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 each year during the annual Orionids meteor shower.
Each time Comet Halley swings by the sun, solar heating evaporates about 6 meters of ice and rock from the nucleus. Comet debris particles are usually no bigger than grains of sand, and much less dense. Although they are very small, these tiny 'meteoroids' make brilliant shooting stars when they strike Earth's atmosphere because they travel at tremendous speeds. The Orionids meteor shower happens each year when Earth passes through the debris stream of Comet Halley, and meteoroids hit the atmosphere at nearly 90,000 mph.
In 1985 five spacecraft from Russia, Japan, and the European Space Agency were sent to rendezvous with Halley's comet. The ESA's Giotto probe, pictured left, captured close-up color pictures of Halley's nucleus showing jets of solar-heated debris spewing into space. In fact, just 14 seconds prior to its closest approach, Giotto was hit by a small piece of the comet which altered the spacecraft's spin and permanently damaged the camera. Most of the instruments were unharmed, however, and Giotto was able to make many scientific measurements as it passed within 600 km of the nucleus.
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.
How to View the Orionids
The best time to view the Orionid meteors is after midnight when Earth's rotation aligns our line of sight with the direction of Earth's motion around the Sun. Then we're heading directly into the stream of meteors. To find the Orionids, go outside and face South-southeast. The radiant, indicated by a red dot on the sky map, is near two of the sky's most familiar landmarks: the constellation Orion and Sirius, the brightest star in the sky. At midnight the radiant will be rising in the southeast, and by a.m. Orion will be high in the sky when you face due south.
Experienced meteor observers suggest the following viewing strategy: Dress warmly as the autumn nights are likely to be cold. Spread a thick blanket over a flat spot of ground. Lie down, look up and somewhat toward the south. Meteors can appear in any part of the sky, although their trails will tend to point back toward the radiant. A reclining chair is also handy.
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