Asteroids, Comets and Meteorites

Asteroids, Comets and Meteorites

Important Points1. Small objects in the solar system are leftovers that never

accreted into planets2. Minor planets mostly orbit between Mars and Jupiter3. Comets formed in the outer solar system and were flung

outward by close encounters with other planets4. Comets can be trapped in the inner solar system by

planetary encounters5. The head and tail of a comet glow from sunlight shining on

gases evaporated from the comet6. Meteors and meteorites are pieces broken off larger bodies

by collisions, or dust shed by comets7. Occasionally comets or minor planets collide with larger

objects like the earth

Small Bodies of the Solar System• 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

• 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?


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

2010 SEPT. 2

• 535789 Minor planets catalogued• 251651 Officially numbered• 16154 Named


How We Study Them

• Spacecraft• Ground-Based and Hubble Imaging• Radar Imaging


Spacecraft Images


951 Gaspra (15 km)

243 Ida (40 km) and Dactyl

253 Mathilde (50 km)

Three Asteroids Compared

433 Eros (20 km)

Eros

Eros


Asteroid Itokawa


Spacecraft Shadow

http://antwrp.gsfc.nasa.gov/apod/image/0511/itokawa2_hayabusa_big.jpg

Earth-Based Optical Imaging


4 Vesta (500 km)

Ceres and Vesta

Radar Imaging


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


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


Creation of Long Period Comet


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

http://antwrp.gsfc.nasa.gov/apod/image/0801/mcnaught_guisard_big.jpg

Comet McNaught 2008

http://antwrp.gsfc.nasa.gov/apod/image/0702/mcnaught_deiriesfulle_comp.jpg

Record-Breaking Hale-Bopp

• 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


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


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


MeteoritePeekskill, 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


Nope

Nope

Nope

Nope

Nope

Nope

Nope

Tektites• 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 Points1. Small objects in the solar system are leftovers that never

accreted into planets2. Minor planets mostly orbit between Mars and Jupiter3. Comets formed in the outer solar system and were flung

outward by close encounters with other planets4. Comets can be trapped in the inner solar system by

planetary encounters5. The head and tail of a comet glow from sunlight shining on

gases evaporated from the comet6. Meteors and meteorites are pieces broken off larger bodies

by collisions, or dust shed by comets7. Occasionally comets or minor planets collide with larger

objects like the earth

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Asteroids, Comets and Meteorites