Asteroids Lecture Notes

Overview of Asteroids

  • Definition: Asteroids are small, rocky or metallic objects that orbit a star. They do not radiate light and are typically too small to reflect significant sunlight.
  • Size Variation: Ranges from baseball-sized to 1/3 the size of the Moon (e.g., Ceres).
  • Detection: Observed photographically by telescopes; their motion appears as streaks against the starry background.
  • Significance: They are remnants from planetary formation, providing insights into solar system history.

Asteroid Characteristics

  • Structure: Most asteroids are not spherical due to insufficient mass for self-gravity to shape them accordingly.
  • Companion Moons: Some have moons, and binaries exist—two comparable asteroids orbiting a common center of mass.

Asteroid Classification by Region

  1. Main Belt Asteroids:

    • Located between Mars (1.5 au) and Jupiter (5.2 au).
    • Primary Region: Asteroid belt, a disk-shaped area with high asteroid concentrations.
    • Size Distribution:
      • Over 200 asteroids larger than 100 km.
      • More than 1,000 larger than 30 km.
      • 1-2 million over 1 km in diameter, along with millions of smaller ones.
    • Combined Mass: Less than that of the Moon (~5% of the Moon's mass).

  2. Trojan Asteroids:

    • Two groups linked with Jupiter; one precedes, and one follows it in orbit.
    • Over 7,000 detected; around 2,000 have diameters > 15 km.

  3. Earth-Crossing Asteroids (ECAs):

    • Paths intersect Earth's orbit, posing collision risks.
    • Classified as near-Earth objects (NEOs) if they enter within 1.3 au of the Sun.
    • 20,000 NEOs identified, with ~1,000 > 1 km in diameter.

NASA's Efforts on NEOs

  • In 1998, Congress mandated NASA to track NEOs.
  • By 2010, the WISE telescope surveyed the sky, identifying over 150,000 main belt asteroids and 2,000 Jovian Trojans.

Detailed Characteristics of Specific Asteroids

Vesta

  • Notable Features:
    • Second most massive in the asteroid belt (30% of Ceres' mass).
    • Mean diameter: 525 km; known for its large, mountain-like structures.
    • Characteristics: Brightest asteroid, visible without aid, visited by NASA's Dawn mission.

Mathilde

  • Density: Small density (specific gravity 1.3 to 1.5) suggests it is a loosely held rock pile rather than a solid body.

Asteroid Classification by Composition

  1. C-type (Carbonaceous):

    • Composition: Mainly carbon and silicate rocks; appear gray to very dark.
    • Characteristics: Most primitive, about 75% of all asteroids, found mainly in the outer belt.

  2. S-type (Siliceous):

    • Composition: Silicate material, often resembling volcanic rocks.
    • Characteristics: 17% of asteroids, typically foraged from larger, differentiated bodies, found in the inner belt.

  3. M-type (Metallic):

    • Composition: Mostly nickel and iron, remnants of a larger differentiated body.
    • Characteristics: Rare (about 8%), reddish in color, found in the middle region of the asteroid belt.

Additional Resources