Asteroids, Comets and
Meteorites
Important Points
1. Small objects in the solar system are leftovers that never
accreted into planets
2. Minor planets mostly orbit between Mars and Jupiter
3. Comets formed in the outer solar system and were flung
outward by close encounters with other planets
4. Comets can be trapped in the inner solar system by planetary
encounters
5. The head and tail of a comet glow from sunlight shining on
gases evaporated from the comet
6. Meteors and meteorites are pieces broken off larger bodies
by collisions, or dust shed by comets
7. Occasionally comets or minor planets collide with larger
objects like the earth
Small Bodies of the Solar System
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Asteroids: within the orbit of Jupiter
Centaurs: Between Jupiter and Neptune
Kuiper Belt Objects (KBO’s): Beyond Neptune
Scattered Disk: Extreme KBO’s
Comets: Icy bodies with elongated orbits
Meteoroids: Small objects
– Meteors: vaporize in Earth’s atmosphere
– Meteorites: survive to reach surface
1. Small objects in the solar system are leftovers that never accreted
into planets
The Bode-Titius Law
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The planets have fairly regular spacings
Start with 0, 3, 6, 12, 24, 48, 96, 192, 384, 788
Add 4: 4, 7, 10, 16, 28, 52, 100, 196, 388, 792
Divide by 10: 0.4, 0.7, 1.0, 1.6, 2.8, 5.2, 10 …
Matches distances of planets in A.U.
What’s at 2.8?
First Asteroid Discovered, 1801
2. Minor planets mostly orbit between Mars and Jupiter
The Bode-Titius Law
• Neptune,discovered in 1846, matched the
predicted 38.8 value poorly (30.1)
• Pluto didn’t match at all (39.5 versus 79.2)
• Coincidence?
• Or is the Bode-Titius pattern due to the
influence of Jupiter and planetary clearing?
2. Minor planets mostly orbit between Mars and Jupiter
Other Hypothetical Planets
• Vulcan
– Hypothetical planet within the orbit of Mercury
– Some asteroids and many comets cross the orbit
of Mercury, but nothing is known to orbit entirely
within the orbit of Mercury
• Planet X
– Hypothetical massive outer planet beyond
Neptune
• Nemesis
– Hypothetical dwarf star companion to Sun
The
Asteroid
Belt,
2001
The
Asteroid
Belt,
2010
Asteroid Discoveries
Asteroids
Year
1
1801
100
1867
500
1902
1,000
1921
2,000
1942
5,000
1972
10,000
1981
20,000
1993
50,000
1999
100,000
2000
200,000
2003
2. Minor planets mostly orbit between Mars and Jupiter
2010 SEPT. 2
• 535789 Minor planets catalogued
• 251651 Officially numbered
• 16154 Named
2. Minor planets mostly orbit between Mars and Jupiter
How We Study Them
• Spacecraft
• Ground-Based and Hubble Imaging
• Radar Imaging
2. Minor planets mostly orbit between Mars and Jupiter
Spacecraft Images
2. Minor planets mostly orbit between Mars and Jupiter
951 Gaspra (15 km)
243 Ida (40 km) and Dactyl
253 Mathilde (50 km)
Three Asteroids Compared
433 Eros (20 km)
Eros
Eros
2. Minor planets mostly orbit between Mars and Jupiter
Asteroid Itokawa
2. Minor planets mostly orbit between Mars and Jupiter
Spacecraft Shadow
Earth-Based Optical Imaging
2. Minor planets mostly orbit between Mars and Jupiter
4 Vesta (500 km)
Ceres and Vesta
Radar Imaging
2. Minor planets mostly orbit between Mars and Jupiter
Double Asteroids
Comets
3. Comets formed in the outer solar system and were flung outward by
close encounters with other planets
Types of Comets
• Short Period (<200 years)
– Record reobserved comet is Comet Ikeya-Zhang
(1661-2002)
• Long Period (>200 years)
– Hale-Bopp (2400 years)
– Need a source very far away
– Oort Cloud
– Too little material very far from the Sun
– Flung out by planetary encounters
3. Comets formed in the outer solar system and were flung outward by
close encounters with other planets
Where Comets Come From
• Ice and Frozen Gases
– Outer Solar System (Kuiper Belt)
• Planetary Encounters perturb Orbits
– Diverted inward to become short-period
– Diverted outward to Oort Cloud
3. Comets formed in the outer solar system and were flung outward by
close encounters with other planets
Creation of Long Period Comet
3. Comets formed in the outer solar system and were flung outward by
close encounters with other planets
Capture
of Short
Period
Comet
4. Comets can be trapped in the inner solar system by planetary
encounters
Anatomy of a Comet
Comet McNaught 2008
Comet McNaught 2008
Record-Breaking Hale-Bopp
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Most-observed comet in history
Discovered the furthest from the Sun
Largest cometary nucleus known
Visible to the naked eye for 18 months – twice
the previous record
• Brighter than magnitude 0 for eight weeks,
longer than any other comet in the last
thousand years.
5. The head and tail of a comet glow from sunlight shining on gases
evaporated from the comet
Comet Hale-Bopp 1997
5. The head and tail of a comet glow from sunlight shining on gases
evaporated from the comet
Halley’s
Comet
5. The head and tail of a
comet glow from
sunlight shining on
gases evaporated from
the comet
Comet Holmes 2007
Comet
Borelly
Comet Tempel I
The Deep Impact Mission
A Comet
Hits The
Sun
Meteoroids
• Meteoroid – Small object orbiting Sun
• Meteor – Meteoroid that becomes incandescent
from friction with atmosphere
• Bolide or Fireball – Exceptionally brilliant meteor
– In impact studies, “bolide” often used for an
impacting meteoroid prior to impact
• Meteorite – A meteoroid that reaches the surface
• Micrometeorite – microscopic meteoroid
6. Meteors and meteorites are pieces broken off larger bodies by
collisions, or dust shed by comets
Bolide
Meteor Showers
• Earth passes through streams of orbiting debris
• Many linked to orbits of known comets
• Occur predictably
– Perseids: August
– Orionids: October (Halley’s Comet)
– Leonids, November
• No known falls
• Generally 20 or so per hour, rarely 1000’s
• Appear to radiate from one point in sky
6. Meteors and meteorites are pieces broken off larger bodies by
collisions, or dust shed by comets
A Meteor Shower
Radiant
Meteorites
• Stony (95%)
– Chondrites: Pellet-like texture
– Carbonaceous Chondrites: Most similar to the Sun
(minus gases), planetary raw material
– Achondrites: Basalt
• Stony-Iron (1%)
• Nickel-Iron (4%)
– Kamacite (>6% Ni) and Taenite (>25% Ni)
– Texture revealed by etching
6. Meteors and meteorites are pieces broken off larger bodies by
collisions, or dust shed by comets
Finding Meteorites
• Finds versus Falls
– Fall: Observed to fall, then recovered
– Find: Identified long after fall
– Stony Meteorites weather and are hard to tell
from natural rocks
– Iron meteorites are more easily recognized
• Prime Scientific Collecting Localities
– Antarctica
– Deserts
6. Meteors and meteorites are pieces broken off larger bodies by
collisions, or dust shed by comets
Meteorite
Peekskill,
NY 1992
Chondrite
Stony-Iron Meteorite
Iron Meteorite
Meteo-Wrongs
• Meteorites Never:
– Have internal cavities
– Have layers
– Have veins
– Flatten on impact
– Mold around objects
– Almost never light in color outside
• If you “think” it’s magnetic, it’s not magnetic
6. Meteors and meteorites are pieces broken off larger bodies by
collisions, or dust shed by comets
Nope
Nope
Nope
Nope
Nope
Nope
Nope
Tektites
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Very silica-rich, water poor glassy rocks
Terrestrial vs. Extraterrestrial origin?
Volcanic vs. Impact origin?
Problems:
– Odd chemistry
– If terrestrial, why are they spread so widely?
– If extraterrestrial, why are they so localized?
• Now considered impact glass
– Atmospheric shock wave evacuates atmosphere
7. Occasionally comets or minor planets collide with larger objects like
the earth
Tektites
Zodiacal Dust
Speaking of Zodiacal Bands….
Chancellor
Brian May,
CBE
(Liverpool
John Moores
University)
Take-Away Points
1. Small objects in the solar system are leftovers that never
accreted into planets
2. Minor planets mostly orbit between Mars and Jupiter
3. Comets formed in the outer solar system and were flung
outward by close encounters with other planets
4. Comets can be trapped in the inner solar system by planetary
encounters
5. The head and tail of a comet glow from sunlight shining on
gases evaporated from the comet
6. Meteors and meteorites are pieces broken off larger bodies
by collisions, or dust shed by comets
7. Occasionally comets or minor planets collide with larger
objects like the earth