Asteroids and the Asteroid Belt

Module 7: Asteroids and the Asteroid Belt

Overview of Asteroids

  • Definition: Remains of planetesimals that were not incorporated into other planets.
  • Quantity:
  • Approximately 100,000 studied with determined orbits.
  • Total known number exceeds 500,000.
  • Orbital Characteristics:
  • Slightly inclined orbits.
  • Elliptical shapes.
  • Total Mass: Equivalent to about 5% of the Moon's mass.

Types of Asteroids

  • Individual Asteroids:
  • 779,736 studied with known orbits.
  • Tens of millions that are less than 1 km across.
  • Approximately 250 asteroids greater than 100 km.
  • Most located in the asteroid belt, between Mars and Jupiter, with some outside this region due to Jupiter's migration.
  • Distance from the Sun varies between 2 to 3.5 AU (astronomical units).
  • All asteroids combined have a diameter half that of the Moon.
  • Majority orbit counter-clockwise, with 0.01% orbiting clockwise (these are captured from different systems).

Notable Asteroids

  • Vesta:
  • Second largest asteroid, visited by the Dawn spacecraft.
  • Features are prominently cratered and rough.
  • Northern Hemisphere shows older terrain due to more cratering, while the Southern Hemisphere features younger terrain.
  • Contains a mountain that rises 22 km high at its south pole, possibly resulting from a collision.
  • Comprised of iron core with rocky material present elsewhere.

Dawn Mission

  • Images and Findings:
  • Vesta and Ceres studied through the Dawn mission.
  • Notable distances from the spacecraft to each asteroid at various points during the mission:
    • Vesta: Ranged from 5,000 km to 265,000 km from June 6, 2011, to July 24, 2011.
    • Ceres: Ranged from 152,000 km to 414,000,000 km from January 24, 2004, to January 25, 2015.

Formation of Asteroids

  • Jupiter's Influence:
  • The gravitational pull of Jupiter creates Kirkwood gaps within the asteroid belt.
  • Distances between asteroids can reach up to 1,000,000 km.
  • Collisions can occur at speeds surpassing 2,000 miles per hour, causing reassembly of fragments.
  • Example: The asteroid Toutatis showcases instability that can lead to its slow breakup into families of fragments that orbit together.

Characteristics of Asteroids

  • Moons:
  • At least 323 asteroids have moons, such as Ida & Dactyl, and Petit-Prince & Romulus & Remus, identified through the Galileo spacecraft.
  • Physical Traits:
  • Generally have irregular shapes, visible craters from impacts, and presence of dust.
  • Other stars have been found to possess asteroid belts.

Example Asteroids

  • Mathilde:
  • Near-Earth asteroid observed by NEAR spacecraft; roughly 60 km across.
  • Mass estimated at 10^17 kg, characterized by low density, likely resembling a rubble pile.
  • Eros:
  • Also visited by NEAR; has a mass of 7 x 10^15 kg and a density of approximately 2400 kg/m³, showing signs of multiple impacts.

Trojan Asteroids

  • Definition: Asteroids located at the Lagrange Points, effectively held in place by the gravitational forces of Jupiter.
  • Cataloguing:
  • 6,729 Trojan asteroids cataloged for Jupiter.
  • Other minor amounts for other planets: 17 for Jupiter, 9 for Mars, and 1 each for Uranus and Earth.

Risks to Earth

  • Impact Potential:
  • Earth-crossing asteroids pose a significant risk.
  • Statistically, impacts are expected, and it is not a matter of 'if' but 'when'.
  • History of 100,000 craters on Earth; notable recollections suggest mass extinction is plausible from a 1 km asteroid impact, equivalent to the strength of 100 times all nuclear weapons combined.

Monitoring by NASA

  • NASA's Eyes on Asteroids:
  • NASA actively monitors Earth-crossing asteroids through its program, providing live updates and information about asteroid locations.