Comets are fascinating celestial objects with unique structures and behaviors. They consist of a solid nucleus and a gaseous coma, developing tails as they approach the Sun. The nucleus, often described as a "dirty snowball," is a mixture of ice and rocky material.

As comets near the Sun, their ices sublimate, forming the coma and tails. The Rosetta mission provided valuable insights into comet composition and behavior. Comets have diverse orbits, ranging from short-period to long-period, and their visibility depends on various factors like proximity and activity level.

Cometary Structure and Composition

Physical features of comets

• Comets have two main components: nucleus (solid, central part) and coma (fuzzy, gaseous atmosphere surrounding nucleus)
• Comets develop tails as they approach the Sun
  • Dust tail composed of dust particles pushed away from nucleus by radiation pressure
    • Appears curved and follows comet's orbit
  • Ion tail composed of ionized gas molecules pushed away from nucleus by solar wind
    • Appears straight and always points directly away from Sun

Components of comet nuclei

• Nucleus is solid, icy core of comet
  • Composed of mixture of ice (mostly water ice) and rocky material
  • Often described as "dirty snowball" due to composition
• Nuclei are typically small, ranging from few hundred meters to few tens of kilometers in diameter
  • Comet 67P/Churyumov-Gerasimenko (visited by Rosetta mission) has nucleus about 4 km in diameter

Behavior of cometary atmospheres

• As comet approaches Sun, nucleus begins to heat up
  • Increased temperature causes ices in nucleus to sublimate (change directly from solid to gas)
• Sublimated gases form coma, which can extend up to 100,000 km from nucleus
  • Coma grows larger and more active as comet gets closer to Sun
• Dust particles are also released from nucleus as ices sublimate
  • These dust particles form dust tail
• Outgassing occurs as volatile materials are released from the nucleus, contributing to the formation of the coma and tails

Findings of Rosetta mission

• Rosetta mission, launched by European Space Agency, studied comet 67P/Churyumov-Gerasimenko from 2014 to 2016
• Key findings
  • Nucleus has complex, "rubber duck" shape with two distinct lobes
  • Surface is covered in variety of terrains, including smooth plains, steep cliffs, and boulder-strewn regions
  • Nucleus has low density (about 0.5 g/cm³), indicating high porosity and mixture of ice and dust
  • Comet's water has different deuterium-to-hydrogen ratio than Earth's oceans, suggesting comets may not have been primary source of Earth's water
  • Organic compounds, including amino acid glycine, were detected in coma, supporting idea that comets may have delivered some of building blocks of life to early Earth
  • Evidence of cryovolcanism was observed, where internal heat causes icy materials to erupt from the surface

Cometary Orbits and Visibility

Variety of cometary orbits

• Comets can have wide range of orbital periods and shapes
  • Short-period comets: orbital periods less than 200 years
    • Examples: Halley's Comet (76 years), Comet Encke (3.3 years)
  • Long-period comets: orbital periods greater than 200 years
    • Example: Comet Hale-Bopp (about 2,500 years)
• Most comets have highly elliptical orbits
  • Spend most of time far from Sun in outer solar system
  • Become visible from Earth only when they approach inner solar system near perihelion (closest approach to Sun)
• Some comets have hyperbolic orbits
  • Not gravitationally bound to Sun
  • Will pass through solar system only once and then escape into interstellar space

Comet visibility and origins

• Comet visibility from Earth depends on several factors
  1. Proximity to Earth and Sun
  2. Size and activity level of nucleus
  3. Viewing geometry (position relative to Sun and Earth)
  4. Albedo (reflectivity) of the comet's surface
• Comet is most visible when it is close to both Sun and Earth, has large and active nucleus, and appears in dark sky away from Sun
• Long-period comets often originate from the Oort Cloud, a spherical cloud of icy bodies at the outer edge of the solar system
• Short-period comets are typically associated with the Kuiper Belt, a region of small, icy bodies beyond Neptune's orbit