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Comet Hale-Bopp

Comets have always captured the imaginations of humans. Probably because of their unexpected appearances, huge apparent sizes on the sky, mysterious and rapid motions about the sky, and, finally, their equally mysterious disappearances, civilizations have been awed and frequently terrified by comets. The history of our understanding of these fascinating objects is quite interesting. Somewhat surprisingly, in the last few decades, comets have taken on new meaning and increased significance in modern astronomy as well.

Nucleus of Halley's Comet
The nucleus of Halley's Comet as imaged by the Giotto spacecraft.
Comets are the largest bodies in the solar system, their tails sometimes reaching between the orbits of planets! These tails, however, are made up of extremely low-density gases and dust particles. The source of the gas and dust is the comet nucleus, which is too small to be seen from Earth-based telescopes. In addition, the nucleus is obscured by the roughly spherical coma, which is the bright gas radiating in all directions from the coma. These nuclei are assumed to similar to dirty snowbanks. A flyby image taken of the nucleus of Halley’s Comet showed it to be a peanut-shaped object with an extremely dark surface. This surface was probably due to residual amounts of hydrocarbons left behind in a tarry residue as the lighter gases sublimated from the icy nucleus.

Because the comet is made up of dust, small particles, and ices, the tail is composed of gas (from the sublimated ices) and small particles and dust.

Comet Hale-Bopp showing a gas tail on the left and a dust tail on the right. Photo by R.S.Hoyle 8 March 1997. The tail often separates into two components. One tail is composed of gas, being pushed away from the Sun by the force of solar radiation; the other tail is composed of dust that stretches back along the orbit of the comet. A spectrum of the gas tail shows a variety of simple molecules that have been excited into fluorescence by the solar radiation; the dust tail shows the solar spectrum, reflected off the dust particles.

Comets are thought to have been created at the same time as the planets of the Solar System. Small icebergs or snowbanks, made of primordial Solar System molecules were probably plentiful throughout the planet formation region near the Sun as well as beyond the planets. Computer simulations of the motions of comets show that their orbits are quickly perturbed by the planets so that they are either captured in the inner solar system, where they quickly evaporate, or they are ejected into orbits that are entirely outside the solar system. Therefore in the early millennia of the Solar System, the resident comets were either destroyed or ejected to distances far from the planets.

In the 1950s, the Dutch astronomer Jan Oort, studying the characteristics of the known periodic comets, showed that there was probably an extended cloud of comets that extended from beyond the orbit of Pluto to about halfway to the nearest stars. Particularly significant in this analysis was the observation that approximately half of the long-term comet orbits were retrograde. This extended cloud of comets, now called the Oort cloud, is considered the reservoir from which many new comets are recruited. Presumably millions of comets wander in huge orbits far from the Sun and easily perturbed by nearby stars and occasional encounters with each other.

Illustration of the Oort cloud and Kuiper Belt. NASA/JPL.

Occasionally one of these perturbations sends one of the comets towards the inner part of the Solar System. Coming from such a large distance, the comet has a very narrow orbit, in the shape of a parabola, rather than the elliptical shapes of the planets or the domestic (periodic) comets of the inner solar system. Now the comet has roughly equal chances of being captured into the inner Solar System, returning to the Oort cloud to continue in its current orbit of millions of years, or being ejected out of the Solar System entirely to become an interstellar comet.

The Dutch-American astronomer Gerad Kuiper postulated a nearer band of comets, just outside the orbit of Neptune. Detailed calculations in the 1980s confirmed that the gravitational effects of the inner Solar System sweep the area clean of comets but, at the edge of the Solar System, a band of comets could congregate. This band, now called the Kuiper Belt, is apparently not affected in a major way by Pluto because of its relatively small mass. In an amazingly difficult series of observations, comets in the Kuiper belt have been observed with the Hubble Space Telescope.

Comets have interesting side effects. Astronomical satellites designed to observe in the far infrared detected narrow strips across the sky. It was determined that these strips were comet orbits marked with dust grains (instead of breadcrumbs) left behind as the dust was released from the snow bank nuclei. The dust, now moving along the comet orbit at slightly different speeds than the comet itself, has been heated by the Sun and glows weakly in the far infrared part of the spectrum.

These comet streams contain particles larger than dust. If the orbit of the Earth intersects with the comet orbit, then, at the same date each year, the comet debris will plunge into Earth’s atmosphere providing a delightful meteor shower. Most meteor showers are associated with known comets.

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