CE

Comets

Comet Structure and Composition

  • Definition: Comets are bright, fuzzy patches in the sky primarily composed of water ice, alongside methane, ammonia, and carbon dioxide.
  • Nickname: Due to their composition, they are sometimes referred to as "dirty snowballs" due to the dust and rock fragments collected on their surface.
  • Nucleus:
    • Main body of the comet, typically a few kilometers across.
    • Covered in dust, giving it a darker appearance.
    • Density is approximately 100 kg/m³, which is very low.
    • Remains frozen for much of the orbit until it approaches the Sun.
  • Temperature:
    • Generally very cold, around tens of Kelvins.

Behavior Near the Sun

  • As comets approach the Sun:
    • The ice sublimates, leading to increased brightness and a larger tail.
    • The coma forms around the nucleus due to sublimation, appearing as a gas and dust cloud.
  • Coma:
    • A significant feature that can include a hydrogen envelope, which can extend millions of kilometers.

Tails of Comets

  • Comets typically possess two distinct tails:
    1. Ion Tail:
    • Composed of charged atoms and molecules that have lost electrons.
    • Directly affected by solar wind.
    1. Dust Tail:
    • Comprised of dust particles that reflect sunlight.
    • Also influenced by solar wind, pushes tail away from the Sun.
  • Some comets may also exhibit rings.

Orbits and Classification

  • Comets have elongated orbits, primarily originating from the Kuiper Belt and the Oort Cloud.
  • Types of Comets:
    • Short-Period Comets:
    • Complete their orbit in less than 200 years.
    • Mainly originate from the Kuiper Belt.
    • Long-Period Comets:
    • Have orbits that can take thousands of years and often originate from the Oort Cloud.
  • Gravitational Interactions:
    • May lead to near-collisions that displace comets into the inner solar system.
    • Oort Cloud comets have random elliptical orbits, allowing them to travel in various directions.

Historical Observations and Missions

  • Some comets, like Halley’s Comet, are predictable and appear regularly (every 76 years).
  • Missions:
    • Stardust (2004): Collected comet dust and found rocky materials and organic material.
    • Deep Impact (2005): Analyzed the icy nucleus, confirming it reflects sunlight.