1445 Introductory Astronomy I

1
1445 Introductory Astronomy I

• Chapter 9
• Asteroids, Comets Objects Beyond Neptune
• R. S. Rubins
  Fall, 2009

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Asteroids 1

• Asteroids are rocky objects (planetesimals)
  orbiting the Sun, typically over about 100 meters
  across. They are thought to be the building
  blocks of planets, which were formed about 4.6
  billion years ago at the same time as the Sun.

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Asteroids 2

• There are probably billions of asteroids less
  than 1 km (1000 m) across, millions larger than 1
  km, with about 200 larger than 100 km (60 miles)
  across.
• The main asteroid belt lies between Mars and
  Jupiter, at 2.2 AU and 3.3 AU from the Sun.
• Asteroids have distinctly elliptical orbits about
  the Sun, moving in the same sense as the planets,
  and inclined to the ecliptic by as much as 30o.
• As of 2005, more than 3300 asteroids, with orbits
  intersecting the Earths orbit, have been found.
• Together, all the material in the asteroid belt
  would form a single planet of diameter of about
  half that of the Moon.
• Within the asteroid belt, the average distance
  between asteroids is about ten million km, so
  that the possibility of a collision with a
  passing spacecraft is negligible.

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Asteroid Orbits
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Asteroids 3

• Asteroids are thought to be the building blocks
  of planets, which were formed with the Sun about
  4.6 billion years ago.
• No major planet was formed in the asteroid belt
  because of Jupiters immense gravity, which also
  created gaps in the asteroid belt, and forced the
  Trojan asteroids to orbit in two clusters in
  roughly the same orbit as Jupiter.
• The largest asteroid, Ceres , now classified as a
  dwarf planet, is almost spherical, with a
  diameter of 930 km (580 miles), making it a
  little smaller than Plutos satellite, Charon.
• Smaller asteroids, Pallas and Vesta, have
  diameters of roughly 600 km and 530 km (330
  miles) respectively.

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The Asteroid Belt

• The Trojan asteroids lie in the range 5.0 to
  5.5 AU, level with Jupiters orbital path.

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Dawn Asteroid Mission 2007-2015 1

• The Dawn asteroid mission was launched in July
  2007, and is scheduled to use Mars for a
  slingshot in 2009, orbit Vesta in 2011, and reach
  Ceres in 2015.
• Vesta, the 4th largest asteroid, was hot and
  volcanic early in its life, so that its original
  surface has probably been obliterated by ancient
  lava flows.
• Vestas surface is dominated by a giant crater at
  its south pole, about 285 miles across and 8
  miles deep.
• Ceres is the more interesting object of study,
  since it is should be close to its original state
  of 4.6 billion years ago.
• Ceres is thought to contain a layer of water ice
  (and possibly liquid water) 40 to 80 miles thick,
  which has kept its surface cool, and eliminated
  volcanic activity.

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Dawn Asteroid Mission 2007-2015 2

• Unlike the chemical rockets of past use, which
  provide bursts of rapid acceleration, Dawn uses
  the gentle, continuous and energy-efficient
  propulsion, provided by xenon ions in an electric
  field, known as ion drive propulsion.
• Taking about 4 days to reach 60 mph, and 1 year
  to reach 5500 mph, it will ultimately escape into
  space at 89,000 mph.
• It will test its instruments during the Mars
  flyby in 2009, and orbit Vesta for 9 months,
  beginning in 2011.
• On hopefully reaching Ceres in 2015, it will
  orbit the dwarf planet for at least 5 months.
• Dawn carries an optical camera, gamma ray and
  neutron detectors, and a mapping spectrometer.

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Ceres, the Largest Asteroid

• Ceres and the Moon have diameters of 930 km
  and 3480 km respectively.

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Asteroid Mathilde
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Ida (53 km across) and its Tiny Moon, Dactyl
Several asteroids have been found to have
orbiting moons.
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Meteoroids, Meteors and Meteorites

• A meteoroid is a small asteroid, which becomes a
  meteor or shooting star as it penetrates the
  Earths atmosphere.
• While there is no precise definition for the size
  of an meteoroid, objects smaller than about 100 m
  across are typically termed meteoroids .
• Most vaporize completely before striking the
  ground.
• A meteorite is a portion of a meteor reaching the
  ground intact, possibly producing an impact
  crater.
• A meteor shower occurs when the Earth moves
  through the debris left behind by a comet.
• Carbon dating shows that the oldest meteorites
  are about 4½ billion years old i.e. the same age
  as the solar system.

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80-Ton Asteroid Hits the Nubian Desert

• In October 2008, an asteroid (which could also be
  called a meteorite) about 12 ft across with a
  mass of about 80,000 kg (80 metric tons) exploded
  at an altitude of about 37 km, the parts
  scattering in the Nubian desert of Sudan.
• By a lucky accident, asteroid 2008 TC3 became the
  first such object to be observed before impact.
• Fifteen fragments were recovered were later
  recovered over a region of length 29 km.
• The black jagged rocks collected contained
  metals, such as iron and nickel, graphite, and
  nanodiamonds.

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Stony Meteorite Found in Texas
Stony meteorites look like ordinary rocks,
often covered with a dark crust, caused by
the melting of the outer surface during its
descent through the atmosphere.
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Iron Meteorite Found in Australia

• Iron meteorites contain iron-nickel compounds.

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Impact Crater Formed by a 12 kg Meteorite
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Meteor Crater, Arizona

• Caused some 50,000 years ago by a meteoroid about
  50 m in diameter.
• Meteor crater is about 1.2 km across and 200 m
  deep.

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Pluto Data

• Average distance from Sun 39.5 AU.
• Mass 0.2 of Earth (0.002 ME).
• Diameter 2370 km (19 of Earth, 49 of Mercury).
• Average density 36 Earth density.
• Orbital eccentricity 0.25.
• Siderial revolution period 248 Earth years.
• Rotation period 6.4 Earth days (retrograde).
• For 20 years of its 248 year orbit, Pluto is
  closer to the Sun than Neptune, to which it is
  locked in a 32 orbital resonance.

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Plutos Orbit
There is an angle of 17o between the orbits of
Pluto and Neptune.
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Pluto and Neptunes Orbits
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About Pluto and Charon 1

• Plutos orbit is significantly more elliptical
  and more tilted with respect to the ecliptic than
  is any other planet.
• Plutos satellite Charon has a diameter of 1190
  km (just half that of Pluto), and its distance
  from Pluto is less than 5 of the Moon-Earth
  distance.
• Plutos surface contains frozen nitrogen, methane
  and carbon monoxide (CO), while Charons may be
  covered with water ice.
• Pluto and Charon move in a unique form of
  synchronous rotation, in which each has a face
  locked to the other body.
• Thus, observed from Pluto, Charon, would appear
  to hover in the sky, and vice-versa.

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Discovery of Charon, 1978
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Images of Pluto and Charon 1

• These 2006 Hubble images were the first to
  show two tiny moons orbiting Pluto and Charon.

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Dwarf Planets

• In an astronomical conference, held in Prague in
  2006, the as smaller planet-like objects, such as
  Pluto, Eris and Ceres were termed Dwarf Planets.
• The largest of the dwarf planets is the Kuiper
  belt object Eris, which is currently at its
  aphelion, about 100 AU from the Sun.
• Eriss orbit is very elliptical, and at its
  perihelion in about 280 years, it will reach 38
  AU from the Sun, which is not much larger than
  Neptunes distance of 30 AU.
• The most recent objects to be identified as dwarf
  planets are Makemake and Haumea, the latter
  having been first observed in 2003.
• Haumea has the elongated shape of an American
  football, possibly due to its rapid rotation,
  with a period of about 4 hours.

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Dwarf Planet Haumea

• Haumea has about 1/3 the mass of Pluto and two
  moons.
• It is a trans-Neptunian object (TNO), with a very
  elliptical orbit, varying between 35 AU to 50 AU
  from the Sun.

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Plutoids, Plutinos and SSSBs

• The International Astronomical Union (IAU)
  adopted other definitions in 2006, which are
  somewhat simplified below.
• Plutoids are dwarf planets with mean orbits
  around the Sun, further than that of Neptune, so
  that all the dwarf planets except Ceres are
  plutoids.
• There are, probably, well over 100 plutoids.
• Plutinos are those objects of the Kuiper Belt,
  which (like Pluto) have a 23 orbital resonance
  with Neptune.
• About 25 of the known Kuiper Belt objects,
  including Charon, are plutinos.
• Small solar-system bodies (SSSBs) refer to all
  objects in the solar system that are not planets,
  dwarf planets, or moons.

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Eris and its Moon, Dysnomia

• Artists rendition of Eris and its moon, which
  are roughly at 100 AU (9 billion miles) from the
  Sun.

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New Horizons Pluto Mission

• This mission should reach Pluto-Charon in
  2015.

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What are the Planets?

• Historically, the Sun, Moon, Mercury, Venus,
  Mars, Jupiter and Saturn were considered planets
  circling the Earth. Following Copernicus work in
  1543, scientists placed the Sun at the center of
  our universe, so that the Sun and the Moon were
  dropped as planets and the Earth included.
  Uranus was added in 1781, and Neptune in 1846.
  Pluto was considered a normal planet from 1930
  to 2006.
• An answer to the question,how does one define
  planets planets?, was given in a 2007 Scientific
  American article. Not only must they be massive
  enough for gravity to give them an essentially
  spherical shape, but it must also sweep up or
  scatter most of their immediate neighbors, at the
  same time holding smaller bodies (moons) in
  stable orbits.

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Maria Mitchell, Astronomer

• Maria Mitchell (1818-1889) was the first
  woman elected to the American Academy of Arts and
  Sciences, after she discovered a new comet in
  1847, using her familys 2 inch telescope.
• She received a medal from the King of Denmark
  for her discovery, which was known as Miss
  Mitchells Comet.
• In 1848, she became the first female
  astronomy prof. in the US, when she joined the
  faculty of Vassar College.
• In addition to her scientific work, she was
  outspoken in her opposition to slavery, and as an
  advocate of womens rights.
• Mitchell Crater on the Moon is named after
  her.

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Comets 1

• A comet consists of a solid nucleus, surrounded
  by a gaseous coma, distinguished by a long dust
  and ion tails, which are caused by the
  vaporization of the nucleus when it approaches
  the Sun.
• Comets passing the Sun lose between 1 and 2 of
  their masses to evaporation during each passage
  by the Sun, so that they would be expected to
  make fewer than 100 orbits.
• The nucleus (10-20 km across), is a fluffy
  amalgum of rock, dust and water ice, with a
  density about 1/5 th that of ice.
• The coma (about 100,000 km in diameter) consists
  of very low pressure gas and dust surrounding the
  nucleus.
• UV light breaks water molecules into hydrogen and
  oxygen, so that the coma is surrounded by an
  invisible H2 envelope.

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Comets 2

• The low pressure gas and dust also form two
  diaphanous tails an ion (or plasma) tail and a
  dust tail (each about 100 million km long).
• Because of pressure of the solar wind, which
  consists of fast-moving charged particles from
  the Sun, comet tails point away from the Sun.
• The less massive plasma tail, which often emits
  the blue light of carbon monoxide (CO) ions,
  always points away from the Sun because of the
  pressure of the Suns radiation, regardless of
  the direction of the comets motion.
• The heavier dust tail also points away from the
  Sun, but because of the inertia of the dust
  particles, bends away from the direction of the
  comets motion.

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Comets 3

• In August 2009, extraterrestrial amino acid
  glycene was reported in material obtained from a
  comet.
• The material was collected in 2004, when the
  spacecraft Stardust through the dust and gas
  tails of the comet Wild 2, and analyzed later
  after the samples were parachuted to the ground.
• Amino acids, of which glycene is the simplest,
  when strung together in chains, form a diversity
  of proteins.
• The research group at The Goddard Flight Center
  in Maryland, were able to confirm that the
  carbon-isotope concentrations in the samples were
  typical of an extraterrestrial origin, and so
  could not have been contamination, coming from
  the Earth.

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Comet Orbit and Tails
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Structure of a Comet
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Two Tails of Comet Mrkos
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Comet Kohoutek by Visible Light
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Comet Kohoutek by UV Showing H Cloud
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Comet Ikeya-Seki, 1965
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Comet West, 1976
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Comet Hayakutake, 1996
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Two Tails of Comet Hale-Bopp
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Kuiper Belt

• The solar system appears to contain two comet
  reservoirs.
• The Kuiper Belt, estimated to contain over a
  hundred million comets, begins near Neptune in
  the plane of the ecliptic, extending from 30 AU
  to at least 55 AU from the Sun.
• For reasons not yet known, Kuiper Belt objects
  show a wide range of color, from slightly blue to
  very red.
• The dwarf planets, Pluto, Eris, Makemake and
  Haumea are Kuiper Belt objects.
• Eris, which is about 50 larger than Pluto, is
  further from the Sun, with an elliptical orbit,
  of period about 650 years.
• Other large objects in the Kuiper Belt are Sedna
  and Quaoarh.

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Oort Cloud

• The Oort cloud is a spherical distribution of icy
  bodies orbiting the Sun, which extends from about
  10,000 AU to about 100,000 AU.
• 100,000 AU is about 9.3 trillion miles (or 1.6
  ly), which is almost 40 of the way to the
  nearest star, Proxima Centauri.
• The periods of most comets from the Oort cloud
  may be millions of years, compared to about 100
  years for the Kuiper belt comets.
• The Oort cloud is estimated to contain trillions
  of comets, only very few of which get
  gravitational pushes into the inner solar system,
  where there become short-period comets with
  altered orbits, such as Comet Halley.

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Kuiper Belt

• Pluto is the second largest known object in the
  Kuiper belt.

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The Oort Cloud
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Oort Cloud and Kuiper Belt
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Changing a Comets Orbit
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The Heliosphere 1
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The Heliosphere 2

• The heliosphere is a bubble in space produced by
  the Suns solar wind, which is an outward flow of
  plasma (largely protons and electrons) from the
  Sun.
• In addition, the heliosphere contains the solar
  magnetic field and the solar system, and extends
  well beyond the orbit of Pluto.
• The particles are bound together by the magnetic
  field surrounding them, while the pressure of the
  solar wind depends on the particle density and
  the strength of the magnetic field, both of which
  get weaker as the plasma spreads out from the
  Sun.
• The outer boundary of the heliosphere would be
  expected to occur when the pressure of the solar
  wind is balanced by that of the surrounding
  interstellar medium.

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The Heliosphere 3
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Termination Shock and Voyager Spacecraft 1

• The speed of the solar wind in the vicinity of
  the Earth is about a million mph, but slows down
  as it moves away from the Sun.
• The first boundary of the heliosphere occurs when
  the solar wind drops from supersonic to subsonic
  speeds, producing a shock wave, known as the
  termination shock.
• Voyager 1, launched in 1977, traveling north from
  the Sun, passed through the termination shock in
  2004, at a distance of about 94 AU (almost 9
  billion miles) from the Sun, as indicated by
  changes in its magnetic readings.
• Voyager 2, also launched in 1977, traveling
  south, reached the termination shock in 2006, at
  a distance of about 76 AU (7 billion miles) from
  the Sun.
• Voyager 2 made about five shock crossings in a
  couple of days.
• Voyager 2, unlike Voyager 1, has the advantage of
  having a plasma measuring instrument, which is
  still functioning.

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Termination Shock and Voyager Spacecraft 2
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Termination Shock and Voyager Spacecraft 3
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The Heliosheath and Heliopause

• The Heliosheath is the region of thr heliosphere
  beyond the termination shock.
• Here, the solar wind is slowed, compressed and
  made turbulent by interacting with the
  interstellar medium.
• The Heliosheath is shaped like the coma of a
  cometand bulges out in the direction opposite to
  the Suns path through space.
• Its closest distance from the Sun lies roughly
  between 80 AU and 100 AU.
• The Heliopause is the theoretical boundary at
  which the solar wind is finally stopped by the
  interstellar medium.

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The Bow Shock

• The bow shock is associated with the slowing of
  the interstellar medium as it comes into contact
  with the heliosphere.
• The bow shock is so-called because of its
  resemblance to the wake left by a ships prow,
  which its projecting front part.
• The bow shock has been suggested to lie at about
  230 AU (22 billion miles) from the Sun.
• Experimentally, a bow shock has been observed by
  the NASA orbiting GALEX telescope, ahead of the
  star Mira in the constellation Cetus.

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The IBEX Mission

• Launched in 2008, the Interstellar Boundary
  Explorer (IBEX) is finding results which do not
  fit the current theoretical models.
• IBEX measures energetic neutral atoms (ENAs)
  which are created, when the solar wind and the
  interstellar medium interact in the heliosheath,
  and are emitted in all directions.

IBEX is 23 high and 38 Across about the size
of two stacked bus tires. It orbits the Earth
at 5/6 of the distance to the Moon.
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IBEX Data

• The initial IBEX data reveals a narrow ribbon,
  which is two to three times brighter than the
  surrounding regions.
• The IBEX results do not match the current
  theoretical models.

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The Earth

• There is a photograph taken from one of the
  early interplanetary probes, looking back toward
  Earth. Earth appears as a tiny blue sphere
  surrounded by an immensity of blackness. It is
  a photograph that makes tears flow. There is no
  sharper visual statement of the loneliness of our
  planet. Earth is an insignificant speck in a
  vast and overwhelmingly hostile universe.
• Bryce DeWitt in
  Gods Rays, 2004

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(No Transcript)
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Epilogue I am a
theoretical physicist, and it is common knowledge
that theoretical physicists often start out as
amateur theologians.Albert Einstein said The
Lord God is subtle, but He is not malicious. I
like to turn this around by saying, The Lord God
is not malicious, but he is subtle.There is
nothing to suggest that human beings have a
special role to play in this universe. Steven
Weinberg is absolutely right when he says The
more the universe is comprehensible, the more it
also seems pointless.love..that raises our
existence above the level of farce. And it
needs no religious framework whatever to exert
its power.
Bryce DeWitt in Gods
Rays, 2004