Halley's comet returns in bits and pieces
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Halley's comet returns in bits and pieces
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.
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.
Shooting stars should be visible October 21st &
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.
This image of Comet Halley's nucleus was taken by the Giotto spacecraft during a flyby on March 13, 1986. Scientists estimate that about 10% of the surface was boiling off into space. The stuff that boiled off Halley in 1986 may one day be seen again during an Orionid 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 nights of October 21st & 22nd are the best times to watch.
- Maximum hours rates are typically 20/hr and meteors are described as "fast".
- The radiant is at RA=06h20m, DEC=+16o, just above the left shoulder of Orion.
- The average magnitude of an Orionid meteor is 3.
This week's new moon makes for ideal observing conditions. You don't need binoculars or a telescope, the naked eye is usually best for seeing meteors which can streak more than 45o across the sky. The field of view of most binoculars and telescopes is simply too narrow for good meteor observations.
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. Web LinksOctober 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
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