Transcript Discovery of Asteroids - High Energy Physics at Wayne State

Chapter 12:
Comets and
Asteroids:
Debris of the
Solar System
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Discovery of Asteroids
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Most asteroid orbits lie in the asteroid belt – between Mars
and Jupiter.
Too small to be visible without a telescope.
First discovered when astronomers were hunting for a
planet between Mars and Jupiter
1st discovered in the 1801
– Name: Ceres
– Distance from the Sun : 2.8 AU
– Discoverer : Giovanni Piazzi
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Followed in subsequent years by the discovery of other
small planets in similar orbits
By 1890, more than 300 objects had been discovered.
More than 10000 asteroids now have well determined
orbits.
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Asteroid Nomenclature
Asteroids are given a number and a name
 Names originally chosen from Greek/Roman
goddesses; other female names; all names go!
 Asteroids 2410, and 4859 named after Morrison
and Fraknoi
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Asteroid Census
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Total number of asteroids in the solar system very large.
Must be estimated on the basis of systematic sampling of the
sky.
Studies indicate there are 106 asteroids with diameters
greater than 1 km!
 Largest: Ceres - Diameter: ~1000 km
 Pallas, and Vesta – Diameter: ~ 500 km
 15 more larger than 250 km.
 100 times more objects of 10 km size than 100 km.
 Total mass of asteroids is less than the mass of the Moon
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Asteroid Orbits
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Revolve around the sun in west-toeast.
Most lie in or near the ecliptic.
Asteroid belt defined as region
that contains all asteroids with
semi-major axes 2.2 to 3.3 au.
Periods: 3.3 to 6 years.
75% of known asteroids in the
main belt.
Not closely spaced – typically
>million km between them.
Japanese astronomer K. Hirayama
found in 1917 that asteroids fall
into families.
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Asteroid Families
Groups with similar characteristics
 Each family may result from explosion of larger
body (most likely by a collision)
 In a family, asteroids have similar velocities
 Several dozen families are found.
 Physical similarities between largest asteroids
of given families.
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Asteroid Physical Appearance
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Majority: very dark
– Do not reflect much light.
– Reflectivity ~ 3-4%.
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Some:
– Sizable group
– Typical reflectivity ~ 15-20% (similar to Moon)
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Few:
– Reflectivity ~ 60%
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Understanding of the reasons for the above difference
provided by spectral analysis.
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Asteroid Classification - 1
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Primitive bodies
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Dark asteroids
Chemically unchanged since beginning of Solar System
Composed of silicates with dark organic carbon compounds
Ceres, Pallas, and most object in outer third of the belt.
Most primitive asteroids part of Class “C”
 Where C stands for carbonaceous – carbon-rich
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Asteroid Classification - 2
Class “S”
 S stands for “Stony” composition.
 No dark carbons.
 Higher reflectivity.
 Most asteroids of this type believed to be also
primitive.
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Asteroid Classification - 3
Class “M”
 M stands for “metal”
 Identification difficult
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– Done by radar for the largest asteroids such as Psyche.
Much less numerous
 Suspected to originate from collision of a parent body
that had previously differentiated.
 Enough metal in 1-km M-type asteroid to supply the
world with iron for a long period of time.
 Mines in Sudbury, ON, Canada originate from
collision with class-M asteroid.
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Trojan Asteroids
Located far beyond main belt
 ~ 5.2 AU, nearly same distance as Jupiter
 Unstable orbits because of Jupiter.
 Two points on the orbit where asteroids can stay
indefinitely.
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– 2 points make equilateral triangle with Jupiter and the Sun
– Collectively called trojans (Homer – Illiad)
Discovered 1906 Several hundreds found.
 Dark, primitive, appear faint, but are nonetheless
sizeable.
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Outer Solar System Asteroids
Many asteroids with orbits beyond Jupiter
 Example:
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– Chiron, just inside the orbit of Saturn, to almost the
distance of Uranus
– Pholus (1992) 33 AU, red surface, of unknown composition.
– Named after Centaurs (half horse, half human)
 so named because these objects have some attributes of comets,
and asteroids.
1988, on closest approach to the Sun, Chiron’s
brightness doubled, much like the comets, which
contain abundant volatile materials such as water ice,
or carbon monoxide ice.
 Chiron is however much bigger than comets.
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Earth-Approaching Asteroids
1989 – a 200-m object passed within 800000
km of the Earth.
 1994 – a 10-m object passed 105000 km away.
 Some of these objects have collided with the
Earth in the past, some are likely to do so again
in the future.
 Referred to as Near-Earth Objects (NEOs)
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Near Earth Objects (NEOs)
640 NEOs larger than 1km located by the end of 2002
 Actual population more likely to be > millions.
 Unstable orbits
 Fate:
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– Collide with our planet – and be destroyed
– Be ejected from the Solar System
Probability of impact – once every 100 million years.
 None of the known NEOs will end up crashing into the
Earth in the foreseeable future…
 Larger impacts likely to generate environmental
catastrophes
 A good argument towards further investigation of
NEOs.
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NEO observation
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5-km NEO Toutatis,
– approached the Earth at 3 million km in
1992
– less than 3 times the distance to the
Moon
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Radar images show it is a double
object (two irregular lumps) 3 and 2
km objects squashed together.
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Comets
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Appearance of Comets
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Observed since antiquity
Typical comets appear as rather faint, diffuse spot of light –
smaller than the Moon, and many times less brilliant.
Small chunk of icy material that develop an atmosphere as they
get closer to the Sun.
As they get “very close” they may develop a faint, nebulous tail
extending far from the main body of the comet.
Appearance seemingly unpredictable
Typically remain visible for periods from a few days to a few
months.
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Comet Orbits
Scientific study of comets dates back to Newton who
first recognized their orbits are elongated ellipses.
 Edmund Halley (a contemporary of Newton)
calculated/published 24 cometary orbits (1705).
 Noted that the orbits of bright comets seen in 1531,
1607, 1682 were quite similar – and could be the
same comet – returning to the perihelion every 76
years. Predicted a return in 1758.
 When the comet did appear in 1758, it was given the
name Comet Halley.
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Comet Halley
Observed/Recorded on every passage at intervals
from 74 to 79 years since 239 B.C.
 Period variations caused by Jovian planets
 1910, Earth was brushed by the comet tail. – causing
much public concern…
 Last appearance in our skies – 1986.
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– Met by several spacecrafts
Return in 2061.
 Nucleus approximately 16x8x8 kilometers.
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Comet Census
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Records exist for ~1000 comets
Comets are discovered at an average rate of 5- 10 per
year.
Most visible only on photos made with large
telescopes.
Every few years, a comet appears that is bright
enough to be seen with the naked eye.
Recent flybys:
– Comet Hyakutake, long tail, visible for about a month, March
(1996)
– Hale-Bopp (1997)
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Comet Structure
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nucleus:
relatively solid and stable, mostly ice and gas with a small amount of dust
and other solids
coma:
dense cloud of water, carbon dioxide and other neutral gases sublimed off of
the nucleus
hydrogen cloud:
huge (millions of km in diameter) but very sparse envelope of neutral
hydrogen
dust tail:
up to 10 million km long composed of smoke-sized dust particles driven off
the nucleus by escaping gases; this is the most prominent part of a comet
to the unaided eye
ion tail:
as much as several hundred million km long composed of plasma and laced
with rays and streamers caused by interactions with the solar wind
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Comet Structure
ion tail
dust tail
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Nucleus and Coma
Nucleus: ancient ice, dust
and gaseous core material
 nucleus has low gravity –
cannot keep dust and gas
from escaping
 Coma: the bright head of
the comet – seen from the
Earth.
 The coma is a temporary
atmosphere of gas and
dust around the nucleus.
 The coma is 100,000's of
kilometers across
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halley's nucleus
halley's coma
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Ion Tail
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Sun spews out charged particles, called the solar wind. The
solar wind travels along solar magnetic field lines extending
radially outward from the Sun.
UV sunlight ionizes gases in the coma. These ions (charged
particles) are pushed by solar wind particles along magnetic
field lines to form the ion tail millions of kilometers long.
The blue ion tail acts like a "solar" wind sock. The ion tail
always points directly away from the Sun, because the
ions move at very high speed.
When the comet is moving away from the Sun, its ion tail will
be almost in front of it!
The blue color is mostly from the light emitted by carbon
monoxide ions but other types of ions also contribute to the
light. Since the gas is so diffuse, the observed spectrum is an
emission-line spectrum.
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Dust Tail and Hydrogen Cloud
The dust tail forms when solar photons collide with the dust in
the coma. Ejected dust particles form a long, curved tail that
lies slightly farther our from the Sun than the nucleus' orbit.
 The dust tail has a yellow-white color from reflected sunlight.
Both of the tails will stretch for millions of kilometers.
 The dust tail curves gently away from comet’s head, because
dust particles are more massive than individual ions. They are
accelerated more gently by the solar wind and do not reach the
same high speeds as ions.
 The hydrogen cloud forms when water vapor in the jets from
the nucleus is dissociated by solar UV into oxygen and
hydrogen.
 The hydrogen cloud can be tens of millions of kilometers across
– the largest things in the solar system!
 All of this is coming from a dirty snowball the size of a city!
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Stardust Mission
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
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Origin and Evolution of Comets
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Originate from very great distances
Aphelia of new comets ~ 50000 AU
Clustering of aphelia first noted by Dutch astronomer Jan Oort
(1950).
Oort’s Comet Origin Model
– Star’s sphere of influence extends a little beyond 50000 AU or 1 LY
– Objects in orbit about the Sun at this distance can be easily perturbed by
passing Stars.
– Some perturbed object take on orbits that bring them much closer to the
Sun.
– Reservoir of ancient icy objects from which comets are derived is called
Oort Comet Cloud.
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QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
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Oort Cloud
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Estimated 1012 comets in the Oort cloud.
10 times this number of comets could be orbiting the Sun
between the planets and the Oort cloud.
Such objects undiscovered because to small, to reflect sufficient
light to be detectable at large distances, and because their
stable orbit do not bring them closer to the Sun.
Total number of comets in the sphere of influence of our Sun
could be of the order of 1013!
Represents a mass the order of 1000 Earths.
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Kuiper Belt
Second source of comets just beyond the orbit of
Pluto.
 First object discovered in 1992.
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– Diameter ~ 200 km.
– Period ~ 300 years.
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60 objects found since then.
Share orbital resonance with Neptune – two orbits
completed for three by Neptune.
Nicknamed Plutinos for this reason.
Speculated that Pluto is the largest example of this
group.
They may share a common origin.
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Fate of Comets
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Comets spend nearly all their existence in the Oort cloud or Kuiper belt
– At a temperature near absolute zero.
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As comet enters the inner Solar System, their “life” changes altogether!
– If they survive the initial passage near the Sun, they return towards the cold
aphelia – and may follow a quasi-stable orbit for a “while”.
– May impact the Sun
– May be completely vaporized as they fly by the Sun
– May interact with a planet
 Final impact
 Speed up and ejection
 Perturbed into an orbit of shorter period.
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Each flyby the Sun reduces the size and mass of the nucleus of the comets.
Some comets end their life catastrophically by breaking apart.
– Shoemaker-Levy 9 broke into ~20 pieces when it passed close to Jupiter in July
1992.
– Fragments of Shoemaker-Levy captured into a very elongated 2 year around
Jupiter – In 1994 the comet fragments crashed into Jupiter.
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Comet Shoemaker-Levy 9
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Impact!
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Recorded by
HST-WFP in
different
wavelengths
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Shoemaker-Levy 9 Impact Site
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Sequence of Impact Sites
The rotation of
Jupiter left a trail
of impacts.
 We see the
debris left in the
upper
atmosphere.
 Debris fields
would cover
Earth!
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Time Evolution of Debris Fields
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