Transcript Document

Chapter 8
Pluto, Comets, and
Space Debris
Introduction



We have learned about the Solar
System’s giant planets, which
range in size from about 4 to
about 11 times the diameter of
the Earth.
We have seen that our Solar
System has a set of terrestrial
planets, which range in size from
the Earth down to 40 per cent the
diameter of the Earth.
This size range includes the four
inner planets as well as seven
planetary satellites.
Introduction



The remaining object that has long had the name
“planet,” Pluto, is only 20 per cent the diameter of
Earth but is still over 2300 km across, so there is
much room on it for interesting surface features.
Recently, additional objects like it, but smaller, have
been found in the outer reaches of the Solar
System.
We shall see how we determined Pluto’s odd
properties, and what the other, similar objects are.
Introduction





Besides the planets and their moons, many
other objects are in the family of the Sun.
The most spectacular, as seen from Earth,
are comets (see figure).
Bright comets have been noted throughout
history, instilling great awe of the heavens.
Comets have long been seen as omens,
usually bad ones.
As Shakespeare wrote in Julius Caesar,
“When beggars die, there are no comets
seen; The heavens themselves blaze forth
the death of princes.”
Introduction






Asteroids, which are minor planets, and chunks of rock known
as meteoroids, are other residents of our Solar System.
We shall see how they and the comets are storehouses of
information about the Solar System’s origin.
Asteroids, meteoroids, and comets are suddenly in the news as
astronomers are finding out that some come relatively close to
the Earth.
We are realizing more and more that collisions of these objects
with the Earth can be devastating for life on Earth.
Every few hundred thousand years, one large enough to do very
serious damage should hit, and every few tens of millions of
years, an enormous collision can produce a mass extinction of
life on Earth.
Apparently, a comet or an asteroid caused the dinosaurs to
become extinct some 65 million years ago.
Introduction



Should we be worrying about asteroid,
meteoroid, or comet collisions?
Should we be monitoring the sky around
us better?
Should we be planning ways of diverting
an oncoming object if we were to find
one?
8.1 Pluto

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 (the “ecliptic” plane, defined in Chapter 4),
near which the other planets revolve.



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.


Pluto’s elliptical orbit is so eccentric that part lies inside the
orbit of Neptune.
It hasn’t been as bright for over 200 years.
It is barely visible through a medium-sized telescope
under dark-sky conditions.
8.1 Pluto


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).


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.
8.1a Pluto’s Mass and Size


Even such basics as the mass and diameter of Pluto are
very difficult to determine.
It had been hard to deduce the mass of Pluto because to
do so was, at first, thought to require measuring Pluto’s
effect on Uranus, a far more massive body. (The orbit of
Neptune, known for less than a hundred years at the time
Pluto was discovered, was too poorly known to be of
much use.)


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.
8.1a Pluto’s Mass and Size


The situation changed drastically in 1978 with
the surprise discovery (see figure) that Pluto has
a satellite.
The moon was named Charon, after the
boatman who rowed passengers across the River
Styx to the realm of Pluto, god of the underworld
in Greek mythology. (Its name is informally
pronounced “Shar´on,” similarly to the name of
the discoverer’s wife, Charlene, by astronomers
working in the field.)


The presence of a satellite allows us to deduce the
mass of the planet by applying Newton’s form of
Kepler’s third law (Chapter 5).
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.
8.1a Pluto’s Mass and Size



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.


Such mutual occultations were the case from 1985 through
1990.
When we measured their apparent brightness, we
received light from both Pluto and Charon together (they
are so close together that they appeared as a single point
in the sky).

Their blocking each other led to dips in the total brightness
we received.
8.1a Pluto’s Mass and Size


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.


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.

So Pluto/Charon is almost a “double-planet”
system.
8.1a Pluto’s Mass and Size



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.
8.1a Pluto’s Mass and Size


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).
8.1a Pluto’s Mass and Size

But we don’t know whether the bright areas are bright
because they are high clouds near mountains or low haze
and frost.


We merely know that there are extreme contrasts on Pluto’s
surface.
If we were standing on Pluto, the Sun would appear over
a thousand times fainter than it does to us on Earth.

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.
8.1b Pluto’s Atmosphere



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.


The observations showed that the starlight diminished
gradually and unevenly.


Any atmosphere would make the starlight diminish more
gradually.
Thus Pluto’s atmosphere has layers in it.
Another such occultation wasn’t observed until 2002,
when (again) Pluto was seen to make the star wink out
for a minute or so on two separate occasions.
8.1b Pluto’s Atmosphere

From the 1988 occultation, astronomers were also able to
conclude that the bulk of Pluto’s atmosphere is nitrogen.



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.
The data from the first occultation seemed to show a
change at a certain height in Pluto’s atmosphere, leading
to the deduction that either the atmosphere had a
temperature inversion or that the lower atmosphere
contained a lot of dust.

The lone high-quality scan obtained in July 2002 showed no
such change at a certain height in the rate at which the
star’s light was dimming as it passed through Pluto’s
atmosphere.
8.1b Pluto’s Atmosphere

Then, in August 2002, a group of scientists, of which one of the authors
(J.M.P.) was a member, succeeded in observing an occultation of a star by
Pluto on ten different telescopes, several of them on Mauna Kea (see
figures).


J.M.P.’s team from
Williams College
obtained a thousand
data points in a 5-minute
interval of the
occultation, part of a 20minute data run.
Further work in 2005 on a
similar occultation of a
star but this time by
Pluto’s moon, Charon,
gave the MIT-Williams
College consortium
success on all but one of
the five telescopes in
South America they used.
8.1b Pluto’s Atmosphere


Our Pluto results showed an expansion of its atmosphere,
which would result from a global warming since 1988.
Perhaps some contribution to that warming comes from
the changing orientation of Pluto’s darker spots with
respect to incoming solar radiation.


We also saw some bright spikes in the light curve, which
could be signs of waves or turbulence in Pluto’s atmosphere.
Further, observations from several telescopes showed that
Pluto’s atmosphere is not quite round, undoubtedly
resulting from strong winds.

Our Charon results pinned down its size, and therefore
density, better than ever before, but even the high-timeresolution observations did not show an atmosphere.
8.1b Pluto’s Atmosphere

As Pluto goes farther from the Sun, as it is now doing, its
atmosphere is generally predicted to freeze out and snow
onto the surface.


Though some calculations indicate that this might not be so,
it is still possible that if we want to find out about the
atmosphere, we had better get a spacecraft there within a
decade or two, or we’ll have to wait another 200 years for
the atmosphere to form again.
NASA’s New Horizons mission, after a period of on-again,
off-again for funding reasons, is a small satellite that at
the time of this writing is scheduled to be launched in
2006 and to reach Pluto a decade later.

Its investigators used Hubble to find two additional, small
(under 100-km) moons of Pluto.
8.1c What Is Pluto?

From Pluto’s mass and radius, we calculate its density.


Since ices have even lower densities than Pluto, Pluto must be
made of a mixture of ices and rock.



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.
Ironically, now that we know Pluto’s mass, we calculate that it is
far too small to cause the deviations in Uranus’s orbit that
originally led to Pluto’s discovery.
The discrepancy probably wasn’t real:


It turns out to be about 2 g/cm3, twice the density of water and
less than half the density of Earth.
The wrong mass had been assumed for Neptune when predicting
the orbit of Uranus.
The discovery of Pluto was purely the reward of Clyde
Tombaugh’s hard work in conducting a thorough search in a
zone of the sky near the ecliptic.
8.1c What Is Pluto?

Pluto, with its moon and its atmosphere, has some similarities
to the more familiar planets.


Increasingly, Pluto is being identified with a newly discovered
set of objects in the outer Solar System, which we will now
study.




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.
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.”
As we will see in the next section, another such object even
bigger than Pluto turned up in 2005.
Should both be called planets, leaving the possibility that we
may soon know of even more?


Or should Pluto be demoted to asteroid or the mere status of a
Trans-Neptunian Object?
As of this writing, the matter is undecided.
8.2 Kuiper-belt Objects



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.
8.2 Kuiper-belt Objects



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.
8.2 Kuiper-belt Objects

They are generally very dark, with albedoes of only about 4 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.


Pluto, by contrast, has an albedo of about 60 per cent.
The newer one, tentatively and unofficially named Quaoar (pronounced
“kwa-whar”) after the Indian tribe that inhabited today’s Los Angeles, was
even imaged with the Hubble Space Telescope, so we have a firmer grasp
of its diameter, 1300 km, slightly over half that of Pluto.
The size, in turn, gives us the albedo (12 per cent), which is larger than
had been assumed for Kuiper-belt objects.
8.2 Kuiper-belt Objects

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.


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, as this book was going to press, with their discovery of an outersolar-system object even larger than Pluto (see figures).

Initially named 2003 UB313, it was first sighted in 2003 but not confirmed
until 2005.
8.2 Kuiper-belt Objects

The object is now 97 A.U. out from the Sun, more than
twice as far out as Pluto.




Undoubtedly, it was thrown into that highly inclined orbit
after a close gravitational encounter with Neptune.
Is it a 10th planet?


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.
That is really a matter of semantics, but words can count.
Keep in touch with this book’s website or with other
sources to find out the latest on it.
8.2 Kuiper-belt Objects


A few objects may once have been Kuiper-belt objects but now
come somewhat closer to the Sun, crossing the orbits of the
outer planets.
About 100 of these “centaur” objects a few hundred kilometers
across may exist.




Since they are larger and come closer to the Earth and Sun than
most Kuiper-belt objects, we can study them better.
On at least one, a coma (typical of comets, as we will soon see)
was seen, so these centaurs are intermediate between comets
and asteroids.
NASA’s New Horizons mission is to go to some Kuiper-belt
objects after it visits Pluto.
Our MIT-Williams consortium certainly hopes to pick up an
occultation of a star by one or more of these Kuiper-belt
objects, which would accurately determine its diameter and
albedo.
8.3 Comets


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.


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.

With binoculars or a telescope, however, an observer
can accurately note the position of the comet’s head
and after a few hours can detect that the comet is
moving at a slightly different rate from the stars.
8.3 Comets


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,
although it can often be followed
for additional months with
binoculars and then for additional
months with telescopes.
8.3 Comets

Most comets are much fainter than the one we have just
described.



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.
Hundreds of comets that go very close to the Sun or even
hit it, destroying themselves, have been discovered by
(and named after) the Solar and Heliospheric Observatory
(SOHO) spacecraft, since it can uniquely monitor a region
of space too close to the Sun to be seen from Earth given
our daytime blue skies.
8.3a The Composition of Comets


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, advanced in 1950 by Fred L. Whipple of the
Harvard and Smithsonian Observatories, is that the
nucleus is like a “dirty snowball.”

It may be made of ices of such molecules as water (H2O),
carbon dioxide (CO2), ammonia (NH3), and methane (CH4),
with dust mixed in.
8.3a The Composition of Comets



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).

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.
8.3a The Composition of Comets


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.


The dust tail is caused by dust particles released from the ices of the
nucleus when they are vaporized.


Many comets actually have two tails (■ Fig. 8 –11).
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.
8.3a The Composition of Comets

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 (see our discussion in Chapter 10).


As puffs of gas are blown out and as the solar wind varies,
the gas tail takes on a structured appearance.


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.

It has jokingly been said that comets are as close as
something can come to being nothing.
8.3b The Origin and Evolution of Comets

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.


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.

In current models, most of the Oort cloud’s mass is in the
inner 1000 to 10,000 A.U.
8.3b The Origin and Evolution of Comets

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.



In the early years of the Oort model, it was thought that
sometimes the gravity of a nearby star tugged an incipient
comet out of place.
In any case, the comet generally gets directly ejected from
the Solar System, but in some cases the comet can
approach the Sun.
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.
8.3b The Origin and Evolution of Comets



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.


Perhaps the discrepancy has to do with the way comets die.
New calculations show that since so few dormant comets are
found, the comets must mainly break up and disappear.

Maybe Oort-cloud comets, coming from so far out in the Solar
System, change temperature regimes so much more quickly than
Kuiper-belt comets that they are preferentially disrupted.
8.3b The Origin and Evolution of Comets

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).


Part of the reason for this division was merely that we had observed
comet orbits reliably for only about 200 years.
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.
8.3b The Origin and Evolution of Comets

This radius corresponds to the peak of the Oort
cloud, and comets from there are considered
“new.”


However, once comets are dislodged from the Oort
cloud and come into the inner Solar System, the
semimajor axes of the orbits of these “returning”
comets are reduced.
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.
8.3b The Origin and Evolution of Comets


A new comet classification basically depends on
the influence of Jupiter.
One of the two major classes consists of those that
come from all directions.


Comets in the other major class are called
“ecliptic,” since the comet orbits are aligned close
to the plane of the Solar System, the ecliptic plane
(see figure), rather than being highly tilted.


Almost all of these come from the Oort cloud.
Almost all of these ecliptic comets come from the
Kuiper belt.
In the new scheme, fewer comets change their
classifications over time.

Notice that comets on highly eccentric orbits spend
most of their time far away from the Sun, an
excellent example of Kepler’s second law (Chapter
5).
8.3b The Origin and Evolution of Comets

As a comet gets closer to the Sun than those distant regions, the
solar radiation begins to vaporize the ice in the nucleus.



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.

But some comets hit the Sun and are
destroyed (see figure).
8.3b The Origin and Evolution of Comets

We shall see in the following section that meteoroids
can be left in the orbit of a disintegrated comet.


Some of the asteroids, particularly those that cross
the Earth’s orbit, may be dead comet nuclei.
In recent years, a handful of asteroids—notably
Chiron in the outer Solar System—have shown
comas or tails, making them comets; conversely, a
few comets have died out and seem like asteroids.

So we may have misidentified some of each in the
past.
8.3b The Origin and Evolution of Comets



How did comets get where they are?
We will say more about the formation of the Solar System
in Chapter 9.
There, we will see that there were many small particles
that clumped together in the early eras.



Some of these clumps interacted gravitationally with other
clumps and even with Jupiter and other planets as they
were formed.
Many of these clumps were ejected from the region of
their formation, often where the asteroid belt now is
between Mars and Jupiter, and wound up forming the Oort
comet cloud.
Other clumps were already beyond the orbit of Neptune,
where fewer interactions took place.

Those clumps formed the Kuiper belt.
8.3b The Origin and Evolution of Comets

Because new comets come from the places in the
Solar System that are farthest from the Sun and thus
coldest, they probably contain matter that is
unchanged since the formation of the Solar System.


So the study of comets is important for understanding
the birth of the Solar System.
Moreover, some astronomers have concluded that
early in Earth’s history, the oceans formed when an
onslaught of water-bearing comets collided with
Earth, although this view is still controversial.
8.3c Halley’s Comet

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.


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.
8.3c Halley’s Comet


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).


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.
8.3c Halley’s Comet

It seems probable that the bright comets reported every
74 to 79 years since 240 b.c. were earlier appearances of
Halley’s Comet.


Halley’s Comet came especially close to the Earth during
its 1910 return, and the Earth actually passed through its
tail.


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.
8.3c Halley’s Comet

The most recent close approach of Halley’s Comet was in 1986.


Since we knew long in advance that the comet would be
available for viewing, special observations were planned for
optical, infrared, and radio telescopes.


It was not as spectacular from the ground in 1986 as it was in
1910, for this time the Earth and comet were on opposite sides of
the Sun when the comet was brightest.
For example, spectroscopy showed many previously undetected
ions in the coma and tail.
When Halley’s Comet passed through the plane of the Earth’s
orbit, it was met by an armada of spacecraft.


The best was the European Space Agency’s spacecraft Giotto
(named after the 14th-century Italian artist who included Halley’s
Comet in a painting), which went right up close to Halley.
Giotto’s several instruments also studied Halley’s gas, dust, and
magnetic field from as close as 600 km from the nucleus.
8.3c Halley’s Comet


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.
8.3c Halley’s Comet

The “dirty snowball” theory of comets was confirmed in
general, but the snowball is darker than expected.


Further, the evaporating gas and dust is localized into jets
that are stronger than expected.



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.
Giotto carried 10 instruments in addition to its camera.

Among them were mass spectrometers to measure the types
of particles present, detectors for dust, equipment to listen
for radio signals that revealed the densities of gas and dust
in the coma, detectors for ions, and a magnetometer to
measure the magnetic field.
8.3c Halley’s Comet



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.
8.3c Halley’s Comet




Many valuable observations were also obtained from the Earth.
For example, radio telescopes were used to study molecules.
Water vapor is the most prevalent gas, but carbon monoxide
and carbon dioxide were also detected.
The comet was bright enough that many telescopes obtained
spectra (see figure).
8.3c Halley’s Comet

The next appearance of Halley’s Comet, in 2061, again
won’t be spectacular.



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.

Some bright comets are at their best for only a few days or
a week.
8.3d Comet
[Shoemaker-]Shoemaker-Levy 9


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. (The
authors of this book like to give each
Shoemaker individual credit for the discovery,
as in the chapter subheading, though the
comet is generally and formally called
Shoemaker-Levy 9.)

This comet looked weird—it seemed squashed.
8.3d Comet
[Shoemaker-]Shoemaker-Levy 9

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.


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.
8.3d Comet
[Shoemaker-]Shoemaker-Levy 9


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.


When they could view Jupiter’s surface, they saw a dark ring
(see figure on next slide).


Even before then, scientists were enthralled by a plume rising
above Jupiter’s edge.
Infrared telescopes detected a tremendous amount of radiation
from the heated gas.
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.

The largest dark spots could be seen for a few months even with
small backyard telescopes. (On one of the April 2005 solar eclipse
cruises, David Levy sometimes wore a T-shirt that said “My comet
crashed.”)
8.3d Comet
[Shoemaker-]Shoemaker-Levy 9
8.3d Comet
[Shoemaker-]Shoemaker-Levy 9




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.
Had any of the fragments hit Earth, they would have
made a crater as large as Rhode Island, with dust thrown
up to much greater distances.


Had the entire comet (whose nucleus was 10 km across) hit
Earth at one time, much of life could have been destroyed.
So Comet Shoemaker-Levy 9 made us even more wary
about what may be coming at us from space.
8.3e Recently Observed Comets


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.

It lived up to its advance
billing (see figure).
8.3e Recently Observed Comets



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.
8.3f Spacecraft to Comets

NASA’s Deep Space 1 mission flew close to Comet 19P/Borrelly in 2001.


This comet’s surface, and therefore probably the surfaces of comet nuclei
in general, was rougher and more dramatic than expected.


It obtained more detailed images of the bowling-pin-shaped nucleus (see
figure) than even Giotto’s views of Halley’s nucleus.
Deep Space 1 found smooth, rolling plains that seem to be the source of the
dust jets, which are more concentrated than Halley’s.
Darkened material, perhaps extruded from underneath, covers some
regions and accentuates grooves and faults.

Borrelly’s albedo in these
places is less than 1 per
cent, while Borrelly’s
overall albedo is only 4 per
cent.
8.3f Spacecraft to Comets

Borrelly is thought to have originated in the Kuiper belt, in
contrast to Halley’s Comet’s origin in the Oort cloud.


Still, compared with Halley, Borrelly gives off relatively
little water, perhaps because so much of its surface is
inactive.


This difference would explain why Halley’s Comet gives off
many carbon compounds while Borrelly gives off more water
and ammonia than carbon.
Scientists have yet to explain why the solar wind is deflected
around Borrelly’s nucleus in an asymmetric fashion.
The center of the plasma in Borrelly’s coma is some 2000
km off to the side, as strange as if a supersonic jet’s shock
wave were displaced far to the airplane’s side.
8.3f Spacecraft to Comets

NASA’s Stardust mission, launched in 1999, went to Comet
Wild 2 (pronounced Vilt-too), a periodic comet with a sixyear orbit.


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
(see figure), and flew through the comet with the aerogel
exposed so that the comet dust could stick in it.

Stardust’s orbit will bring it back near Earth in January 2006,
when it will parachute the aerogel down to the Utah desert.
(A parachute that didn’t open in a 2004 mission to gather
solar wind particles, Genesis, makes everybody worried.)
8.3f Spacecraft to Comets

A major European Space Agency spacecraft, Rosetta, was
launched in 2004 to orbit with a comet for some years and to
land a probe on the comet’s nucleus in 2014.


It will use three gravity assists from Earth and one from Mars to
reach the comet, passing asteroids (2867) Steins in 2008 and
(21) Lutetia in 2010, both in the asteroid belt, on the way.
(Asteroids are discussed in Section 8.5.)


It is heading for Comet 67P/Churyumov-Gerasimenko.
Rosetta will drop a lander, Philae, onto the comet’s nucleus.
Just as the Rosetta Stone, now in the British Museum, enabled
Egyptian hieroglyphics to be deciphered by having the same
text in three scripts (hieroglyphics, Demotic, and Greek),
scientists hope that the Rosetta spacecraft will prove to be the
key to deciphering comets. (Philae was an island in the Nile on
which an obelisk was found that helped to decipher the
hieroglyphics of the Rosetta Stone.)
8.3f Spacecraft to Comets

Rosetta is to orbit the comet at an altitude of only a few kilometers,
mapping its surface and making other measurements, for 18 months,
including the comet’s closest approach to the Sun and therefore, it is
hoped, its increasing activity.



The lander is to work for some weeks, taking photographs and drilling into
the surface.
NASA’s Deep Impact spacecraft crashed a 370-kg projectile into Comet
Tempel 1 in 2005.
The remainder of the spacecraft studied the
impact, which should have formed a footballfield-sized crater some 7 stories deep.

Astronomers were at telescopes all around the
Earth, and were using telescopes in space like
Hubble, to record the impact (see figure).
8.4 Meteoroids

There are many small chunks of matter orbiting in the Solar System,
ranging up to tens of meters across and sometimes even larger.


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.

Such events result in streaks of light in the sky (see figure), which we call
meteors (popularly, and incorrectly, known as shooting stars or falling
stars).

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.

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.
8.4a Types and Sizes of Meteorites

Space is full of meteoroids of all sizes, with the smallest
being most abundant.




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 (what
little there is) 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.

They are often sufficiently slowed down by Earth’s
atmosphere to avoid being vaporized before they reach the
ground.
8.4a Types and Sizes of Meteorites


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).
8.4a Types and Sizes of Meteorites

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.


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.

Some meteorites are rich
in carbon, and some of
these even have complex
molecules like amino
acids.
8.4a Types and Sizes of Meteorites

A large terrestrial crater that is obviously meteoritic
in origin is the Barringer Meteor Crater in Arizona
(see figure, left).



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)!
8.4a Types and Sizes of Meteorites

Often the positions in the sky of extremely bright meteors
are tracked in the hope of finding fresh meteorite falls.


The newly discovered meteorites are rushed to laboratories
in order to find out how long they have been in space by
studying their radioactive elements.
Over 10,000 meteorites have been found in the Antarctic,
where they have been well preserved as they accumulated
over the years.

Though the Antarctic ice sheets flow, the ice becomes
stagnant in some places and disappears, revealing
meteorites that had been trapped for over 10,000 years.
8.4a Types and Sizes of Meteorites


Some odd Antarctic meteorites are now known to have come
from the Moon or even from Mars.
Recall that in Chapter 6 we even discussed controversial
evidence for ancient primitive life-forms on Mars, found in one
such meteorite.




As of mid-2005, the conclusion hasn’t been entirely ruled out, but
few scientists accept it.
As the late Carl Sagan said, “Extraordinary claims require
extraordinary evidence,” and the evidence from this meteorite is
not convincing, at least not yet.
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.

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.
8.4b Meteor Showers


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.


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.
8.4b Meteor Showers

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.)


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.)
8.4b Meteor Showers


Though the Perseids and Geminids can be counted on each year, the
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.
8.5 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
(see figure).
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.
8.5 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.
8.5a 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.
8.5a 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.




The asteroids at the inner edge of the asteroid belt are mostly stony in
nature, while the ones at the outer edge are darker (because they
contain more carbon).
Most of the small asteroids that pass near the Earth belong to the stony
group.
Three of the largest asteroids belong to the high-carbon group.
A third group is mostly composed of iron and nickel.
8.5a 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.
8.5a 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.

For example, ground-based
astronomers found a 13-km
satellite orbiting 200-kmdiameter (45) Eugenia every
five days. (Note that
Eugenia’s low number shows
that it was one of the first
asteroids discovered.)
8.5b 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.
8.5b 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.
8.5b 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.



The impact that formed the largest crater, 8 km
across and now named Shoemaker, is thought to
have formed most of the large boulders found across
Eros’s surface.
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.
8.5b Near-Earth Objects

Scientists analyzing the data have found abundances of
elements similar to that of the Sun and of a type of
primitive meteorite known as chondrites that are the most
common type of meteorite found on Earth.


NEAR Shoemaker’s observations show that Eros was
probably broken off billions of years ago from a larger
asteroid as a uniformly dense fragment.


They have concluded that Eros is made of primitive material,
unchanged for 4.5 billion years, so we are studying the early
eras of the Solar System with it.
This solidity contrasts with Mathilde’s rubble-pile nature.
Besides providing much detailed information, the close-up
studies of these objects are allowing us to verify whether
the lines of reasoning we use with ground-based asteroid
observations give correct results.
8.5b 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.
8.5b 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.
8.5b 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.