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Chapter 12

AST101H1 Chapter Notes - Chapter 12: Asteroid Belt, Oort Cloud

Astronomy & Astrophysics
Course Code
Clifford Orwin

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Chapter 12: Asteroids, Comets, and Dwarf Planets
12.1 – Asteroids and Meteorites
Asteroids, comets, + meteorites arescraps left over from formation of solar system
(S.S.) Gives understanding of how planets + larger moons came to exist.
Asteroids are planetesimals left over from birth of solar system.
Meteorites: Pieces of rock that have fallen to ground from space.
Most meteorites = pieces of asteroids that orbited Sun for billions of yrs bef. falling to
Asteroid’s orbit calculated by law of gravity (Section 4.4).
Ceres: Largest asteroid in SS (under 1000km in diameter).
Probably more than a million asteroids w/ diameters > 1 km.
If all asteroids compressed together + compressed into sphere by gravity then = object <
2000 km in diameter.
If asteroid is relatively large, gravity can mold its solid rock into spherical or near-
spherical shape. *Only Ceres is large enough for gravity to have compressed it into
roughly spherical shape Thus Ceres is qualified as dwarf planet.
Asteroids (like moons + planets) have many craters Thus they have also been battered.
Mathilde: Asteroid w/ huge crater in centre Most likely bound (by gravity)rubble
pile” b/c impact that made that crater would have shattered it.
Asteroids shine with reflected sunlight brightness depends on (1) Size (2) Distance (3)
Asteroid’s distance can be determined from its position in orbit.
Asteroid’s reflectivity determined by comparing asteroids brightness in visible light to
brightness in infrared light Visible light is reflected sunlight while infrared is thermal
radiation emitted from asteroid itself. Thus infrared brightness depends on
temperature which depends on how much sunlight it absorbs Thus comparison btw.

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infrared and visible brightness tells us proportions of incoming sunlight that asteroid
reflects and absorbs.
Asteroid’s (1) Brightness and (2) Distance tell us its size.
Sometimes we can determine shapes of asteroids by monitoring brightness variations as
asteroid rotates: Nonspherical asteroid with uniformly bright surface reflects more light
when it presents its larger side toward the Sun and our telescopes.
Asteroids made mostly of metal and rock because they condensed within the frost line
in the solar nebula (S.N.).
Those near outskirts of asteroid belt (A.B.) contain larger portions of dark, carbon-rich
A few asteroids appear to be made mostly of metals (ex. iron) suggesting they may be
fragments of metal cores of shattered worlds.
Only direct way to measure distant object’s mass is to observe gravitational effect on
another object.
For asteroids with moons (ex. Ida) we can determine mass of central asteroid by applying
Newton’s version of Kepler’s third law to orbital characteristics of moon.
Density can offer insight into asteroid’s origin and makeup Ex. Density of Mathilde is
1.5g/cm3 Too low for it to be a solid chunk of rock (w/ a density of 2.4g/cm3.)
All asteroids orbit sun in same direction as planets.
Average distance between asteroids in asteroid belt is millions of kilometres.
Orbital Resonance: Occurs whenever two objects periodically line up with each other.
Because gravity tugs at the objects in the same direction at each alignment, the effects
can build up over time. Objects will periodically line up – and hence have an orbital
resonance – whenever one object’s orbital period is simple ratio of another object’s
period, such as ½, ¼, 2/5.
Trojan Asteroids: Two sets of asteroids which share Jupiter’s 12-year orbit around Sun.
One clump always stays 60 degrees ahead and other clump stays 60 degrees behind.
When the S.S. was forming, the asteroid belt contained enough rocky material to form
another planet as large as Earth or Mars, but the orbital resonance with Jupiter disrupted

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the orbits of this region’s planetesimals, prevented them from accreting into a full-
fledged planet.
Most orbital resonances cause gaps, but is not the case for the Trojan Asteroids Their
orbits remain stable in 60 degrees zones ahead of and behind Jupiter b/c any asteroid that
wanders away from one of these zones is nudged back by Jupiter’s gravity.
Meteor: A flash of light caused by a particle entering our atmosphere at high speed, not
the particle itself.
Meteorite: An actual rock that has crashed to Earth.
Rocks from space large enough to become meteorites make unusually bright meteors,
called fireballs, as they fall through our atmosphere.
Meteorites usually covered w/ dark, pitted crust resulting from their fiery passage through
the atmosphere.
Meteorites often have iridium Very rare on Earth rocks + Meteorites tend to have
different rations among their isotopes that are found in rocks on Earth.
Two Types of Meteorites: (1) Primitive (2) Processed
Primitive: The first type of meteorite to have formed, nearly 4.6 billion years old. They
are the remnants of our solar system’s birth. Subtypes: (1) Stony primitive (2)
Carbon-rich primitive (Pg. 355).
Subtype: (1) Stony primitive: Composed of rocky minerals w/ small but noticeable
fraction of pure metallic flakes mixed in. (2) Carbon-rich: Similar to stony primitive but
also contain substantial amounts of carbon compounds and, sometimes, a small amount
of water.
Processed: These meteorites were apparently part of a larger object thatprocessed” the
original material of solar nebula into another form. Processed meteorites are generally
younger than primitive ones by a few hundred million years. Subtypes: Metal-rich
processed (2) Rocky processed.
Metal-rich processed: Made mostly of high-density iron and nickel mixed with smaller
amounts of other metals.
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