Transcript File

Week #6
Pluto, Comets, and
Space Debris
Pluto
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Pluto, the outermost known planet, is a deviant. Its elliptical orbit is the
most out of round (eccentric) and is inclined by the greatest angle with
respect to the Earth’s orbital plane, near which the other planets
revolve.
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Pluto’s elliptical orbit is so eccentric that part lies inside the orbit of
Neptune.
Pluto was closest to the Sun in 1989 and moved farther away from the
Sun than Neptune in 1999.
So Pluto is still relatively near its closest approach to the Sun out of its
248-year period, and it appears about as bright as it ever does to
viewers on Earth.
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It hasn’t been as bright for over 200 years.
It is barely visible through a medium-sized telescope under dark-sky
conditions.
Pluto’s Orbit:
Pluto
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The discovery of Pluto was the result of a long search for an additional
planet that, together with Neptune, was believed to be slightly
distorting the orbit of Uranus.
Finally, in 1930, Clyde Tombaugh, hired at age 23 to search for a new
planet because of his experience as an amateur astronomer, found the
dot of light that is Pluto (see figure).
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It took him a year of diligent study of the photographic plates he obtained
at the Lowell Observatory in Arizona.
From its slow motion with respect to the stars over the course of many
nights, he identified Pluto as a new planet.
Pluto’s Mass and Size
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Even such basics as the
mass and diameter of Pluto
are very difficult to
determine.
Moreover, Pluto has made
less than one revolution
around the Sun since its
discovery, thus providing
little of its path for detailed
study.
As recently as 1968, it was
mistakenly concluded that
Pluto had 91 per cent the
mass of the Earth, instead of
the correct value of 0.2 per
cent.
Pluto’s Mass and Size
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The situation changed drastically in 1978 with
the surprise discovery (see figure) that Pluto has
a satellite.
The moon was named Charon,
The presence of a satellite allows us to deduce
the mass of the planet by applying Newton’s
form of Kepler’s third law.
Charon is 5 to 10 per cent of Pluto’s mass, and
Pluto is only 1/500 the mass of the Earth, ten
times less than had been suspected just before
the discovery of Charon.
Pluto’s Mass and Size
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Pluto’s rotation axis is nearly in the ecliptic, like that of
Uranus.
This is also the axis about which Charon orbits Pluto every
6.4 days.
Consequently, there are two five year intervals during
Pluto’s 248-year orbit when the two objects pass in front
of (that is, occult) each other every 3.2 days, as seen from
Earth.
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Such mutual occultations were the case from 1985 through
1990.
Pluto’s Mass and Size
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From the duration of fading, we deduced how
large they are.
Pluto is 2300 km in diameter, smaller than
expected, and Charon is 1200 km in diameter.
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Charon is thus half the size of Pluto.
Further, it is separated from Pluto by only about
8 Pluto diameters, compared with the 30 Earth
diameters that separate the Earth and the
Moon.
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So Pluto/Charon is almost a “double-planet”
system.
Pluto’s Mass and Size
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The rate at which the light
from Pluto/Charon faded also
gave us information that
revealed the reflectivities
(albedoes) of their surfaces,
since part of the surface of
the blocked object remained
visible most of the time.
The surfaces of both vary in
brightness (see figure).
Pluto seems to have a dark
band near its equator, some
markings on that band, and
bright polar caps.
Pluto’s Mass and Size
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In 1990, the Hubble Space Telescope took an image that showed Pluto
and Charon as distinct and separated objects for the first time, and they
can now be viewed individually by telescopes on Mauna Kea in Hawaii
(see figure, top) and elsewhere where the “seeing” is exceptional.
The latest Hubble views show that Pluto has a dozen areas of bright and
dark, the finest detail ever seen on Pluto, whose diameter is smaller than
that of the United States (see figure, below).
Pluto’s Mass and Size
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If we were standing on Pluto, the Sun would appear over
a thousand times fainter than it does to us on Earth.
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Consequently, Pluto is very cold; infrared measurements
show that its temperature is less than 60 K. From Pluto, we
would need a telescope to see the solar disk, which would
be about the same size that Jupiter appears from Earth.
Pluto’s Atmosphere
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Pluto occulted — passed in front of and hid—a star on one
night in 1988.
Astronomers observed this occultation to learn about
Pluto’s atmosphere.
If Pluto had no atmosphere, the starlight would wink out
abruptly.
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Any atmosphere would make the starlight diminish more
gradually.
The observations showed that the starlight diminished
gradually and unevenly.
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Thus Pluto’s atmosphere has layers in it.
Pluto’s Atmosphere
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From the 1988 occultation, astronomers were also able to
conclude that the bulk of Pluto’s atmosphere is nitrogen.
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A trace of methane must also be present, since the methane
ice on Pluto’s surface, detected from its spectrum, must be
evaporating.
Still, Pluto’s atmospheric pressure is very low, only
1/100,000 of Earth’s.
What Is Pluto?
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From Pluto’s mass and radius, we calculate its density.
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It turns out to be about 2 g/cm3, twice the density of water and
less than half the density of Earth.
Since ices have even lower densities than Pluto, Pluto must be
made of a mixture of ices and rock.
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Its composition is more similar to that of the satellites of the giant
planets, especially Neptune’s large moon Triton, than to that of
Earth or the other inner planets.
What Is Pluto?
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Pluto remains strange in that it is so small next to the giants,
and that its orbit is so eccentric and so highly inclined to the
ecliptic.
Increasingly, Pluto is being identified with a newly discovered
set of objects in the outer Solar System, which we will now
study.
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Is Pluto even a planet?
It is so small, so low in mass, and in such an inclined orbit with
respect to the eight inner planets that perhaps it should only be
called an asteroid, a “Kuiper-belt object,” or a “Trans-Neptunian
Object.”
Kuiper-belt Objects
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Beyond the orbit of Neptune, a population of icy objects
with diameters of a few tens or hundreds of kilometers is
increasingly being found.
The planetary astronomer Gerard Kuiper (pronounced
koy´per) suggested a few decades ago that these objects
would exist and should be the source of many of the
comets that we see.
As a result, these objects are now known as the Kuiperbelt objects, or, less often, Trans-Neptunian Objects.
Kuiper-belt Objects
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The Kuiper belt is
probably about 10 A.U.
thick and extends from
the orbit of Neptune
about twice as far out
(see figure).
About 1000 Kuiper-belt
objects have been found
so far, and tens of
thousands larger than
100 km across are
thought to exist.
The objects may be left
over from the formation
of the Solar System.
Kuiper-belt Objects
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They are generally very dark, with albedoes of only about 4 per cent.
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Pluto, by contrast, has an albedo of about 60 per cent.
Still, Pluto is one of the largest of the Kuiper belt objects, so much
larger than most of the others that it is covered with frost.
Triton may have initially been a similar object, subsequently captured by
Neptune.
A Kuiper-belt object larger than Pluto’s moon Charon was found in
2001, about half of Pluto’s diameter.
One that may be even somewhat larger was found in 2002, though the
uncertainty limits of these two Kuiper-belt objects overlap.
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The newer one, tentatively and unofficially named Quaoar (pronounced
“kwa-whar”)
Kuiper-belt Objects
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David Jewitt of the University of Hawaii and Jane Luu, now at MIT’s
Lincoln Lab, have been the discoverers of most of the known Kuiper-belt
objects.
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They found the first one in 1992 and they and several other astronomers are
looking for more.
Michael Brown of Caltech and his colleagues stunned the world in July
2005, with their discovery of an outer-solar-system object even larger
than Pluto (see figures).
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Initially named 2003 UB313, it was first sighted in 2003 but not confirmed
until 2005.
Kuiper-belt Objects
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UB313 is now 97 A.U. out from the Sun, more than twice
as far out as Pluto.
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It takes over 500 years to orbit the Sun.
Its orbit is tilted an incredible 44°, taking it so high out of
the ecliptic that no previous planet hunter found it.
Undoubtedly, it was thrown into that highly inclined orbit
after a close gravitational encounter with Neptune.
Is it a 10th planet?
Comets
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Nearly every decade, a bright comet appears in our
sky.
From a small, bright area called the head, a tail may
extend gracefully over one-sixth (30°) or more of the
sky.
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The tail of a comet is always directed roughly away
from the Sun, even when the comet is moving outward
through the Solar System.
Although the tail may give an impression of motion
because it extends out only to one side, the comet
does not move noticeably with respect to the stars as
we casually watch during the course of a night.
Comets
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Still, both comets and stars rise
and set more or less together
(see figure).
Within days, weeks, or (even less
often) months, a bright comet
will have become too faint to be
seen with the naked eye.
Comets
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Most comets are much fainter than the one we have just
described.
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About two dozen new comets are discovered each year, and
most become known only to astronomers.
If you should ever discover a comet, and are among the
first three people to report it to the International
Astronomical Union Central Bureau for Astronomical
Telegrams at the Smithsonian Astrophysical Observatory in
Cambridge, Massachusetts, it will be named after you.
The Composition of Comets
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At the center of a comet’s head is its nucleus, which is
composed of chunks of matter.
The most widely accepted theory of the composition of
comets, is that the nucleus is like a “dirty snowball.”
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It may be made of ices of such molecules as water (H2O),
carbon dioxide (CO2), ammonia (NH3), and methane (CH4),
with dust mixed in.
The Composition of Comets
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The nucleus itself is so small that we
cannot observe it directly from Earth.
Radar observations have verified in
several cases that it is a few kilometers
across.
The rest of the head is the coma
(pronounced coh´ma), which may grow
to be as large as 100,000 km or so
across (see figure).
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The coma shines partly because its gas
and dust are reflecting sunlight toward
us and partly because gases liberated
from the nucleus get enough energy
from sunlight to radiate.
The Composition of Comets
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The tail can extend 1 A.U. (150,000,000 km), so comets can be the
largest objects in the Solar System.
But the amount of matter in the tail is very small—the tail is a much
better vacuum than we can make in laboratories on Earth.
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The dust tail is caused by dust particles released from the ices of the
nucleus when they are vaporized.
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Many comets actually have two tails
The dust particles are left behind in the comet’s orbit, blown slightly away
from the Sun by the pressure of sunlight hitting the particles.
As a result of the comet’s orbital
motion, the dust tail usually
curves smoothly behind the
comet.
The Composition of Comets
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The gas tail is composed of gas blown outward from the
comet, at high speed, by the “solar wind” of particles
emitted by the Sun.
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As puffs of gas are blown out and as the solar wind varies,
the gas tail takes on a structured appearance.
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It follows the interplanetary magnetic field.
Each puff of matter can be seen.
A comet—head and tail together—contains less than a
billionth of the mass of the Earth.
The Origin and Evolution of Comets
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It is now generally accepted that trillions of tail-less
comets surround the Solar System in a sphere perhaps
50,000 A.U. (that is, 50,000 times the distance from the
Sun to the Earth, or almost 1 light-year) in radius.
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This sphere, far outside Pluto’s orbit, is the Oort comet
cloud (named after the Dutch scientist Jan Oort).
The total mass of matter in the cloud may be only 1 to 10
times the mass of the Earth.
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In current models, most of the Oort cloud’s mass is in the
inner 1000 to 10,000 A.U.
The Origin and Evolution of Comets
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Occasionally one of these comets leaves the comet cloud.
Currently, astronomers tend to think that gravity from the
disk of our Milky Way Galaxy does most of the tugging.
The comet’s orbit may be altered, sometimes into an
elliptical orbit, if it passes near a giant planet, most
frequently Jupiter.
Because the comet cloud is spherical, comets are not
limited to the plane of the ecliptic, which explains why one
major class of comets comes in randomly from all
directions.
The Origin and Evolution of Comets
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Another group of comets has orbits that are much more limited
to the plane of the Solar System (Earth’s orbital plane).
They probably come from the Kuiper belt beyond the orbit of
Neptune, a flatter distribution of objects ranging from about 25
to 50 A.U.
We seem to discover more of these Kuiper-belt-origin comets
than we expect compared with Oort-cloud-origin comets.
The Origin and Evolution of Comets
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Until recently, astronomers tended to say that the long-period
comets, those with orbital periods longer than 200 years, came
from the Oort cloud while comets with periods shorter than 200
years came from the Kuiper belt (see figure).
Most of the long-period comets have
semimajor axes close to 20,000 A.U.,
5000 times the 40 A.U. semimajor axis
of Pluto’s orbit.
The Origin and Evolution of Comets
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The short-period comets, those with periods less
than 200 years, were divided into “Jupiter-family”
comets, whose orbits were made so small by
encounters with Jupiter that their periods were
less than 20 years, and “Halley-type” comets,
which suffered less influence by Jupiter.
The Origin and Evolution of Comets
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As a comet gets closer to the Sun than those distant regions, the
solar radiation begins to vaporize the ice in the nucleus.
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The tail forms, and grows longer as more of the nucleus is vaporized.
Even though the tail can be millions
of kilometers long, it is still so
tenuous that only 1/500 of the mass
of the nucleus may be lost each time
it visits the solar neighborhood.
Thus a comet may last for many
passages around the Sun.
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But some comets hit the Sun and are
destroyed (see figure).
Halley’s Comet
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In 1705, the English astronomer Edmond
Halley (Halley is pronounced to rhyme with
“Sally,” and not with “say´lee”) (see figure)
applied a new method developed by his
friend Isaac Newton to determine the orbits
of comets from observations of their positions
in the sky.
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He reported that the orbits of the bright
comets that had appeared in 1531, 1607, and
1682 were about the same.
Moreover, the intervals between appearances
were approximately equal, so Halley
suggested that we were observing a single
comet orbiting the Sun, and he accounted for
the slightly different periods with Newton’s
law of gravity from interactions with planets.
Halley’s Comet
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Halley predicted that this bright comet would again return in
1758.
Its reappearance on Christmas night of that year, 16 years after
Halley’s death, was the proof of Halley’s hypothesis (and
Newton’s method).
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The comet has thereafter been known as Halley’s Comet (see
figure).
Since it was the first known “periodic comet” (i.e., the first
comet found to repeatedly visit the inner parts of the Solar
System), it is officially called 1P, number 1 in the list of periodic
(P) comets.
Halley’s Comet
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It seems probable that the bright comets reported every
74 to 79 years since 240 b.c. were earlier appearances of
Halley’s Comet.
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Halley’s Comet came especially close to the Earth during
its 1910 return, and the Earth actually passed through its
tail.
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The fact that it has been observed dozens of times endorses
the calculations that show that less than 1 per cent of a
cometary nucleus’s mass is lost at each passage near the
Sun.
Many people had been frightened that the tail would
somehow damage the Earth or its atmosphere, but the tail
had no noticeable effect.
Even then, most scientists knew that the gas and dust in
the tail were too tenuous to harm our environment.
Halley’s Comet
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The most astounding observations were undoubtedly the
photographs showing the nucleus itself (see figure,
bottom left), which turns out to be potato-shaped (see
figure, bottom right).
It is about 16 km in its longest dimension, half the size of
Manhattan Island.
Halley’s Comet
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The “dirty snowball” theory of comets was confirmed in
general, but the snowball is darker than expected.
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Further, the evaporating gas and dust is localized into jets
that are stronger than expected.
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It is as black as velvet, with an albedo of only about 3 per
cent.
They come out of fissures in the dark crust.
We now realize that comets may shut off not when they
have lost all their material but rather when the fissures in
their crusts close.
Halley’s Comet
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About 30 per cent of Halley’s dust particles are made only of hydrogen,
carbon, nitrogen, and oxygen (see figure).
This simple composition resembles that of the oldest type of meteorite.
It thus indicates that these particles may be from the earliest years of
the Solar System.
Halley’s Comet
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The next appearance of Halley’s Comet, in 2061, again
won’t be spectacular.
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Not until the one after that, in 2134, will the comet show a
long tail to earthbound observers.
Fortunately, though Halley’s Comet is predictably
interesting, a more spectacular comet appears every 10
years or so.
When you read in the newspaper that a bright comet is
here, don’t wait to see it another time.
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Some bright comets are at their best for only a few days or
a week.
Comet
Shoemaker-Levy 9
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A very unusual comet gave thrills to people
around the world.
In 1993, Eugene Shoemaker, Carolyn
Shoemaker, and David Levy discovered their
ninth comet in a search with a wide-field
telescope at the Palomar Observatory.
This comet looked weird—it seemed
squashed.
Comet
Shoemaker-Levy 9
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Higher-resolution images taken with other telescopes,
including the Hubble Space Telescope (see figure),
showed that the comet had broken into bits, forming a
chain that resembled beads on a string.
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Even stranger, the comet was in orbit not around the Sun
but around Jupiter, and would hit Jupiter a year later.
Apparently, several decades earlier the comet was
captured in a highly eccentric orbit around Jupiter, and in
1992, during its previous close approach, it was torn apart
into more than 20 pieces by Jupiter’s tidal forces.
Comet Shoemaker-Levy 9
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Telescopes all around the world and in space were trained on
Jupiter when the first bit of comet hit.
The site was slightly around the back side of Jupiter, but rotated
to where we could see it from Earth after about 15 minutes.
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Even before then, scientists were enthralled by a plume rising
above Jupiter’s edge.
Over a period of almost a week, one bit of the comet after
another hit Jupiter, leaving a series of Earth-sized rings and
spots as Jupiter rotated.
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The largest dark spots could be seen for a few months even with
small backyard telescopes.
Comet
Shoemaker-Levy 9
Comet Shoemaker-Levy 9
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The dark material showed us the hydrocarbons and other
constituents of the comet.
Spectra showed sulfur and other elements, presumably
dredged up from lower levels of Jupiter’s atmosphere than
we normally see.
The biggest comet chunk released the equivalent of 6
million megatons of TNT—100,000 times more than the
largest hydrogen bomb.
So Comet Shoemaker-Levy 9 made us even more wary
about what may be coming at us from space.
Recently Observed Comets
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In 1995, Alan Hale and
Thomas Bopp independently
found a faint comet, which was
soon discovered to be quite far
out in the Solar System.
Its orbit was to bring it into the
inner Solar System, and it was
already bright enough that it
was likely to be spectacular
when it came close to Earth in
1997.
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It lived up to its advance
billing (see figure).
Recently Observed Comets
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Telescopes of all kinds were trained on
Comet Hale-Bopp, and hundreds of
millions of people were thrilled to step
outside at night and see a comet just by
looking up.
Modern powerful radio telescopes were
able to detect many kinds of molecules
that had not previously been recorded in
a comet.
Occasionally, other bright comets, such
as C /2002 C1, Comet Ikeya-Zhang (see
figure), turn up and are fun to watch.
Spacecraft to Comets
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NASA’s Deep Space 1 mission flew close to Comet 19P/Borrelly in 2001.
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It obtained more detailed images of the bowling-pin-shaped nucleus (see
figure) than even Giotto’s views of Halley’s nucleus.
This comet’s surface, and therefore probably the surfaces of comet nuclei
in general, was rougher and more dramatic than expected.
Spacecraft to Comets
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NASA’s Stardust mission, launched in 1999, went to Comet
Wild 2 (pronounced Vilt-too), a periodic comet with a sixyear orbit.
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When it got there in 2004, it not only photographed the
comet but also gathered some of its dust.
It carries an extremely lightweight material called aerogel,
and flew through the comet with the aerogel exposed so
that the comet dust could stick in it.
Meteoroids
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There are many small chunks of matter orbiting in the Solar System,
ranging up to tens of meters across and sometimes even larger.
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When these chunks are in space, they are called meteoroids.
When one hits the Earth’s atmosphere, friction and the compression of
air in front of it heat it up—usually at a height of about 100 km—until
all or most of it is vaporized.
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Such events result in streaks of light in the sky, which we call meteors
(popularly, and incorrectly, known as shooting stars or falling stars).
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When a fragment of a meteoroid
survives its passage through the
Earth’s atmosphere, the remnant that
we find on Earth is called a meteorite.
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Counting even tiny meteorites,
whose masses are typically a
milligram, some 10,000 tons of this
interplanetary matter land on Earth’s
surface each year.
Types and Sizes of Meteorites
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Space is full of meteoroids of all sizes, with the smallest
being most abundant.
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Most of the small particles, less than 1 mm across, may
come from comets.
The large particles, more than 1 cm across, may generally
come from collisions of asteroids in the asteroid belt (see
Section 8.5).
Tiny meteorites less than a millimeter across,
micrometeorites, are the major cause of erosion on the
Moon.
Micrometeorites also hit the Earth’s upper atmosphere
all the time, and remnants can be collected for analysis
from balloons or airplanes or from deep-sea sediments.
Types and Sizes of Meteorites
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Some of the meteorites
that are found have a
very high iron content
(about 90 per cent); the
rest is nickel.
These iron meteorites
are thus very dense—
that is, they weigh quite
a lot for their volume
(see figure).
Types and Sizes of Meteorites
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Most meteorites that hit the Earth are stony in nature. Because
they resemble ordinary rocks (see figure) and disintegrate with
weathering, they are not easily discovered unless their fall is
observed.
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That difference explains
why most meteorites
discovered at random are
made of iron.
But when a fall is observed,
most meteorites recovered
are made of stone.
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Some meteorites are rich
in carbon, and some of
these even have complex
molecules like amino
acids.
Types and Sizes of Meteorites
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A large terrestrial crater that is obviously meteoritic
in origin is the Barringer Meteor Crater in Arizona
(see figure, left).
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It resulted from what was perhaps the most recent
large meteoroid to hit the Earth, for it was formed only
about 50,000 years ago.
Every few years a meteorite is discovered on Earth
immediately after its fall.
The chance of a meteorite landing on someone’s
house or car is very small, but it has happened (see
figure, below)!
Types and Sizes of Meteorites
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Some odd Antarctic meteorites are now known to have come
from the Moon or even from Mars.
Meteorites that have been examined were formed up to 4.6
billion years ago, the beginning of the Solar System.
The relative abundances of the elements in meteorites thus tell
us about the solar nebula from which the Solar System formed.
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In fact, up to the time of the Moon landings, meteorites and cosmic
rays (charged particles from outer space) were the only
extraterrestrial material we could get our hands on.
Meteor Showers
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Meteors sometimes occur in showers, when meteors are
seen at a rate far above average.
Meteor showers are named after the constellation in
which the radiant, the point from which the meteors
appear to come, is located.
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The most widely observed—the Perseids, whose radiant is in
Perseus—takes place each summer around August 12 and
the nights on either side of that date.
The best winter show is the Geminids, which takes place
around December 14 and whose radiant is in Gemini.
Meteor Showers
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On any clear night a naked-eye observer with a dark sky
may see a few sporadic meteors an hour—that is,
meteors that are not part of a shower. (Just try going out
to a field in the country and watching the sky for an hour.)
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During a shower, on the other hand, you may typically see
one every few minutes.
Meteor showers generally result from the Earth’s passing
through the orbits of defunct or disintegrating comets and
hitting the meteoroids left behind. (One meteor shower
comes from an asteroid orbit.)
Meteor Showers
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Leonid meteor shower (whose radiant is in Leo) peaks every 33 years,
when the Earth crosses the main clump of debris from Comet TempelTuttle.
On November 17/18, 1998, one fireball (a
meteor brighter than Venus) was visible each
minute for a while (see figure), and on
November 17/18, 1999 through 2001,
thousands of meteors were seen in the peak
hour.


We will now have to wait until about 2031 for
the next Leonid peak.
The visibility of meteors in a shower depends in large part on how bright
the Moon is; you want as dark a sky as possible.

Meteors are best seen with the naked eye; using a telescope or binoculars
merely restricts your field of view.
Asteroids

The nine known planets were not the only bodies to result from the gas
and dust cloud that collapsed to form the Solar System 4.6 billion years
ago.



Thousands of minor planets, called asteroids, also resulted.
We detect them by their small motions in the sky relative to the stars.
Most of the asteroids have elliptical orbits between the orbits of Mars and
Jupiter, in a zone called the asteroid belt.

It is thought that Jupiter’s gravitational tugs perturbed the orbits of asteroids,
leading to collisions among them that were too violent to form a planet.
Asteroids

Asteroids are assigned a number in order of discovery and
then a name: (1) Ceres, (16) Psyche, and (433) Eros, for
example.


Often the number is omitted when discussing well-known
asteroids.
Though the concept of the asteroid belt may seem to
imply a lot of asteroids close together, asteroids rarely
come within a million kilometers of each other.

Occasionally, collisions do occur, producing the small chips
that make meteoroids.
General Properties of Asteroids


Only about 6 asteroids are larger than 300 km in diameter. Hundreds are
over 100 km across (see figure), roughly the size of some of the moons
of the planets, but most are small, less than 10 km in diameter.
Perhaps 100,000
asteroids could be
detected with Earthbased telescopes;
automated searches
are now discovering
asteroids at a
prodigious rate.

Yet all the asteroids
together contain
less mass than the
Moon.
General Properties of Asteroids

Spacecraft en route to Jupiter and beyond travelled
through the asteroid belt for many months and showed
that the amount of dust among the asteroids is not much
greater than the amount of interplanetary dust in the
vicinity of the Earth.


So the asteroid belt is not a significant hazard for space travel to
the outer parts of the Solar System.
Asteroids are made of different materials from each other,
and represent the chemical compositions of different
regions of space.
General Properties of Asteroids

The differences may be telling us about conditions in the early Solar
System as it was forming and how the conditions varied with distance
from the young Sun.

Many of the asteroids must have broken off from larger, partly
“differentiated” bodies in which dense material sank to the center (as in the
case of the terrestrial planets; see our discussion in Chapter 6).

The path of the Galileo
spacecraft to Jupiter sent it
near the asteroid (951) Gaspra
in 1991 (see figure).

It detected a magnetic field
from Gaspra, which means
that the asteroid is probably
made of metal and is
magnetized.
General Properties of Asteroids


Galileo passed the asteroid (243) Ida in 1993, and discovered
that the asteroid has an even smaller satellite (see figure), which
was then named Dactyl.
Other double asteroids have since been discovered, and
astronomers newly recognize the frequency of such pairs.
Near-Earth Objects

Some asteroids are far from the asteroid belt; their orbits
approach or cross that of Earth.



We have observed only a small fraction of these types of
Near-Earth Objects, bodies that come within 1.3 A.U. of
Earth.
The Near Earth Asteroid Rendezvous (NEAR) mission
passed and photographed the main-belt asteroid (253)
Mathilde in 1997.
The existence of big craters that would have torn a solid
rock apart, and the asteroid’s low density, lead scientists
to conclude that Mathilde is a giant “rubble pile,” rocks
held together by mutual gravity.
Near-Earth Objects


NEAR went into orbit around (433) Eros on Valentine’s Day,
2000 (see figures), when it was renamed NEAR Shoemaker after
the planetary geologist Eugene Shoemaker.
Eros was the first near-Earth asteroid that had been discovered.


It is 33 km by 13 km by 13 km in size.
NEAR Shoemaker photographed craters, grooves, layers, housesized boulders, and a 20-km-long surface ridge.
Near-Earth Objects



The existence of the craters and ridge, which
indicates that Eros must be a solid body, disagrees
with the previous suggestions of some scientists
that most asteroids are mere rubble piles as
Mathilde seems to be.
Eros’s density, 2.4 g /cm3, is comparable to that of
the Earth’s crust, about the same as Ida’s, and
twice Mathilde’s.
From orbit, NEAR Shoemaker’s infrared, x-ray, and
gamma-ray spectrometers measured how the
minerals vary from place to place on Eros’s
surface.

The last of these even survived the spacecraft’s
landing on Eros (see figures), and radioed back
information about the composition of surface rocks.
Near-Earth Objects
Near-Earth asteroids (see figure) may well
be the source of most meteorites, which
could be debris of collisions that occurred
when these asteroids visit the asteroid belt.
Eventually, most Earth-crossing asteroids will
probably collide with the Earth.




Statistics show that there is a 1 per cent chance of a collision of
this tremendous magnitude per millennium.


Over 1000 of them are greater than 1 km in diameter, and none are
known to be larger than 10 km across.
This rate is pretty high on a cosmic scale.
Such collisions would have drastic consequences for life on Earth.
Near-Earth Objects

Smaller objects are a hundred times more common, with
a 1 per cent chance that an asteroid greater than 300 m
in diameter would hit the Earth in the next century.


Such a collision could kill thousands or millions of people,
depending on where it lands.
The question of how much we should worry about NearEarth Objects hitting us is increasingly discussed, including
at a meeting sponsored by the United Nations.

Even Hollywood movies have been devoted to the topic,
though at present we can’t send out astronauts to deflect or
break up the objects the way the movies showed.
Near-Earth Objects



Several projects are under way to find as many Near-Earth
Objects as possible.
Current plans are to map 90 per cent of them in the next
couple of decades, and the pace of discovery is
accelerating.
Several projects use CCD detectors, repetitive scanning,
and computers to locate asteroids and are discovering
thousands each year, some of which are Near-Earth
Objects.