Comet, any of many astronomical objects identified by a long, luminous tail that appears in the part of a comet's orbit when it passes closest to the sun. Comet
Hale-Bopp's Ion Tail
Explanation: What's happening to Comet Hale-Bopp's blue ion tail? The comet's ion tail is fluctuating more rapidly as it passes a region of changing solar wind. As the comet passes from north to south, it crosses the plane of the Sun's equator, where the solar magnetic field changes direction. Ions from the solar wind, which cause Comet Hale-Bopp's ion tail, to act unpredictably here. Therefore, Comet Hale-Bopp's ion tale may show unusual structure or even a disconnection - where the tail appears to break off and then reestablish itself later. The picture above, taken April 30th, 1996, indeed shows unusual structure in the blue ion tail
A comet is generally thought to have a small, sharp core called a nucleus, surrounded by a cloudy disk called the coma. American astronomer Fred L. Whipple proposed that the nucleus, containing practically all the mass of the comet, is a "dirty snowball" of ices and dust. The snowball theory is supported by various data, including the fact that comets move in orbits that depart from Newtonian laws. This is evidence that escaping gases produce a jet action, propelling the nucleus away from its predicted path. The head of a comet, including the hazy coma, may exceed the planet Jupiter in size. The solid portion of most comets, however, would measure only a few cubic kilometers. Large comets were regarded as atmospheric occurrences until Danish astronomer Tycho Brahe proved that they were astronomical. British scientist Sir Isaac Newton demonstrated that the movements of comets follow the same laws that control the planets. British astronomer Edmund Halley successfully predicted the return of an earlier comet.
The Core of the Comet
Until the Russian "VEGA" and the European "Giotto" spacecraft flew by Halley's core, people had never really seen the heart of a comet! By the time a comet like Halley becomes visible to us, the little "dirty snowball" that is its core is completely veiled by the much larger coma -- the cloud of evaporated ice and loosened dust produced by the action of sunlight on the core. As a comet nears the Sun, its coma can stretch some 100,000 kilometers or more across (10,000 times larger than the iceberg that produced it.) Thus, it is fair to say that the most eagerly awaited result from the space missions was our first glimpse of what the comet looked like at its heart.
That glimpse produced a great surprise the core is not round, but instead has an irregular shape rather like a huge unshelled peanut. Photographs taken by the European Giotto probe - which passed closer than any of the other probes showed that the comet's nucleus was about 15 kilometers long and 8 kilometers wide. The pictures returned by Giotto (and the Russian VEGA probes) also reveal the comet nucleus to be much darker than we expected. Most of the surface is so black that it effects only about 4% of the light that hits it - less than black velvet reflects! Astronomers hypothesize that this dark material is dust and complex molecules left over when the comet's more volatile ices evaporated as it came close to the Sun. The photographs show bright geysers of gas and dust spewing out from the dark surface.
In fact, Giotto's cameras almost missed the comet's nucleus as a result of its unexpected darkness. Since Earthbound controllers couldn't communicate with the distant spacecraft quickly enough to make changes during the crucial moments of the encounter, the sequence of photographic instructions had been coded into onboard computers. Since astronomers expected the comet's nucleus to be made of bright reflective ices, they programmed the cameras to center the photos on the brightest object they saw". That brightest object turned out to be a bright geyser of escaping material but, luckily, the nucleus can still be seen off to the side of each frame!
Giotto's photographs, which are being computer processed to bring out every possible detail, also show some craters on the nucleus and several features that look like Earth mesas. The vents from which the material for the coma and tail was escaping when the comet was near the Sun made up only 10% or so of its surface. The vents seem to be active only when exposed to the Sun, shutting off very quickly as the rotation of the comet brings them to its night" side.