Download The Main Points Asteroids

3/14/2008
The Main Points
Lecture #22: Asteroids
• Discovery/Observations
Asteroids are small, rocky objects that
exhibit surprisingly diverse compositions
and surface geologies
• Where are they?
• How many are there?
• What are they like?
Most asteroids are found in the Main Belt
between Mars and Jupiter or in the
Kuiper Belt beyond Neptune, but many
exist in near-Earth space too
• Where did they come from?
• Reading: Chapter 12.1
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Asteroids
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Asteroids are discovered in
telescopic images because they
move at a different speed across the
sky than the stars
• "Asteroid" is Greek for "star-like"
• Asteroids are small, rocky objects that look
star-like in telescopes, except they move
across the sky like the planets do
• The first asteroid was discovered in 1801
"Vermin of the skies" (!)
QuickTime™ and a
GIF decompressor
are needed to see this picture.
– Ceres, about 940 km diameter
• There may be 1,000,000+ >1 km diameter
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The Largest Asteroids
Where are the Asteroids?
• Asteroids can be found throughout the solar
system, but there are two main populations:
• 1 Ceres: Diam. = 940 km, a = 2.77 AU
– Discovered by Guiseppe Piazzi in 1801
– Piazzi was searching for a "missing planet"
between Mars and Jupiter
– The Main Belt between Mars and Jupiter
– The Kuiper Belt beyond Neptune's orbit
• Most of the asteroids with well-determined
orbits are in the main belt
• Other smaller populations exist too
• 2 Pallas: D=540 km, a=2.77 AU, disc. 1802
• 4 Vesta: D=510 km, a=2.36 AU, disc. 1807
• 13 main belt asteroids have D > 250 km
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– Trojans, Apollos, Amors, Centaurs, ...
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The Main Asteroid Belt
A snapshot of
the locations of
all known inner
solar system
asteroids as of
March 2007
• The main belt extends from
about 2.2 to 3.3 AU
• Most of the orbits lie at or
near (±10° to 20°) the
plane of the ecliptic
(outer circle is
the orbit of
Jupiter)
• As of 2008, almost 200,000
main belt asteroids have
well-known orbital
parameters
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Edge on
• The number has been
recently increasing by a
few thousand per year
because of new telescopic9
search programs
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http://cfa-www.harvard.edu/iau/lists/InnerPlot.html
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Gaps in the Belt...
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Other Asteroid Groups
• Astronomer Daniel
Kirkwood (1886)
noticed that the main
belt is not uniformaly
populated
• Asteroids "missing"
f
from
places
l
where
h
disturbances by
Jupiter are strongest
• These places are
where resonances
with Jupiter's orbit
occur
• May explain why no
planet here: Jupiter
only allowed small
bodies to coalesce...
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A snapshot of the
locations of all known
outer solar system
asteroids as of today
(outer circle is the
orbit of Neptune)
Astronomers believe
that there may by
100,000 or more
Kuiper Belt Objects
still to be discovered...
The solar system does
not end at Pluto!
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http://cfa-www.harvard.edu/iau/lists/OuterPlot.html
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• Trojans
– About 1000 asteroids leading and trailing Jupiter by 60°
• Centaurs
– About 100 asteroids found between Jupiter & Neptune
• Kuiper Belt Objects
– More than 1000 large asteroids beyond Neptune
• Near-Earth Asteroids (NEAs)
– Atens (6%): orbits interior to Earth's
– Apollos (62%): orbits that cross Earth's
– Amors (32%): orbits exterior to Earth's (e.g. 433 Eros)
• All these populations are incompletely counted
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How Many Asteroids are There?
Number of
Near-Earth Asteroids
(a model)
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• The number of asteroids of a
given diameter D is
proportional to 1/D2: a
collisional distribution
• For example:
3 > 500 km
13 > 250 km
hundreds > 100 km
10,000+ > 10 km
• Total: >1,000,000 > 1 km
• Most of the MASS is in the
largest few asteroids
• Total mass of all asteroids is
only ~ 5-10% mass of the 14
Moon
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Discovering Asteroids
• For many years, most discoveries were random
• Within the past decade, dedicated surveys have
begun in response to the possible threat of asteroid
and comet impacts on the Earth
–
–
–
–
–
NASA Spacewatch Program
Air Force Near Earth Asteroid Tracking Program (NEAT)
Lowell Observatory Near Earth Object Survey (LONEOS)
Lincoln (MIT) Near Earth Asteroid Research Project (LINEAR)
Catalina (Arizona) Sky Survey
Dedicated search programs started in the last few years have
located perhaps 70% of the >1 km Potentially Hazardous
Asteroids (PHAs) that could pose a threat to life on Earth.
• Tens of thousands of objects discovered, many NEAs...
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Asteroid Physical Properties
•
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Asteroid Composition
• Spectroscopy and radar observations of asteroids
reveal four main classes:
Many diagnostic properties can be determined by
telescopic remote sensing:
– Albedo (% of reflected sunlight)
–
–
–
–
• darkest are 1% to 2%, brightest are 40% to 50%
– Spin period (length of an asteroid "day")
– Shape
Sh
(f
(from radar
d & variations
i ti
in
i the
th light
li ht curve))
C Type: dark, "carbon rich", albedo ~5%
S Type: brighter, "stony", albedo ~20%
D Type: dark, very red spectra
M Type: rare "metallic" type, very radar bright
• C,S,D Types probably all primitive (?)
• M Types and other anomalous classes (like Vesta)
from differentiated parent bodies
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(controversial!)
Asteroid Geology
• 4 asteroids visited up close by spacecraft
–
–
–
–
• Also: Spacecraft images of Martian moons Phobos
and Deimos: captured asteroids?
• Abundant evidence for impacts, and surprising
evidence for erosion and tectonism on these bodies
• Composition varies systematically with distance from the Sun
• Implies that asteroids preserve the conditions of planetary
formation in the early solar system
• But primitive vs. differentiated issue is controversial!
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951 Gaspra: Galileo flyby in 1991
243 Ida: Galileo flyby in 1993
253 Mathilde: NEAR flyby in 1997
433 Eros: NEAR orbital mission in 2000-2001
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951 Gaspra
S Type
16 x 11 x 10 km
253 Mathilde
C Type
66 x 48 x 46 km (large!)
Many large craters
Density only 1.3 g/cm3
May be 50% "empty"
243 Ida
S Type
60 x 25 x 19 km
Dactyl
Ida's
moon!
D = 1.5 km
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433 Eros
S Type
31 x 13 x 13 km
Many large craters
Density about 2.6
g/cm3
Geology!
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Tectonic ridges:
Eros appears to have been
compressed and possibly even
twisted during its history...
Fine-grained regolith
appears to be moving
downhill on Eros: erosion!
Last image from NEAR
before its landing: very finegrained regolith!
(Compare with basket ball)
Eros movies from NEAR
• craters, boulders, & more
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Summary
Next Lecture…
• Meteorites
– Samples from the Early Solar System
– Age,
g , origin
g and evolution
– Classification
– Parent Bodies: Asteroids & Planets
• Reading
• Asteroids are small, rocky, primitive objects thought to be
remnants from the formation of the solar system
• There are probably more than 1 million asteroids > 1 km in
size in the solar system, but their total mass is only a small
fraction of the mass of the Moon
• Remote sensing shows that not all asteroids are alike
– Different compositional classes (C, S, D, M)
– Some asteroids have had complex internal histories
• Spacecraft observations show that asteroids have interesting
and complex geologic histories: impacts, tectonics, erosion
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– 12.1
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