Pluto, Comets, and Space Debris

1
Chapter 8

• Pluto, Comets, and Space Debris

2
Introduction

• We have learned about the Solar Systems 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.

3
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 Plutos odd
  properties, and what the other, similar objects
  are.

4
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.

5
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
  Systems 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.

6
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?

7
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 Earths orbital plane (the
  ecliptic plane, defined in Chapter 4), near
  which the other planets revolve.
• Plutos 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.
• It hasnt been as bright for over 200 years.
• It is barely visible through a medium-sized
  telescope under dark-sky conditions.

8
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.

9
8.1a Plutos 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 Plutos 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.

10
8.1a Plutos 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
  Sharon, similarly to the name of the
  discoverers wife, Charlene, by astronomers
  working in the field.)
• The presence of a satellite allows us to deduce
  the mass of the planet by applying Newtons form
  of Keplers third law (Chapter 5).
• Charon is 5 to 10 per cent of Plutos 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.

11
8.1a Plutos Mass and Size

• Plutos 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 Plutos 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.

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8.1a Plutos 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.

13
8.1a Plutos 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.

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8.1a Plutos 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).

15
8.1a Plutos Mass and Size

• But we dont 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 Plutos 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.

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8.1b Plutos Atmosphere

• Pluto occultedpassed in front of and hida star
  on one night in 1988.
• Astronomers observed this occultation to learn
  about Plutos atmosphere.
• If Pluto had no atmosphere, the starlight would
  wink out abruptly.
• Any atmosphere would make the starlight diminish
  more gradually.
• The observations showed that the starlight
  diminished gradually and unevenly.
• Thus Plutos atmosphere has layers in it.
• Another such occultation wasnt observed until
  2002, when (again) Pluto was seen to make the
  star wink out for a minute or so on two separate
  occasions.

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8.1b Plutos Atmosphere

• From the 1988 occultation, astronomers were also
  able to conclude that the bulk of Plutos
  atmosphere is nitrogen.
• A trace of methane must also be present, since
  the methane ice on Plutos surface, detected from
  its spectrum, must be evaporating.
• Still, Plutos atmospheric pressure is very low,
  only 1/100,000 of Earths.
• The data from the first occultation seemed to
  show a change at a certain height in Plutos
  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 stars light was dimming as it
  passed through Plutos atmosphere.

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8.1b Plutos 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 20-minute data run.
• Further work in 2005 on a similar occultation of
  a star but this time by Plutos moon, Charon,
  gave the MIT-Williams College consortium success
  on all but one of the five telescopes in South
  America they used.

19
8.1b Plutos 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 Plutos 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 Plutos atmosphere.
• Further, observations from several telescopes
  showed that Plutos 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-time-resolution observations did
  not show an atmosphere.

20
8.1b Plutos 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
  well have to wait another 200 years for the
  atmosphere to form again.
• NASAs 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.

21
8.1c What Is Pluto?

• From Plutos mass and radius, we calculate its
  density.
• 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.
  
• Its composition is more similar to that of the
  satellites of the giant planets, especially
  Neptunes large moon Triton, than to that of
  Earth or the other inner planets.
• Ironically, now that we know Plutos mass, we
  calculate that it is far too small to cause the
  deviations in Uranuss orbit that originally led
  to Plutos discovery.
• The discrepancy probably wasnt real
• The wrong mass had been assumed for Neptune when
  predicting the orbit of Uranus.
• The discovery of Pluto was purely the reward of
  Clyde Tombaughs hard work in conducting a
  thorough search in a zone of the sky near the
  ecliptic.

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8.1c What Is Pluto?

• Pluto, with its moon and its atmosphere, has some
  similarities to the more familiar planets.
• 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.
• 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.

23
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 koyper) 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
  Kuiper-belt objects, or, less often,
  Trans-Neptunian Objects.

24
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.

25
8.2 Kuiper-belt Objects

• They are generally very dark, with albedoes of
  only about 4 per cent.
• 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 Plutos moon
  Charon was found in 2001, about half of Plutos
  diameter.
• One that may be even somewhat larger was found in
  2002, though the uncertainty limits of these two
  Kuiper-belt objects overlap.
• The newer one, tentatively and unofficially named
  Quaoar (pronounced kwa-whar) after the Indian
  tribe that inhabited todays 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.

26
8.2 Kuiper-belt Objects

• David Jewitt of the University of Hawaii and Jane
  Luu, now at MITs 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
  outer-solar-system object even larger than Pluto
  (see figures).
• Initially named 2003 UB313, it was first sighted
  in 2003 but not confirmed until 2005.

27
8.2 Kuiper-belt Objects

• The object is now 97 A.U. out from the Sun, more
  than twice as far out as Pluto.
• 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?
• That is really a matter of semantics, but words
  can count.
• Keep in touch with this books website or with
  other sources to find out the latest on it.

28
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.
• NASAs 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.

29
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
  comets head and after a few hours can detect
  that the comet is moving at a slightly different
  rate from the stars.

30
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.

31
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.

32
8.3a The Composition of Comets

• At the center of a comets 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.

33
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
  cohma), 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.

34
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
  smallthe tail is a much better vacuum than we
  can make in laboratories on Earth.
• Many comets actually have two tails ( Fig. 8
  11).
• The dust tail is caused by dust particles
  released from the ices of the nucleus when they
  are vaporized.
• The dust particles are left behind in the comets
  orbit, blown slightly away from the Sun by the
  pressure of sunlight hitting the particles.

• As a result of the comets orbital motion, the
  dust tail usually curves smoothly behind the
  comet.

35
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).
• It follows the interplanetary magnetic field.
• As puffs of gas are blown out and as the solar
  wind varies, the gas tail takes on a structured
  appearance.
• Each puff of matter can be seen.
• A comethead and tail togethercontains 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.

36
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 Plutos 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 clouds mass
  is in the inner 1000 to 10,000 A.U.

37
8.3b The Origin and Evolution of Comets

• Occasionally one of these comets leaves the comet
  cloud.
• 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.
• Currently, astronomers tend to think that gravity
  from the disk of our Milky Way Galaxy does most
  of the tugging.
• In any case, the comet generally gets directly
  ejected from the Solar System, but in some cases
  the comet can approach the Sun.
• The comets 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.

38
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
  (Earths 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.

39
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 Plutos orbit.

40
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.

41
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.
• Almost all of these come from the Oort cloud.
• 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 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 Keplers second law
  (Chapter 5).

42
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).

43
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 Earths orbit, may be dead comet
  nuclei.
• In recent years, a handful of asteroidsnotably
  Chiron in the outer Solar Systemhave 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.

44
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.

45
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 Earths history, the oceans formed when
  an onslaught of water-bearing comets collided
  with Earth, although this view is still
  controversial.

46
8.3c Halleys Comet

• In 1705, the English astronomer Edmond Halley
  (Halley is pronounced to rhyme with Sally, and
  not with saylee) (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 Newtons law of gravity from
  interactions with planets.

47
8.3c Halleys Comet

• Halley predicted that this bright comet would
  again return in 1758.
• Its reappearance on Christmas night of that year,
  16 years after Halleys death, was the proof of
  Halleys hypothesis (and Newtons method).
• The comet has thereafter been known as Halleys
  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.

48
8.3c Halleys Comet

• It seems probable that the bright comets reported
  every 74 to 79 years since 240 b.c. were earlier
  appearances of Halleys Comet.
• The fact that it has been observed dozens of
  times endorses the calculations that show that
  less than 1 per cent of a cometary nucleuss mass
  is lost at each passage near the Sun.
• Halleys Comet came especially close to the Earth
  during its 1910 return, and the Earth actually
  passed through its tail.
• 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.

49
8.3c Halleys Comet

• The most recent close approach of Halleys Comet
  was in 1986.
• 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.
• Since we knew long in advance that the comet
  would be available for viewing, special
  observations were planned for optical, infrared,
  and radio telescopes.
• For example, spectroscopy showed many previously
  undetected ions in the coma and tail.
• When Halleys Comet passed through the plane of
  the Earths orbit, it was met by an armada of
  spacecraft.
• The best was the European Space Agencys
  spacecraft Giotto (named after the 14th-century
  Italian artist who included Halleys Comet in a
  painting), which went right up close to Halley.
• Giottos several instruments also studied
  Halleys gas, dust, and magnetic field from as
  close as 600 km from the nucleus.

50
8.3c Halleys 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.

51
8.3c Halleys Comet

• The dirty snowball theory of comets was
  confirmed in general, but the snowball is darker
  than expected.
• It is as black as velvet, with an albedo of only
  about 3 per cent.
• Further, the evaporating gas and dust is
  localized into jets that are stronger than
  expected.
• 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.

52
8.3c Halleys Comet

• About 30 per cent of Halleys 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.

53
8.3c Halleys 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).

54
8.3c Halleys Comet

• The next appearance of Halleys Comet, in 2061,
  again wont be spectacular.
• Not until the one after that, in 2134, will the
  comet show a long tail to earthbound observers.
• Fortunately, though Halleys 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, dont wait to see it another time.
  
• Some bright comets are at their best for only a
  few days or a week.

55
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 weirdit seemed squashed.

56
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 Jupiters tidal forces.

57
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.
• Even before then, scientists were enthralled by a
  plume rising above Jupiters edge.
• When they could view Jupiters surface, they saw
  a dark ring (see figure on next slide).
• 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.)

58
8.3d Comet Shoemaker-Shoemaker-Levy 9
59
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
  Jupiters atmosphere than we normally see.
• The biggest comet chunk released the equivalent
  of 6 million megatons of TNT100,000 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.

60
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).

61
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.

62
8.3f Spacecraft to Comets

• NASAs Deep Space 1 mission flew close to Comet
  19P/Borrelly in 2001.
• It obtained more detailed images of the
  bowling-pin-shaped nucleus (see figure) than even
  Giottos views of Halleys nucleus.
• This comets surface, and therefore probably the
  surfaces of comet nuclei in general, was rougher
  and more dramatic than expected.
• Deep Space 1 found smooth, rolling plains that
  seem to be the source of the dust jets, which are
  more concentrated than Halleys.
• Darkened material, perhaps extruded from
  underneath, covers some regions and accentuates
  grooves and faults.

• Borrellys albedo in these places is less than 1
  per cent, while Borrellys overall albedo is only
  4 per cent.

63
8.3f Spacecraft to Comets

• Borrelly is thought to have originated in the
  Kuiper belt, in contrast to Halleys Comets
  origin in the Oort cloud.
• This difference would explain why Halleys Comet
  gives off many carbon compounds while Borrelly
  gives off more water and ammonia than carbon.
• Still, compared with Halley, Borrelly gives off
  relatively little water, perhaps because so much
  of its surface is inactive.
• Scientists have yet to explain why the solar wind
  is deflected around Borrellys nucleus in an
  asymmetric fashion.
• The center of the plasma in Borrellys coma is
  some 2000 km off to the side, as strange as if a
  supersonic jets shock wave were displaced far to
  the airplanes side.

64
8.3f Spacecraft to Comets

• NASAs Stardust mission, launched in 1999, went
  to Comet Wild 2 (pronounced Vilt-too), a periodic
  comet with a six-year 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.
• Stardusts orbit will bring it back near Earth in
  January 2006, when it will parachute the aerogel
  down to the Utah desert. (A parachute that didnt
  open in a 2004 mission to gather solar wind
  particles, Genesis, makes everybody worried.)

65
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
  comets nucleus in 2014.
• It is heading for Comet 67P/Churyumov-Gerasimenko.
  
• 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.)
• Rosetta will drop a lander, Philae, onto the
  comets 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.)

66
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 comets 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.
• NASAs 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
  football-field-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).

67
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 Earths atmosphere, friction
  and the compression of air in front of it heat it
  upusually at a height of about 100 kmuntil 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 Earths 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 Earths surface
  each year.

68
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 Earths 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
  Earths atmosphere to avoid being vaporized
  before they reach the ground.

69
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 densethat
  is, they weigh quite a lot for their volume (see
  figure).

70
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.

71
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 someones
  house or car is very small, but it has happened
  (see figure, below)!

72
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.

73
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 hasnt 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.

74
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 observedthe Perseids, whose
  radiant is in Perseustakes 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.

75
8.4b Meteor Showers

• On any clear night a naked-eye observer with a
  dark sky may see a few sporadic meteors an
  hourthat 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 Earths
  passing through the orbits of defunct or
  disintegrating comets and hitting the meteoroids
  left behind. (One meteor shower comes from an
  asteroid orbit.)

76
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
  Tempel-Tuttle.

• 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.

77
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 Jupiters gravitational tugs
  perturbed the orbits of asteroids, leading to
  collisions among them that were too violent to
  form a planet.

78
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.

79
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
  Earth-based telescopes automated searches are
  now discovering asteroids at a prodigious rate.
• Yet all the asteroids together contain less mass
  than the Moon.

80
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.

81
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.

82
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-km-diameter (45)
  Eugenia every five days. (Note that Eugenias low
  number shows that it was one of the first
  asteroids discovered.)

83
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 asteroids low
  density, lead scientists to conclude that
  Mathilde is a giant rubble pile, rocks held
  together by mutual gravity.

84
8.5b Near-Earth Objects

• NEAR went into orbit around (433) Eros on
  Valentines 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, house-sized boulders, and a 20-km-long
  surface ridge.

85
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 Eross surface.
• Eross density, 2.4 g /cm3, is comparable to that
  of the Earths crust, about the same as Idas,
  and twice Mathildes.
• From orbit, NEAR Shoemakers infrared, x-ray, and
  gamma-ray spectrometers measured how the minerals
  vary from place to place on Eross surface.
• The last of these even survived the spacecrafts
  landing on Eros (see figures), and radioed back
  information about the composition of surface
  rocks.

86
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.
• 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.
• NEAR Shoemakers observations show that Eros was
  probably broken off billions of years ago from a
  larger asteroid as a uniformly dense fragment.
• This solidity contrasts with Mathildes
  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.

87
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.

• Over 1000 of them are greater than 1 km in
  diameter, and none are known to be larger than 10
  km across.
• Statistics show that there is a 1 per cent chance
  of a collision of this tremendous magnitude per
  millennium.
• This rate is pretty high on a cosmic scale.
• Such collisions would have drastic consequences
  for life on Earth.

88
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 ques