Lecture 21 - Solar System Debris Notes

Solar System Debris

Asteroids

  • Asteroids are cosmic debris, remnants from the Solar System's formation. They are too small to be classified as planets.
  • Most asteroids are small, rocky bodies orbiting the Sun, larger than 100 m across, but mostly less than 300 km. They reside in the asteroid belt between Mars and Jupiter (2.1 to 3.3 AU).
  • Over 700,000 asteroids have been identified; however, their total mass is less than the mass of the Moon.
  • Largest asteroid: Ceres
    • Diameter: 940 km
    • Mass: 10−4M⨁10^{-4} M_{\bigoplus}
  • Other large asteroids: Pallas (580 km), Vesta (540 km).
  • Mass Distribution:
    • Most mass is in the larger asteroids.
    • Ceres, Pallas, and Vesta are over 500 km across.
    • Only two dozen asteroids exceed 200 km.
    • 99% of asteroids over 100 km are known.
    • 50% of asteroids over 10 km are known.
  • Due to their size and faintness, their size and shape can only be determined using indirect methods (sunlight reflection).
  • Classification of Asteroids:
    • C-type: Carbonaceous (carbon-rich), dark in color, ancient (75%)
    • S-type: Silicate (rocky material), light in color, reprocessed (15%)
    • M-type: Metallic (iron/nickel), (10%)
    • S-types dominate the inner belt, C-types the outer belt
  • Examples of Asteroid Missions:
    • Galileo probe: Provided first close-up pictures.
      • Gaspra: Irregular shape, 20 km across, ~200 million years old.
      • Ida: 60 km across, mass ~ 101710^{17} kg, density 2500 kg/m3m^3, has moon Dactyl, ~1 billion years old.
    • NEAR (Near Earth Asteroid Rendezvous):
      • Mathilde: C-type, mass ~101710^{17} kg, density 1400 kg/m3m^3, porous rubble pile.
      • Eros: Density 2700 kg/m3m^3, rocky composition, craters from 50 m to 5 km.
    • Dawn spacecraft:
      • Vesta: 500 km across, deep grooves, towering mountain, evidence of volcanism, differentiated interior, may be a protoplanet.
      • Ceres: Craters with bright spots made of ice and salt.
  • Asteroid Orbits:
    • Most asteroids have eccentricities between 0.05 and 0.3, staying between Mars and Jupiter in the asteroid belt.
    • Trojan asteroids orbit at Lagrangian points of the Jupiter-Sun system.
    • Apollo asteroids:
      • Eccentricity > 0.4, semimajor axis > 1 AU, cross Earth's orbit.
      • Raise concern of potential Earth collisions.
      • ~15,000 discovered, 1700 potentially hazardous.
      • Most are 1-10 km in size.
  • Collision Risks:
    • 250 potentially hazardous asteroids came within 0.05 AU of Earth between 2006 and 2015.
    • Asteroid 4179 Toutatis is the largest of these.
    • A 350 m asteroid (2004 MN4) will pass within 23,000 km in April 2029.
  • Impact Frequency:
    • Earth is hit by 3 asteroids per million years on average.
    • A 1 km asteroid impact ≈ 100 times all nuclear weapons = destroys 100 km area, climate change, potential extinction.
    • 65 million years ago, the dinosaurs were probably wiped out by a 10 to 15 km asteroid.
      • Evidence: impact crater, iridium layer.
  • Orbital Resonance:
    • Asteroids have orbital resonances with Jupiter (Lagrangian points).
    • L1, L2, and L3 are unstable; L4 and L5 are stable (Trojan asteroids).
    • Kirkwood gaps: