Transcript Document

Unit 41
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The Asteroid Belt
• Using Bode’s Rule (a simple
mathematical formula that
seemed to predict where planets
would be found), the asteroid
Ceres was discovered between
the orbits of Jupiter and Mars
• Soon many more small bodies
were discovered
Bode’s Rule
Bode’s Rule
Number
Planet
True Distance
(0 + 4)/10 =
0.4
Mercury
0.39
(3 + 4)/10 =
0.7
Venus
0.72
(6 + 4)/10 =
1.0
Earth
1.00
(12 + 4)/10 =
1.6
Mars
1. 52
(24 + 4)/10 =
2.8
Ceres (dwarf)
2.78
(48 + 4)/10 =
5.2
Jupiter
5.20
(96 + 4)/10 =
10.0
Saturn
9.58
(192 + 4)/10 =
19.6
Uranus
19.2
(384 + 4)/10 =
38.8
Neptune
30.1
(768 + 4)/10 =
77.2
Pluto (dwarf)
39.5
(1536 + 4)/10 =
154.0
Eris (dwarf)
67.7
The Asteroid Belt
• Now we know of 210,453 asteroids,
located in the asteroid belt
The Asteroid Belt
– Types of Asteroids (by location)
• Near-Earth Asteroids (NEAs): ones that
closely approach the Earth
– Amors: perihelion distances between
1.017 and 1.3 AU;
» Have orbits that cross the orbit of
Mars, but not Earth, at perihelion
» Example: Eros
The Asteroid Belt
– Types of Asteroids (by location)
• Near-Earth Asteroids (NEAs): ones that
closely approach the Earth
– Atens: semimajor axes less than 1.0
AU and aphelion distances greater
than 0.983 AU;
» have orbits that NEVER cross
the orbit of Earth at perihelion
The Asteroid Belt
– Types of Asteroids (by location)
• Near-Earth Asteroids (NEAs): ones that
closely approach the Earth
– Apollos: semimajor axes greater than
1.0 AU and perihelion distances less
than 1.017 AU
» have orbits that cross the orbit of
Earth at perihelion
» Example: Apollo
The Asteroid Belt
– Types of Asteroids (by location)
• Main Belt: located between Mars and Jupiter
roughly 2 - 4 AU from the Sun;
– Further divided into subgroups: Hungarias,
Floras, Phocaea, Koronis, Eos, Themis,
Cybeles and Hildas (which are named after
the main asteroid in the group).
– Located between mars and Jupiter
The Asteroid Belt
– Types of Asteroids (by location)
• Main Belt: located between Mars and Jupiter
roughly 2 - 4 AU from the Sun;
– Between the main concentrations of
asteroids in the Main Belt are relatively
empty regions known as the Kirkwood
gaps.
» These are regions where an object's
orbital period would be a simple
fraction of that of Jupiter.
» An object in such an orbit is very likely
to be accelerated by Jupiter into a
different orbit.
The Asteroid Belt
– Types of Asteroids (by location)
• Trojans: located near Jupiter's Lagrange points
(60 degrees ahead and behind Jupiter in its
orbit).
– Several hundred such asteroids are now
known; it is estimated that there may be a
thousand or more altogether.
The Asteroid Belt
– Types of Asteroids (by location)
• Trojans: located near Jupiter's Lagrange points
(60 degrees ahead and behind Jupiter in its
orbit).
– Curiously, there are many more in the
leading Lagrange point (L4) than in the
trailing one (L5). (There may also be a few
small asteroids in the Lagrange points of
Venus and Earth (see Earth's Second Moon)
that are also sometimes known as Trojans;
5261 Eureka is a "Mars Trojan".)
The Asteroid Belt
– Types of Asteroids (by location)
• Trojans: located near Jupiter's Lagrange points
(60 degrees ahead and behind Jupiter in its
orbit).
» Located orbiting 60 degrees ahead
or behind the planet
» Martian
» Jovian
» Neptunian
The Asteroid Belt
– Types of Asteroids (by location)
• There also a few "asteroids" (designated as
"Centaurs") in the outer solar system: 2060
Chiron (aka 95 P/Chiron) orbits between
Saturn and Uranus;
– The orbit of 5335 Damocles ranges from
near Mars to beyond Uranus;
– 5145 Pholus orbits from Saturn to past
Neptune.
–
The Asteroid Belt
– Types of Asteroids (by location)
• There also a few "asteroids" (designated as
"Centaurs") in the outer solar system: 2060
Chiron (aka 95 P/Chiron) orbits between
Saturn and Uranus;
– There are probably many more, but such
planet-crossing orbits are unstable and they
are likely to be perturbed in the future.
– The composition of these objects is probably
more like that of comets or the Kuiper Belt
objects than that of ordinary asteroids. In
particular, Chiron is now classified as a
comet.
The Asteroid Belt
• The Main Asteroid belt was once believed to be the
remains of a shattered planet due to Jupiter, or once a
part of Mars. But…
The Asteroid Belt
• The asteroids are not the remains of a shattered planet
or a part of Mars.
– All the asteroids mass added together is ~1% of the
Earth’s mass
The Shapes and Sizes of Asteroids
• Asteroids come in all shapes and sizes
– Ceres is the largest, only 578 miles across
– Ceres is massive enough to pull itself into a sphere
– Most asteroids are not massive enough
– Eros is potato shaped
The Shapes and Sizes of Asteroids
• Most asteroids are very small but would still cause
tremendous damage if one impacted Earth!
The Shapes and Sizes of Asteroids
• Spacecrafts have
only recently visited
asteroids
– NEAR landed on
Eros
– Discovered
craters and a
regolith-covered
surface
Artist’s concept of the Dawn spacecraft (NASA)
http://dawn.jpl.nasa.gov/
• NASA’s Dawn Mission
will be the first to orbit
a main belt asteroid,
doing a detailed and
extensive study of the
two largest asteroids
Ceres and Vesta
• Scientists will study
their surface features
and gain insights on
their internal structure
• Dawn is scheduled for
launch on September 27th
at the Cape Canaveral Air
Force Station on a Delta II
7925H launch vehicle
• With a Mars gravity assist in
February 2009, it will arrive
at Vesta in October 2011
• It will depart Vesta in May
2012 and arrive at Ceres in
August 2015
• The end of the mission is
planned for January 2016
NASA technicians install the instruments
aboard the Dawn spacecraft
The instruments aboard
the spacecraft are:
• A visible camera and an
infrared mapping
spectrometer to reveal
surface minerals
• A gamma ray and
neutron spectrometer to
determine the elements
that make up the
asteroids
• The spacecraft will also
measure the gravity field
of each asteroid
• Dawn’s ion
propulsion
system allows it
to undertake a
mission that
would have
been
unaffordable
using other
technologies
Dawn spacecraft shown with
Ceres (right) and Vesta (left)
in an artist’s image
• Two large solar
panels
stretching 19.7
meters (65 feet)
from tip to tip
harness the
Sun’s energy
and power the
ion engines
Dawn spacecraft shown with
Ceres (right) and Vesta (left)
in an artist’s image
• The energy
ionizes the
onboard fuel
(xenon),
accelerates the
ions, which in
turn accelerate
the spacecraft
Dawn spacecraft shown with
Ceres (right) and Vesta (left)
in an artist’s image
Hubble Telescope images of
Ceres (top) and Vesta (bottom)
• These asteroids were
chosen because they
are two contrasting
planetesimals—Ceres
is wet and icy and may
have subsurface water.
Vesta is dry and rocky
and may have been
formed by volcanoes
• Ceres is about 578
miles in diameter and
Vesta is about 329
miles in diameter
Hubble Telescope images of
Ceres (top) and Vesta (bottom)
• Both bodies formed
early in the history of
the solar system
• Exploring a new
frontier, Dawn will
journey “back in
time,” so to speak,
over 4.5 billion years
to the beginning of
our solar system
• Many thousands of
bodies in the main
asteroid belt between
Mars and Jupiter
formed at the same
time and in similar
environments as the
rocky planets
(Mercury, Venus,
Earth, and Mars)
• Scientists theorize that
asteroids were
budding planets that
were never given the
chance to grow due to
the gravitational
effects of Jupiter
• By investigating these
two asteroids, Dawn
hopes to unlock some
of the mysteries of
planetary formation:
the conditions and the
building blocks under
which they were
formed
Artist’s concept of Dawn spacecraft (NASA)
• Dawn will also
contrast the
formation and
evolution of these
two small
planetesimals that
followed very
different
evolutionary paths
so we can
understand what
controls that
evolution
• This can help us in
our observation of
exoplanets in
other planetary
systems
Artist’s concept of Dawn spacecraft (NASA)
Asteroid Composition
• Asteroids can be grouped into three compositional
types:
– Carbonaceous bodies
• Carbon rich, coal-like substance
• Located in the outer part of the asteroid belt
– Silicate bodies
• Composed primarily of silicates (low-density
rock)
– Metallic iron-nickel bodies
• Composed mostly of dense metals
• Located in the inner part of the asteroid belt
Asteroid Composition
Classification of Asteroid’s
• Asteroids are classified into a number of types according
to their spectra (and hence their chemical composition)
and albedo:
• By examining the spectra of light reflected from these
objects, we can classify asteroids as follows
Asteroid Composition
Classification of Asteroid’s
• C - Dark, probably carbon-containing (carbonaceous)
– C-type, includes more than 75% of known asteroids: extremely
dark (albedo 0.03); similar to carbonaceous chondrite
meteorites; approximately the same chemical composition as
the Sun minus hydrogen, helium and other volatiles; would
appear darkest and reflect the least light
Asteroid Composition
Classification of Asteroid’s
• S - Twice as bright as C, probably made of stony iron
– S-type, 17%: relatively bright (albedo .10-.22); metallic nickeliron mixed with iron- and magnesium-silicates; Silicaceous
• M - Similar to iron meteorites
– M-type, most of the rest: bright (albedo .10-.18); pure nickeliron; Metallic; densest
• P and D - Low brightness, reddish
– There are also a dozen or so other rare types.
• Where did these three kinds of
asteroids come from?
Asteroid Composition
Origin of Asteroids
• Asteroids are probably fragments of planetesimals
• The planetesimal had collected a mixture of rock and
metals, and then differentiated, creating a dense
metallic core and a lighter, silicate-rich outer shell
Origin of Asteroids
• A collision with another asteroid could have shattered the
planetesimal
– Fragments of the inner core would form the ironnickel asteroids
– Fragments of the outer shell would form the silicate
asteroids
Asteroid Orbits
• It is likely that the asteroids were unable to form a
planet (however small) due to the gravitational
influence of Jupiter
Asteroid Orbits
• Jupiter “stirs up” the asteroids, keeping them apart
• There are empty regions in the asteroid belt, called
Kirkwood Gaps
Asteroid Orbits
• These gaps are present at orbital resonances of Jupiter
Asteroid Orbits
• Asteroids with an orbital resonance get periodic
tugs from Jupiter, pulling them out of position
DAWN Asteroids:
1 Ceres and 4 Vesta
By Christina O. Lee
October 13, 2004
Astro 249
DAWN Mission Goals
• To characterize conditions and processes
of the solar system's earliest epoch by
investigating in detail the two largest
protoplanets remaining intact since their
formations. The growth of these bodies
(and others in the belt) were presumably
interrupted by the formation of Jupiter,
whose gravitational forcing countered the
accretionary process.
October 13, 2004
Astro 249
Targets: 1 Ceres & 4 Vesta
• Two largest protoplanets
residing between Mars and
Jupiter.
• Each followed a very
different evolutionary path
constrained by diversity of
processes that operated
during the very first few
Myrs of solar system
evolution
October 13, 2004
Astro 249
Relative sizes
• Ceres is 933 km diameter, 870000 x 1015 kg
• Vesta is 510 km diameter, 300000 x 1015 kg
October 13, 2004
Astro 249
Vesta Quick Facts
• Named after the
Roman Goddess of
Hearth
• The only asteroid
visible with the naked
eye
• 3rd largest asteroid
(510 km diameter)
• Distance: 2.361 AU
• Rotation rate: 5.342 hr
• Evolved and dry,
differentiated body
October 13, 2004
Astro 249
Vesta – some properties
•
•
•
Have rocks more strongly magnetized than Mars (> 100s nT)
– alter ideas of how and when dynamos arise
Have basaltic surface composition
– possibly possessing an early magma ocean like our Moon
Experienced significant excavating events
– indicated by huge crater near southern pole
October 13, 2004
Astro 249
Vesta meteorite
Almost entirely made of pyroxene,
commonly found in lava flows on Earth
Mineral grain structure also indicates it
was once molten, and its oxygen
isotopes are unlike oxygen isotopes
found for all other rocks of the Earth
and Moon.
•
Radioisotope chronology (i.e. cosmic ray dating) from the howardite,
eucrite, and diogenite (HED) meteorites believed to be from Vesta
suggests it accreted in only 5-15 million years (Vesta has same
spectral signature as pyroxene)
– similar evidence indicates that Mars continued to accrete for close to 30
Myrs and Earth for 50 Myrs
•
Dating also indicate that impacts have released meteoritic material at
least 5x in the last 50 Myrs.
October 13, 2004
Astro 249
Ceres – Quick Facts
• Named after the
Roman Goddess of
Agriculture
• Largest asteroid (933
km diameter)
• Distance: 2.769 AU
• Rotation rate: 9.075 hr
• Very primitive surface
w/ water bearing
minerals!
October 13, 2004
Astro 249
Ceres – Some properties
• Have active hydrological processes
leading to seasonal polar caps of water
frost
• Have thin permanent atmosphere of frost
• Have dry clay surface
– indicated by microwave studies)
October 13, 2004
Astro 249
No Ceres Meteorite
• …But it is expected to be like Vesta and
formed ~ 10 Mya.
• Possibly the excavating events or
dynamic that provided the HED meteorites
did not occur, but it's possible that the
reflectance spectrum of the surface is not
indicative of Ceres' crustal rocks
– on Vesta, the basaltic dust layer reflects its
crustal composition
October 13, 2004
Astro 249
Why are Ceres and Vesta so different?
It appears that a rather short additional radial
separation allowed Ceres (further away) to
accrete wet and stay cool while early heat
sources in the accreting material melted Vesta
(slightly closer)
October 13, 2004
Astro 249
Go DAWN mission!
Will determine internal structure, density
and by measuring their mass, shape,
volume and spin rate with both imagery,
laser altimetry and gravity
October 13, 2004
Astro 249