Dec. 20, 2000 -
NASA AMES RESEARCH CENTER, CA.
Astronomers are forecasting a brief shower of Ursid meteors on
Thursday night, when the Earth will hit a dense trail of dust created
in 1405, before the birth of Columbus. Excellent viewing is predicted
over both the United States and Canada.
Dr. Peter Jenniskens of the SETI Institute, who is based at the NASA
Ames Research Center in the heart of California's Silicon Valley, and
researcher Esko Lyytinen, from Helsinki, Finland, have used research
and observation skills honed during the 1999 Leonid meteor storm to
make the prediction.ÿý The shower is expected to hit the Earth at 2:29
a.m. EST on Dec. 22 (or 11:29 p.m.ÿý PST Dec. 21).
"The normally ordinary-looking Ursids have long puzzled researchers
because of two intense showers seen in 1945 and 1986," said
Jenniskens. "Both of these showers lagged the passage of the comet by
as much as 6 years.ÿý By the time these meteors hit the Earth, the
comet was on its way back to the outer reaches of the solar system,
almost as far from Earth as it ever gets."
The Ursid meteors are caused by dust particles ejected from comet
8P/Tuttle that plunge into Earth's atmosphere. The meteors appear to
come from the constellation of Ursa Minor (the Little Bear) close by
the pole star. In its 13.6-year orbit around the sun, comet 8P/Tuttle
never ventures inside Earth's orbit. As a result, its meteor shower
usually is unspectacular. It has been 6 years since the last return
of the comet.
In a paper submitted to WGN, the journal of the International Meteor
Organisation, Jenniskens and Lyytinen explain the 6-year lag of the
meteor shower, and forecast this year's rich display. Once the
meteoroids are ejected into space, they say, it takes as much as six
centuries before their orbits are sufficiently changed by the planet
Jupiter so they can hit the Earth. During that time, the particles
slowly fall behind the orbiting comet that produces them.ÿý After six
centuries, that lag amounts to just about 6 years.
Jenniskens and Lyytinen are the first researchers to link the cycles
of intense Ursid showers with a particular passage of comet
8P/Tuttle.ÿý The 1945 outburst was caused by dust shed in 1392, while
the 1986 shower was dust from 1378, six centuries ago. The
researchers calculate that this year, the Earth will pass the center
of yet another trail at a distance of only halfway to the moon.
On Thursday, the Earth will find in its path the trail of dust
ejected in 1405, they say. The shower is expected to last 2 to 3
hours, and possibly reach rates of one meteor per minute. Many of
these will be faint meteors, so observers are encouraged to go to a
dark location away from city lights for best viewing.
This year's Ursids will provide an unexpected bonanza for
astrobiologists, the scientists who study the origin, evolution,
distribution and future of life in the universe. The shower will
enable researchers to probe the composition and morphology of grains
from a comet not previously sampled. Like the Leonid meteors, the
Ursid meteoroids can be precisely dated, but they are different in
important ways because they have spent six times longer in the solar
system environment and plow into Earth's atmosphere with just half
the Leonids' speed.
NASA Ames Research Center is NASA's lead center for astrobiology.
The central administrative office of the NASA Astrobiology Institute
(NAI), a research consortium involving academic, non-profit and NASA
field centers, also is located at Ames. The NAI has international
affiliate and associate members.
Futher information and a pdf file of the original paper can be found