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Halley's comet is among the most famous of all heavenly bodies. It orbits the sun once every 76 years and has been seen on each visit to the inner solar system since 240 BC. Many of the apparitions have been spectacular and some even play a role in human history. In 1066 the comet was so bright that it terrified millions of Europeans and was widely credited with the Norman victory at the Battle of Hastings. Halley's Comet of 1456 followed the 1453 invasion of Constantinople by the Turks, and was thought by some to be a gesture of heavenly support for their victory. There are accounts that Pope Calixtus III excommunicated the comet and ordered prayers to prevent the Turks from taking over all of Europe. In 1910 Earth passed through Halley's tail, prompting widespread fears that cyanogen gas from the comet would poison the atmosphere. Fortunately comet tails are very tenuous and Earth suffered no ill effects from the encounter. Although the wispy nature of comet tails was already well known to astronomers in 1910, sales of gas masks that year were brisk. 
Left: 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 eta Aquarid meteor shower.
Mark Twain was born in 1835 when Halley's comet was in the sky. Later in life he began to say that "I came in with Halley's Comet in 1835. It is coming again next year (1910), and I expect to go out with it. It will be the greatest disappointment of my life if I don't...." During the spectacular return of 1910 he died [refs. 1, 2]. Each time Halley's comet swings by the sun, solar heating evaporates about 6 meters of ice and rock from the nucleus. The debris particles, usually no bigger than grains of sand, gradually spread along the comet's orbit until it is almost uniformly filled with tiny meteoroids. Twice a year when Earth passes by the debris cloud there is a meteor shower. The first, on May 5th, is called the eta Aquarids and the second, on October 21st is called the Orionids. Sometimes both showers are referred to as "Halleyids" after the parent comet. So, although Halley's comet won't return to Earth until 2061, skywatchers can glimpse bits of the comet in just a few days when the eta Aquarid meteor shower peaks. 
Left:
Comet Halley moves around the sun in an elliptical
retrograde orbit,
opposite to
the direction of Earth's motion. As a result
Halleyid meteoroids and the earth approach one another
at high speed, much like two cars in a head-on collision.
Although the meteoroids are very small and light
they give rise to bright shooting stars because they speed through
the atmosphere at nearly 66 km/s (148,000 mph). The size of Earth's orbit is exaggerated by a factor of two in this diagram.
In 1985 scientists enjoyed the closest-ever view of comet Halley and its meteoroids when five spacecraft from Russia, Japan, and the European Space Agency were sent to rendezvous with the comet. The ESA's Giotto probe 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. 
Right:
An artist's rendering of the European Space Agency's Giotto probe.
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.
The best times to view the eta Aquarids in 1999 are during the early morning
hours of May 5 and May 6. The constellation Aquarius rises above the
horizon at about 2:30 am. The
the best time to look for meteors will be between
about 4 a.m. and dawn. That's when the local sky is pointing
directly into the meteoroid debris stream (see the diagram below),
and also when the elevation of the radiant exceeds 15 degrees as seen from
mid-latitudes.
The nominal
maximum of the eta Aquarids occurs at 11 UT on May 6, but the
shower's peak is broad and there
should be enhanced meteor activity between
10 UT on May 5 and 11 UT on May 6.
The bright gibbous moon
will make all but the brightest meteors difficult to see on both days.
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at a Glance
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Experienced observers suggest the following viewing strategy:
Dress warmly. Bring
a reclining chair, or spread a thick blanket over a flat spot
of ground. Lie down and look up somewhat toward the south. Meteors
can appear in any part of the sky, although their trails will
tend to point back toward the radiant, pictured in the sky map below.
| Web Links | |
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The eta Aquarids - From Gary Kronk's Comets & Meteors Web Site The Orionids - From Gary Kronk's Comets & Meteors Web Site North American Meteor Network - home page Leonids Live! -site of the live webcast of the 1998 Leonids Related Stories: |
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