Asteroids
Not just for kids anymore.
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Goals
• What are asteroids?
• How do we know?
• Why do we care?
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Asteroids
• All planets and moons have
been modified chemically and
geologically.
• Where do you look for a piece
of the original “stuff” of the
solar system?
• Asteroids and comets.
• Small objects
– Little internal heat, little to no geological activity.
– Little gravity, little to no atmosphere.
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Orbits
1. Asteroid belt.
2. Same as Jupiter, but
separated by 60º Trojans
3. Elliptical orbits that pass
Earth
• Earth-crossing asteroids:
– Near-earth asteroids
(NEAs)
– Near-earth
objects (NEOs)
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Asteroid sizes
• How big are they?
–
–
–
–
Largest (Ceres) is 940 km in diameter
Three larger than 500 km
About a dozen larger than 250 km
Number increases rapidly with decreasing size
• Total volume of all asteroids ~ much smaller
than moon.
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How do we know?
• Compare IR light to visible light.
– Visible light: what light a body reflects.
– IR light: what a body emits because of its temperature =
what light it absorbs.
• Albedo = function of vis/(vis + IR)
• Size = function of (vis/albedo)*distance
– Assume all asteroids have same albedo and at same
distance: Size ~ vis
– Allow different distances: Size ~ vis*distance
– Allow different albedo: Size ~ (vis/albedo)
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Concept Test
•
Suppose you discover two asteroids that are
equally bright in the visible but IR observations
tell you Asteroid#1 is more reflective than
Asteroid#2. What can you conclude?
a.
b.
c.
d.
e.
Asteroid#1 is larger than Asteroid#2.
Asteroid#1 has a lower albedo than Asteroid#2.
Asteroid#1 is farther away than Asteroid#2.
Asteroid#1 is warmer than Asteroid#2.
None of the above.
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What do asteroids look like?
• Shape generally depends on
size.
• Gravity tries to make things
spherical (hydrostatic
equilibrium).
– Largest (Ceres) is 940 km in
diameter - spherical
– Three larger than 500 km (Vesta)
– mostly spheroidal
– Smaller than 250 km - irregular
• Gravity Total mass of all
asteroids ~ 5%
of the moon
Ceres - HST
Vesta – Thomas et al. HST
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Shapes
• Asteroid light curves.
• Radar mapping.
Ostro et al. 1995
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Asteroid Encounters
• Three fly-bys of asteroids:
– Gaspra by Galileo in 1991
– Ida by Galileo in 1993
– Mathilde by NEAR in 1999
• Two orbiters:
– Eros by NEAR in 2000
– Itokawa by Hayabusa in 2005
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Eros
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Eros Scale
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Eros Landing
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Mathilde
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Eros
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Itokawa
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Composition
•
Asteroids are classified into a number of types according to
their spectra (and hence their chemical composition) and
albedo:
1. C-type, includes more than 75% of known asteroids:
– extremely dark (albedo 0.03)
– approximately the same chemical composition as the
Sun minus hydrogen, helium and other volatiles
2. S-type, 17%
– relatively bright (albedo .10-.22)
– metallic nickel-iron mixed with iron- and magnesiumsilicates
3. M-type, most of the rest
– bright (albedo .10-.18)
– pure nickel-iron
4. There are also a dozen or so other rare types
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Masses
• Kepler’s
Third Law
– “moon”
– spacecraft
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Density
•
•
•
•
•
•
•
•
Calculate Density
Rock ( ~ 3g/cm3) vs. metal (~7g/cm3).
Solid vs. rubble pile.
Ida = 2.6 g/cm3
Eros = 2.4 g/cm3
Itokawa = 1.9 g/cm3
Mathilde = 1.5 g/cm3
Eugenia = 1.12 g/cm3
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Concept Test
•
I discover an asteroid all by itself. Without
sending a spacecraft there, what can I determine
about the asteroid?
a.
b.
c.
d.
e.
Albedo, size, distance, mass, density, composition.
Albedo, size, distance, mass, density.
Albedo, size, distance, mass.
Albedo, size, distance.
Albedo, size.
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Meteorites
• Want real sample of this
material.
• Hayabusa sample return –
one asteroid.
• Meteorites potentially many
asteroids.
• Really piece of asteroids?
– Compare spectra.
Copyright - Wally Pacholka
– Compare trajectories (observed
falls).
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Meteorites
•
•
•
•
Meteoroid – the particle in space.
Meteor – the fiery streak through the sky.
Meteorite – the rock on the ground.
Types (2 main)
1.
2.
•
Primitive – mix of rock and metals
Processed – rocky or metallic (from differentiated asteroid or
“parent body”).
Compare spectra to find parent body:
–
–
–
Asteroid (e.g. Vesta)
Moon
Mars
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Meteorites
Primitive
Processed: stony-iron
Processed: iron
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Peekskill Meteorite
Copyright – Pierre Thomas (1992)
Copyright – Anne Arundel (1992)
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Peekskill Orbit
• Parent body aphelion = 2.1 AU
• Martin Beech et al. (Univ of Western Ontario) 1995
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Concept Test
•
When you see the bright flash of a meteor, what are you
actually seeing?
a.
b.
c.
d.
e.
Emission of visible light from a particle that has not yet entered
Earth's atmosphere.
The flash that occurs when a speeding rock from space hits the
ground.
A star that has suddenly shot across the sky.
The glow from a pea-size particle and the surrounding air as the
particle burns up in our atmosphere.
None of the above.
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Concept Test
•
I find a meteorite that is composed entirely of
rock (no metal). Assuming it’s from an asteroid,
what type of parent body is it probably from?
a.
b.
c.
d.
e.
A very small asteroid.
A part of the core of a very large asteroid.
A part of the outer layers of a very large asteroid.
From the heart of a differentiated asteroid.
It is not possible to tell.
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Homework #19
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•
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Due Wednesday 19-Nov:
Read Bennett 12.1 - 12.3.
Do 6, 8, 27, 28, and 32.
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