MINOR MEMBERS OF THE SOLAR SYSTEM: Asteroids

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• MINOR MEMBERS OF THE SOLAR SYSTEM Asteroids

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• Images of three asteroids, taken during
  spacecraft flybys, shown to scale (Mathilde is 59
  km wide and 47 km high)

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General Thesis

• Asteroids and Comets are relics of the early
  stages of accretion of the solar nebula

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I History of Discovery

• A gap in the known planetary system between
  Mars (1.5 AU) and Jupiter (5.2 AU)
• The Titius-Bode law (purely numerical relation)
  predicted a planet in the gap between Mars and
  Jupiter

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Titius-Bode Law
Planet Calculated distance True distance
Mercury (04)/10 0.4 0.39
Venus (34)/10 0.7 0.73
Earth (64)/10 1.0 1.00
Mars (124)/10 1.6 1.5
? (244)/10 2.8
Jupiter (484)/10 5.2 5.2
Saturn (964)/10 10.0 9.6
Uranus (1924)/10 19.6 19.2
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Asteroid discoveries

• Discoveries Ceres 1801, Pallas 1802, Juno 1804,
  Vesta 1807
• Largest is Ceres (940 km diameter)
• Photography (after 1890s) allowed discovery of
  many more asteroids
• As many as several million with diameters of 1 km
  or more
• Total mass 1/20 mass of Moon

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II Classification of Asteroids

• Belt Asteroids
• Semimajor axes 2.23.3 AU
• Periods 3.36 years
• Families of asteroids (similar orbits, surface
  appearance) may be fragments of a single asteroid
  produced by collisions
• Some gaps in belt caused by resonances with
  Jupiter

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Belt asteroids
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Trojans

• Orbit at Lagrangian Points (60 ahead and behind
  Jupiter)
• Stable orbits the asteroids will not be swept
  up by Jupiter
• May be several thousand in number
• Size Most are a few km, some are gt100 km

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Trojan Asteroids
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Earth-approaching

• May be several thousand gt1 km diameter
• Radar images are available for several that
  approached Earth
• Amor
• Have orbits crossing Marss orbit
• Perihelion distances between 1.017 and 1.4 AU (ie
  between Earth and Mars)

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Apollo

• Cross Earths orbit but have semimajor axes
  greater than 1.0 AU (elliptical orbits!)
• Aten
• Have orbits with semimajor axes of less than 1.0
  AU (inside Earths orbit!)

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Centaurs

• Orbit beyond Jupiter
• eg Hidalgo A 5.9 AU
• Chiron A 13.7 AU (beyond Saturn)
• Are these asteroids or comets?

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Kuiper Belt Asteroids?

• Beyond Neptune ( 40 AU) a growing number of
  small bodies have been discovered
• Are these asteroids or comets?
• Distinction may be unimportant all bodies out
  here are ice-balls like comets!
• The Kuiper Belt
• A belt of many orbiting icy chunks
• Perturbations alter orbit of Kuiper Belt object
  and can send it into inner solar system ? comet !

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• Several space missions have brought back close-up
  pictures of asteroids
• Galileo spacecraft flew by Gaspra and Ida
• NEAR spacecraft flew by Mathilde.

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Origin of the Asteroids

• Debris left over from formation of solar system.
  (Accretion process limited by tidal effects of
  Jupiter).
• Total mass (1/20 mass of moon) is too low to have
  been remnants of a planet.
• Collisions between asteroids produce smaller
  fragments (families of asteroids) and meteoroids,
  some of which fall on Earth.

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IV Collisions between Asteroids and the Earth?

• Evidence that impact of 10 km asteroid 65
  million years ago, at end of Cretaceous Period
• led to the extinction of the dinosaurs
• enhanced Iridium in layer at K-T
  (Cretaceous-tertiary) boundary in sediments
  worldwide
• Chicxulub crater (200 km) in Yucatan, Mexico
• Tunguska event (30 June 1908) in Siberia, may
  have been an impact with a 100,000 ton body

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Watch Out!

• Estimated that impacts like the K-T impact occur
  about every 100 million years.
• Results could be catastrophic to civilization!