Upwards of 300 professional and amateur astronomers will scatter around the world this week to follow and count the Orionid meteor showers that scientists think are the dust tails from past appearances of Halley's Comet.
At the same time, separate groups of astronomers will gather at meteor radar stations in Canada, Sweden, Czechoslovakia and the Soviet Union to track the ionized trails 60 miles high left by the showers. Research aircraft in Canada and the Soviet Union also will go aloft this week to see if they can catch the "falling stars" and return with samples of the fine dust tails that could have been left in space by Halley's Comet as long ago as 240 B.C.
"There are two meteor showers associated with Halley that occur when the Earth makes its closest approach to Halley's orbit," said Stephen J. Edberg of the Jet Propulsion Laboratory in Pasadena, Calif. "One is the Aquarid shower in May, and the other is the Orionid shower that we are about to observe," said Edberg, a member of the International Halley Watch.
The upcoming meteor shower begins early Monday and continues through the pre-dawn hours each day to Oct. 24. The shower peaks either Monday or Tuesday, when as many as 25 streaks of light will shoot across the eastern sky every hour from 1 a.m. until the sun rises. If the sky is clear, the showers should be visible in the Washington area.
The shower is called Orionid because it appears to come from the constellation Orion, which moves across the eastern sky toward the southeast all this week.
"Orionid meteors occur when the Earth intersects the inbound leg of Halley's dusty orbital track," said Francis Reddy, author of the book "Halley's Comet." "The Aquarid showers occur in May when Earth passes through the outbound leg of the comet."
Curiously, this week's shower will appear to be coming from the same direction as Halley's Comet. Still outside the orbit of Mars, the world's most famous comet is on its way to making its 30th recorded appearance, an apparition it makes every 76 years.
"This is just a coincidence," said Richard Schmidt of the Naval Observatory here. "The comet is crossing the skies very rapidly right now."
Scientists have long suspected that passing comets trigger the meteor showers we see throughout the year as "shooting stars." Comet Encke produces a daylight meteor shower every year at the end of June. The Leonid and Perseid meteor showers that streak the sky with light in August and September are the leftovers of comet tails.
One ancient comet that is now a burned-out asteroid has been linked to the Geminid meteor showers that appear to come from the constellation Gemini every year around Dec. 13. The orbiting Infrared Astronomical Satellite found this ancient hulk inside the orbit of Mercury more than a year ago.
Clearly, the hulk was an asteroid, measuring about 1.2 miles across. But scientists wondered what was it doing so close to Mercury and so near to the sun. Only comets come that close to the sun. Plotting its orbital path out from Mercury, scientists at the Jet Propulsion Laboratory soon found that it coincided exactly with the orbit followed by the Gemini meteor showers.
"To me, this was one of the biggest single surprises we had through the entire IRAS mission," said Gerry Neugebauer. "It lends a lot of support to the idea that many of the asteroids we see are the remnants of ancient comets."