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Halley's Comet Returns In Bits And Pieces


Halley's comet returns in bits and pieces
Marshall Space Flight Center Space Science News
http://science.nasa.gov/newhome/headlines/ast20oct98_1.htm

Shooting stars should be visible October 21st & 22nd

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 it's 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. 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.

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.

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