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October 20, 1998: The last time Halley's comet visited Earth, in 1986, many observers were disappointed because the famous comet was barely visible to the naked eye. Some years are simply better than others, as in 1066 when the comet was so bright that it terrified millions of Europeans and was widely credited with the Norman victory at the Battle of Hastings. Comet Halley isn't officially scheduled to visit Earth again until 2061 when it swings through the inner solar system on its 76-year orbit, but fans of Halley can see bits and pieces of the comet tonight during the annual Orionids meteor shower. |
Some of the most important measurements came from Giotto's 'mass spectrometers', which allowed scientists to analyze the composition of the ejected gas and dust. It's widely believed that comets were formed in the primordial Solar Nebula at about the same time as the sun. If that's true, then comets and the Sun would be made of essentially the same thing -- namely light elements such as hydrogen, carbon and oxygen. Objects like Earth and the asteroids tend to be rich in heavier elements like silicon, magnesium, and iron. True to expectations, Giotto found that light elements on comet Halley had the same relative abundances as the Sun. That's one reason why the tiny meteoroids from Halley are so light. A typical debris particle is about the same size as a grain of sand, but it is much less dense, weighing only 0.01 gram. |
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![]() The image indicates the general region of the sky from which the Orionid meteors appear to emanate (red dot). This point, called the radiant, is really an optical illusion - the meteors are moving along parallel paths, but appear to come from a single point, just as a stretch of parallel railroad tracks will appear to meet at a point on the horizon. |
Web Links | |
October
Meteor Showers -
Washington University in St.Louis (external link) ESA Giotto spacecraft information - all about the comet Halley flyby Comet Halley - from "Views of the Solar System" (external link) International Meteor Organization (external link) |
Related Stories: Weak Impact -- the 1998 Perseid Meteor Shower 28 Aug 1998, NASA Science News Tune up for the Leonids 7 Oct 1998, NASA Science News Giacobinids dazzle observers 14 Oct 1998, NASA Science News More science headlines |
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Phillips |