OORT CLOUD

Comets are cosmic debris, probably planetesimals that originally resided in the vicinity of the orbits of Uranus and Neptune rather than in the warmer regions of the asteroid belt. Thus, the nuclei of comets are icy balls of frozen water, methane, and ammonia, mixed with small pieces of rock and dust, rather than the largely volatile-free stones and irons that typify asteroids. In the most popular theory, icy planetesimals in the primitive solar nebula that wandered close to Uranus or Neptune but not close enough to be captured by them were flung to great distances from the Sun, some to be lost from the solar system while others populated what was to become a great cloud of cometary bodies, perhaps 10 trillion in number.

In the original version of the theory, the Oort cloud extended tens of thousands of times farther from the Sun than the Earth, a significant fraction of the way to the nearest stars. Random encounters with passing stars would periodically throw some of the comets into new orbits, plunging them back toward the heart of the solar system. As a comet nears the Sun, the ices begin to evaporate, loosening the trapped dust and forming a large coma that completely surrounds the small nucleus, which is the ultimate source of all the material. The solar wind blows back the evaporating gas into an ion tail, and radiation pressure pushes back the small particulate solids into a dust tail. Each solid particle is now an independently orbiting satellite of the Sun, and the accumulation of countless such passages by many comets contributes to the total quantity of dust particles and micrometeoroids found in interplanetary space.

The Oort cloud is an immense spherical cloud surrounding the planetary system and extending approximately 3 light years, about 30 trillion kilometers from the Sun. This vast distance is considered the edge of the Sun's orb of physical, gravitational, or dynamical influence.

Within the cloud, comets are typically tens of millions of kilometers apart. They are weakly bound to the sun, and passing stars and other forces can change their orbits, sending them into the inner solar system or out to interstellar space. This is especially true of comets on the outer edges of the Oort cloud. The structure of the cloud is believed to consist of a relatively dense core that lies near the ecliptic plane and gradually replenishes the outer boundaries. One sixth of an estimated six trillion icy objects or comets are in the outer region with the remainder in the relatively dense core.

In addition to stellar perturbations where another star's Oort cloud passes through or close to the Sun's Oort cloud, are the influences of giant molecular clouds and tidal forces. A giant molecular-cloud is by far more massive than the Sun. It is an accumulation of cold hydrogen that is the birthplace of stars and solar systems. These are infrequently encountered, about every 300-500 million years, but when they are encountered, they can violently redistribute comets within the Oort cloud.

Tidal forces affecting the Oort cloud results from the sun and comets being different distances from these massive amounts of matter. The force on the comets from these tides is greater than that of passing stars, and comets beyond 200,000 AU are easily lost to interstellar space. This pull contributes to the steady state which replenishes the outer comets that are randomly distributed away from the ecliptic plane.

The total mass of comets in the Oort cloud is estimated to be 40 times that of Earth. This matter is believed to have originated at different distances and therefore temperatures from the sun, which explains the compositional diversity observed in comets.

Typical temperatures are four degrees Celsius above absolute zero. As temperatures move toward absolute zero, the kinetic energy of the molecules approach a finite value. There still remains some molecular energy, although it is at a minimum, at absolute zero.

Comets entering the planetary region for the first time, come from an average distance of 44,000 astronomical units. The Oort cloud is the source of long-period comets and possibly higher-inclination intermediate comets that were pulled into shorter period orbits by the planets. Comets can also shift their orbits due to jets of gas and dust that rocket from their icy surface as they approach the sun. Comets have initial orbits with widely different ranges, from 200 years to once every million years or more even though they get off course sometimes.

Long period comets can appear at any time and come from any direction. Bright comets can usually be seen every 5-10 years. Hale-Bopp was an unusually large and dynamic comet, ten times that of Halley at comparable distances from the sun, making it appear quite bright, even though it did not approach closer than 1.32 AU (197,000,000 km) to the Earth.

In 1950, Jan H. Oort suggested the existence of the Oort cloud from the physical evidence of long-period comets entering the planetary system. This Dutch astronomer, who determined the rotation of the Milky Way galaxy in the 1920's, interpreted comet orbital distribution with only 19 well-measured orbits to study and successfully recognized where these comets came from. Additional gathered data has since confirmed his studies, establishing and expanding our knowledge of the Oort cloud. Recognition of the Oort cloud has given explanation to what and where comets come from.