroids and possibly deflecting one or
both of the bodies involved onto eccentric, Earth-crossing orbits
The less violent collisions may be responsible for the binary systems we see
(two asteroids orbiting one another as they circle the Sun)
Earth-Crossing Asteroids
Most asteroids have eccentric orbits that ensure that they always remain
between the orbits of Mars and Jupiter
Those that have eccentricities greater than 0.4 are of particular interest to
use as their paths may intersect Earths orbit, leading to the possibility of
collision with our planet
oThese bodies are collectively known as Earth-Crossing asteroids
Those stray asteroids having highly elliptical orbits or orbits that do not lie in
the main asteroid belt have probably been influenced by nearby Mars and,
especially, Jupiter
Orbital Resonances
Although most asteroids orbit in the main belt, between 2 and 3 AU from the
Sun, an additional class of asteroids, called the Trojan asteroids, orbits at the
distance of Jupiter
oThey are locked into a 1:1 orbital resonance with Jupiter by that planets
strong gravity
The Trojan asteroids are orbiting at points known as Lagrangian points (60
degrees behind and in front of Jupiter along the same orbiting path)
oThese Lagrangian points revolve around the Sun at the same rate as
oThe Lagrangian points of the Jupiter-Sun system
One point both behind and infront of Jupiter at 60 degrees, and
then three more that lie on the line joining Jupiter and the Sun
However, only the two 60 degrees in front or behind of Jupiter
are considered stable as an object located at one of the three
points that lie on the line joining Jupiter and the Sun are
unstable in the sense that if an object were to for any reason
stray from one of these three points, it will tend to drift slowing
away from it, not back toward it
Comets are usually discovered as faint, fuzzy patches of light on the sky while they
are still several AU away from the Sun
oTravelling in highly elliptical orbits with the Sun at one focus, a comet
brightens and develops a tail as it nears the Sun (beginning to slowly burn
up from the Suns heat)
oAs the comet departs the Suns vicinity, its brightness and tail diminish until
it once again becomes a faint point of light receding into the distance
Like the planets, comets emit no visible light of their own
oThey shine by reflected (or reemitted) sunlight
Comet Appearance and Structure
Even through a large telescope, the nucleus, or main solid body, of a comet is no
more than a minute point of light
oA typical cometary nucleus is extremely small only a few kilometers in
During most of the comets orbit, far from the Sun, only this frozen
nucleus exits
When a comet comes within a few AU of the Sun its icy surface
becomes too warm to remain stable