Transcript Asteroids and Comets

Asteroids
&
Comets
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Debris of the Solar System
Asteroids are rocky or metallic
objects orbiting the Sun that
are smaller than a major
planet, but that show no
evidence of an atmosphere
and contain little volatile
(easily evaporated) material
Comets are icy bodies that
revolve around the Sun and
are smaller than a major
planet, but that contain frozen
water and other volatile
materials
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Discovery of Asteroids
Most asteroid orbits lie in
the asteroid belt, between
Mars and Jupiter
too small to be visible
without a telescope
They were first discovered
when astronomers were
hunting for a missing planet between Mars and Jupiter
The 1st asteroid, named Ceres and initially thought to
be the “missing planet”, was discovered by Giovanni
Piazzi in 1801
It orbits at 2.8 AU from the Sun
The discovery of other “minor planets” in similar orbits
followed in subsequent years
Now, more than 20,000 asteroids are known to have
well-determined orbits
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Asteroid Nomenclature
Asteroids are given both a
number and a name
The names were originally
chosen from Greek/Roman
goddesses, then other female
names were used, and finally
all names go!
Asteroids 2410 and
4859 are named
after Morrison and
Fraknoi
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Gaspra
Ida
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Asteroid Census
The total number of asteroids in the solar
system is very large
It must be estimated on the basis of systematic
sampling of the sky
Studies indicate that there are 106 asteroids
with diameters greater than 1 km!
The largest is Ceres, with a diameter of ~1000 km
Pallas and Vesta have diameters of ~500 km
15 more are larger than 250 km across
There are 100 times more objects 10-km across
than 100-km across
The total mass of asteroids is less than the
mass of the Moon
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Asteroid Orbits
All asteroids revolve around the Sun in west-to-east
direction, like the planets
Most of their orbits lie near the plane in which the
Earth and the other planets circle
The asteroid belt is defined as the region that contains
all asteroids with semi-major
axes in the range from 2.2
to 3.3 AU
Their orbital periods range
from 3.3 to 6 years
75% of known asteroids are
in the main belt
But they are not closely
spaced
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Asteroid Families
Japanese astronomer Kiyotsuga Hirayama
found in 1917 that some asteroids fall into
families
The families are groups with similar orbital
characteristics
Each family may have resulted from a breakup of a
larger body, or from the collision of two asteroids
Members of each family have similar speeds
There are physical similarities among the larger
members of a given family
Several dozen families are found
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Asteroid Physical Appearance
The majority of asteroids are very dark
They do not reflect much light
Their reflectivities are only 3 to 4 percent
There is, however, a sizable group that is not
very dark
Its typical reflectivities are 15 to 20 percent (similar
that of the Moon)
A few asteroids even have reflectivities as high
as 60%
To understand the reasons for these
differences, astronomers performed spectral
analysis of the light reflected by asteroids
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Asteroid Classification (1)
The dark asteroids
are believed to be primitive bodies
chemically unchanged since the beginning of the
solar system
are composed of silicates mixed with dark,
organic carbon compounds
include Ceres, Pallas, and most objects in
the outer third of the asteroid belt
Most of the primitive asteroids are
classed as C asteroids
C stands for carbonaceous (carbon rich)
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Asteroid Classification (2)
The second most populous group is the S asteroids
S stands for a “stony” or silicate composition
They have no dark carbon compounds and hence higher
reflectivities
Most S asteroids seem to be also primitive
The third group is the M asteroids
M stands for “metal”
Their identification is difficult
done by radar for the largest asteroids such as Psyche
They are much less numerous
Each may have come from a parent body that had
earlier differentiated and later shattered in a collision
There is enough metal in a 1-km M-type asteroid to
supply the world with iron for a long period of time
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Asteroid Classification (3)
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Trojan Asteroids
The Trojans are located far beyond main belt
at ~5.2 AU from the Sun, nearly the same distance
as Jupiter
dark, primitive objects, like some other asteroids
They have stable orbits because of Jupiter
In Jupiter’s orbit, there are two points near
which an asteroid can
stay almost indefinitely
They make equilateral
triangles with Jupiter
and the Sun
Since their first discovery
in 1906, several hundreds
have been found
The larger Trojans can be up to ~200 km across
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Animation: the Trojans
Green circles indicate main-belt
asteroids and blue dots on the
outermost circle are the Trojans
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Asteroids in Outer Solar System
There are asteroids with orbits that carry them far
beyond Jupiter
They are hard to detect and only a few have been found
Examples:
Chiron is 200 km across and the largest of them, with a
path carrying it from just inside the orbit of Saturn to
almost the distance of Uranus
Pholus, with an orbit that takes it 33 AU from the Sun,
beyond Neptune, has the reddest surface of any object
in the solar system, with unknown composition
They are named after centaurs (mythological half
horse, half human) because these objects have some
of the properties of both comets and asteroids
In 1988, on its closest approach to the Sun, Chiron’s
brightness doubled, much like the comets
Chiron, however, is much bigger than comets
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Earth-Approaching Asteroids
Some asteroids may stray far outside the main belt
and travel inward along paths that come close to or
cross Earth’s orbit
Such asteroids, and other objects that come close to
the Earth, are collectively known as Near-Earth
Objects (NEOs)
Needless to say, they are of great interest to us
Some of these NEOs have collided with the Earth in
the past, and some others are likely to do so in the
future
In 1994, a 1-km object passed closer than the Moon
By the end of 2002, more than 640 NEOs larger than
1 km in diameter had been discovered
Astronomers have estimated that there are probably
500 or so NEOs larger than 1 km in diameter that have
not yet been found
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NEOs
NEOs generally have unstable orbits
Each NEO will meet one of 2 fates:
collide with one of the terrestrial planets — and be
destroyed
be ejected gravitationally from the inner solar system
after a near-encounter with a planet
The probability for impact is once every 100 million
years
Hence the likelihood is very remote than any one of the
known NEOs will end up crashing into the Earth in the
foreseeable future …
The larger of these impacts will likely generate
environmental catastrophes for our planet
This is a good reason for further investigation of NEOs
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An NEO Observation
A 5-km-long NEO called Toutatis
approached to within 3 million km
of the Earth in 1992
which is less than 3 times the
distance to the Moon
Radar images indicate that it is a
double object
consisting of 2 irregular lumps,
with diameters of 3 km and 2 km,
squashed together
Animation
Radar images
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Rendezvous with Eros
Eros is an Earth-approaching S-type asteroid
potato-shaped, 34 km long, and 11 km wide
heavily cratered, suggesting that the surface is old
Movies of Eros captured by the
NEAR-Shoemaker spacecraft,
which orbited and then landed
on it in 2000
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Comets
They have been observed since
antiquity
Typical comets appear as rather
faint, diffuse spots of light
smaller than the Moon and many
times less brilliant
They are small chunks of icy
material that develop atmospheres
as they get closer to the Sun
As a comet gets “very close” to the Sun, the comet
may develop a faint, nebulous tail extending far from
the main body of the comet
Their appearance is seemingly unpredictable
Comets typically remain visible for periods from a few
days to a few months
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Comet Orbits
The scientific study of comets dates back to
Newton who first recognized that their orbits
were very elongated ellipses
Edmund Halley (a contemporary of Newton) in
1705 calculated/published 24 cometary orbits
He noted that the orbits of bright comets seen in
1531, 1607, and 1682 were quite
similar — and could belong to the
same comet — returning to the
perihelion every 76 years
He predicted a return in 1758
When the comet did appear in 1758,
it was given the name Comet Halley
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Comet Halley
It has been observed
and recorded on every
passage near the Sun at intervals from 74 to
79 years since 239 B.C.
The period variations are caused by the jovian
planets
In 1910 the Earth was brushed by the comet’s
tail, causing much needless public concern
Its last appearance in our skies was in 1986
met by several spacecraft
It is predicted to return in 2061
Its nucleus is approximately
16 x 8 x 8 km3
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Comet Census
Records exist for ~1000 comets
Comets are discovered at an average rate of 5
to 10 per year
Most of them are visible only on photographs
made with large telescopes
Every few years, a comet may appear that is
bright enough to be seen with the naked eye
Recent flybys:
Comet Hyakutake, with a very long tail, was visible
for about a month in March 1996
Comet Hale-Bopp appeared in 1997
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Comet Components (1)
Nucleus: relatively solid and
stable, composed mostly of ice
and gas, with a small amount of
dust and other solids
Coma: a dense cloud of water,
carbon dioxide, and other neutral
gases sublimed off of the nucleus
Hydrogen cloud: a huge (millions
of km in diameter), but very
sparse, envelope of neutral H gas
Dust tail: up to 10-million km long, composed of smokesized dust particles driven off the nucleus by escaping
gases
This is the most prominent part of a comet to the unaided eye
Ion tail: as much as several-hundred-million km long,
composed of plasma and laced with rays and streamers
caused by interactions with the solar wind
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Comet Components (2)
ion tail
dust tail
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Nucleus and Coma of Comet
The nucleus is composed of
ancient ice, dust, and
gaseous core material
The nucleus has low gravity
It cannot keep dust and gas
from escaping
The coma is the bright head
of the comet, as seen from
the Earth
The coma is a temporary
atmosphere of gas and dust
around the nucleus
The coma is 100,000's of
kilometers across
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Halley's nucleus
Halley's coma
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Nucleus of Comet Wild 2
The images were captured by NASA's Stardust spacecraft
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Comet’s Ion Tail
The Sun spews out charged particles
This solar wind occurs along the solar magnetic-field lines,
extending radially outward from the Sun
Ultraviolet (UV) sunlight ionizes gases in the coma
These ions (charged particles) are pushed by solar-wind
particles along field lines to form a tail millions of km long
The blue ion tail acts like a "solar" wind-sock
The tail always points directly away from the Sun because the
ions move at very high speed
When the comet is moving
away from the Sun, the
comet’s ion tail will be
almost in front of it!
The blue color is mostly
from the light emitted by
carbon-monoxide ions, but
other types of ions also
contribute to the light
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Origin and Evolution of Comets
Comets originate from very great distances
The aphelia of new comets are typically ~50,000 AU
This clustering of aphelia was first noted by Dutch astronomer
Jan Oort in 1950
He then proposed an idea for the origin of those comets,
which is still accepted by most astronomers today
Oort’s model of comet origin:
The Sun’s sphere of influence extends only a little beyond
50,000 AU, or about 1 LY
Objects in orbit about the Sun at this distance can be easily
perturbed by the gravity of passing stars
The comets are some of the perturbed objects, which take on
orbits that bring them much closer to the Sun
The reservoir of ancient icy objects from which such
comets are presumably derived is called the Oort
comet cloud
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Oort Comet Cloud
Astronomers estimate that there may be about a
trillion (1012) comets in the Oort cloud
In addition, 10 times this number of comets could be
orbiting the Sun between the planets and the Oort
cloud
Such cometary objects remain undiscovered probably
because they are too faint to be seen directly and
because their stable orbits do not bring them closer to
the Sun
The total number of comets within the sphere of
influence of our Sun could therefore be on the order
of ten trillion (1013)!
Their total mass would be similar to that of 1000 Earths
Cometary material could thus be the most important
constituent of the solar system after the Sun itself
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Kuiper Belt
Another possible source of comets lies just beyond the
orbit of Neptune
The existence of this region was first suggested by
Gerard Kuiper in 1951
The first object from this region, now called the Kuiper
belt, was discovered in 1992
The object is ~200 km across
Since then, several hundred more Kuiper-belt objects
(KBOs) have been found
It appears that these KBOs are heavily influenced by
the gravity of Neptune
Many of the known KBOs have orbits like that of Pluto
Some astronomers have, therefore, suggested that Pluto
can be considered the largest member of the Kuiper belt
For this reason, KBOs are sometimes called plutinos
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Fate of Comets
Most comets probably spend nearly all their existence in
the Oort cloud or Kuiper belt
at a temperature near absolute zero
But once a comet enters the inner solar system, its life
likely changes dramatically!
If it survives the initial passage near the Sun, it will return
towards the cold aphelion
and may follow a fairly stable orbit for a “while”
It may impact the Sun, or be completely vaporized as it flies
by the Sun
It may interact with one or more planets with 3 possible fates:
destroyed after impacting a planet
speeded up and ejected, leaving the solar system forever
perturbed into an orbit of shorter period
Each time it approaches the Sun, a comet loses part of its
material
It may end its life catastrophically by breaking apart
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Comet Shoemaker-Levy 9
Some comets die very spectacularly
When comet Shoemaker-Levy 9 passed close to
Jupiter in July 1992, the comet broke into about
20 pieces
perhaps due Jupiter’s tidal forces
Fragments of the comets then orbited Jupiter until
July 1994 when they
crashed into Jupiter,
experiencing violent
destruction
Animation
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