Chapter 14
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Chapter 14
Rocks are Asteroids. Dirty Snowballs are
Comet Nuclei
Asteroids are quite small, and most have eccentric orbits in the asteroid belt between
Mars and Jupiter.
Two other families are Apollo Asteroids and Trojan Asteroids twlee2016@gmail.com

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The Titus-Bode Rule predicted that there was a
Planet at 2.8 AU from the
Sun. It was tentatively named
Aster. When the field of rocks instead of a Planet was discovered at that distance the tiny objects were called
Asteroids. Now all rocks in space are called Asteroids.
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Belt Asteroids are rocky; about 200,000 have been identified so far.
Belt Asteroids are classified in types:
C-type: carbonaceous
S-type: silicate
M-type: metallic; iron and nickel
The three largest Belt Asteroids are Ceres (940 km), Pallas (580 km) and Vesta (540 km) twlee2016@gmail.com

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tasked with the exploration and study of the asteroid Vesta and the dwarf planet Ceres, two of the largest members of the Asteroid
Belt. The spacecraft was constructed with some European partners.
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Two small S-type asteroids, Gaspra and Ida, were visited by the Galileo. Gaspra (left) is in false color; it is really gray. Note that Ida
(right) has a small moon, Dactyl.
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A 2 nd family of asteroids have orbits so eccentric that they cross Earth’s orbital plane.
They are called Apollo Asteroids, or Near Earth
Asteroids, and raise the concern of a possible collision. These are survivors left over from creation and were not picked up during the
Early Intense Bombardment.Over 2600 such asteroids have been discovered so far, of which about 600 have been designated as potentially hazardous, due to their size, over one mile.
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The NEAR –Shoemaker spacecraft visited the Ctype asteroid Mathilde, on its way to its main target,
Eros. Mathilde, like many other asteroids, has a very low density, and is probably not solid, but is rather like a compact gravel pile.
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Eros does seem to be solid. It is covered with craters, as expected, but it also has free rocks sitting on it, not expected.
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The NEAR program presented an interesting challenge.
Not only did we need to go to a new orbit but we had to go to a new orbital plane. The maneuver was successful and we also landed on the Asteroid.
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5 twlee2016@gmail.com

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Comet Nuclei are described as “Dirty
Snowballs”. They reside in the Oort
Cloud which is a spherical cloud surrounding the Sun. It came from the Tsunami caused when the Sun went nuclear. The nuclei are
Irregular shaped objects of ices laced with dust and rock twlee2016@gmail.com

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The spherical Oort Cloud of
Comet Nuclei is very large but not densely populated. The trajectories go in every direction.
The Kuiper Belt Objects are more in the plane of the planets and are closer to the orbit of
Neptune. They are roughly the same composition as the Comet
Nuclei but many are much larger.
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The Kuiper Belt begins around 40 AU from the
Sun but continues out until it finally blends in with the Oort Cloud. The KBO orbits are irregular but move roughly in the prograde direction.
Some of these
Objects are quite large, larger even than Pluto. twlee2016@gmail.com

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Comets have a very small
Nucleus, a Coma of gas and dust that is the most visible part that can be very large, a Hydrogen Envelope, a dust tail, and an gas
(ionized) tail.
The Nucleus and the
Hydrogen Envelope are not normally seen.
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Comets have very eccentric orbits.
The comet’s tail always points away from the sun, due to the sunshine and the solar wind. The ion tail is straighter than the dust tail.
Long-period comets; over 2000 years
Short-period comets; less common, and have periods of less than 200 years.
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The theory of
Jupiter adjusting the orbit of an incoming comet would be much less viable if we had not seen
Jupiter capture an incoming comet.
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Halley’s comet is one of the most famous; it has a period of 76 years and has been observed since antiquity. Its most recent visit, in 1986, was not spectacular.
Left: The comet in 1910, as seen with the naked eye.
Right: the comet in 1986, seen through a telescope.
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Halley’s comet has a short period, just 76 years. Its orbit is not in the plane of the solar system so as it revolves around the Sun it passes through the plane of the Earth’s orbit at a spot closer to the Sun than the Earth. Some passes it is closer to the Earth than on other passes.
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This close up image was made by a Japanese spacecraft in 1986. It shows how the gas
Coma is developed from the Sun side of the
Nucleus as it rotates on its axis.
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Occasionally the Earth passes through a debris cloud left behind in the path of Comets. The interaction of the Atmosphere with these tiny particles looks like a
Shower of Meteors.
Meteor Showers repeat every year as the Earth passes through that part of the sky. [The + shown simply marks the apparent location of the radiant and is not seen in the sky] twlee2016@gmail.com

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Here are some of the best known Meteor Showers twlee2016@gmail.com

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Meteorites are defined as any particle from space that survives to reach the surface.
Most matter from space falls in slowly and does not make a crater.
These come mostly from the asteroid belt.
Meteorites are mostly very old –
4.4 to 4.6 billion years – and carry valuable information about the early stages of the solar system.
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Rarely does a meteorite strike near humans. This one hit a car.
This one arrived in
New England before the Whites did.
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Most meteorites are rocky and often have a polished look from passing through the atmosphere They are distinguishable from Earth rock by their internal structure.
A few are made of iron and can be identified by their grain structure, Iron like this can not be duplicated on
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Some large objects impact the earth when their orbits intersect the Earth’s orbit. These are often larger, from the Apollo (Near Earth) asteroids, and their impact velocity can be very large and their impact craters also large.
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The event called Cretaceous-Tertiary
Extinction likely resulted from a giant meteorite impact near Mexico 65 million years ago. It caused a 10 year long “Nuclear
Winter” that probably killed the dinosaurs along with 90% of the living things on Earth and from which only 25% of the species recovered. The event was likely due to an impact of a Near Earth Asteroid twlee2016@gmail.com

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