Comets

  • Unlike asteroids, comets shine by bothreflected sunlightand the re-emission of absorbed sunlight. The gaseous outer parts of comets “glow” due to the action of sunlight.
  • Comets display a characteristic structure –

Nucleus – the main body, a few km in diameter, a solid to slushy ice/rock mixture

Coma – the expanding, evaporating envelope of gas and embedded dust, produced by heating of the nucleus as the comet approaches the Sun. The object we see from afar is essentially the coma.

Ion tail - straight “streamers”, always pointing away from the Sun, composed of ionized atoms & molecules, “pushed” away from the comet by the pressure of the solar wind.

Dust tail - curved, diffuse tail, made of dust particles, which are released as the nucleus of the comet evaporates. This dust is made visible by reflecting sunlight

  • Comets are composed of dust to sand to gravel-sized particles trapped within a matrix of water, carbon dioxide, methane, and ammonia ices. Often referred to as “dirty snowballs”.
  • Comet orbit within the solar system according to Kepler and Newton’s laws, but their orbits are unlike most other solar system bodies:

highly elliptical

not confined to the ecliptic plane

enter the inner solar system from all directions

  • Conforming to Kepler’s laws, comets spend most of their time far from the Sun, in the frigid outer fringes of the solar system. Orbital periods range from a few years to literally millions of years.
  • Short-period comets (orbital periods less than 200 years) originate in the Kuiper Belt, a donut-shapedregion of space out beyond Neptune.
  • Long-period comets (orbital periods up to millions of years) originate in the Oort Cloud, an enormous spherical region surrounding the entire solar system.
  • Starting in the 1990s, astronomers began to identify objects in the Kuiper belt, some of considerable size. It has become clear that many, large icy KBOs exist, and that Pluto, Triton & Charon are just perhaps the largest examples of this population of small solar system objects.
  • Chance collisions and small gravitational pulls from passing stars sometimes deflect these iceballs into orbits which then pass through the inner solar system.
  • With each passage near the Sun, comets partially evaporate as the ices are heated by sunlight. Eventually a comet will completely disintegrate.
  • As a comet “dies”, the solid materials will disperse along the path of the comet, sometimes producing a meteor shower, when they intersect Earth. Most comet dust ends up as a disk of interplanetary dust which extends throughout the inner solar system.
  • Famous comets:

Comet Halley: 76 yr period, visited by spacecraft in 1986. First comet predicted to return. In 1705 Edmond Halley predicted that a comet seen in 1687 would return in 1758. It did, which helped validate Newton’s Law of Gravity.

Comet Shoemaker-Levy: ripped apart by Jupiter, it collided with the planet in 1994. Cosmic collisions in action.

  • Comets are believed to be largely pristine chunks of the material from which the solar system formed. 4.6 billion years ago.
  • Several spacecraft have flown by comets, most notably comet Halley. NASA’s Stardust mission passed by Comet Wild 2 in January 2004, and will return bits of that comet to Earth in January 2006. ESA’s Rosetta mission is on the way to rendezvous with a comet in 2010, and will be the first spacecraft to fly along size a comet as it approaches the Sun.