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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. |
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. |
Close-Up  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 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.
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. |
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 | |
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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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