Arctic Asteroid!
June
1, 2000 -- On January 18, 2000, residents of Western Canada
were surprised when a fireball as bright as the Sun streaked
across the morning sky. Exploding with an estimated yield of
5-10 thousand tons of TNT, the brilliant meteor attracted the
attention of defense satellites, seismic monitoring stations,
and just about anyone who happened to be standing outdoors within
700-800 km of the dazzling meteor's path.
"People described it as coming over the mountains, over
their heads, and then disappearing over the horizon," says
Dr. Peter Brown of the University of Western Ontario. "It
was very long-lasting and unusual. We estimate that this object
was about 7 meters across and 200 to 250 metric tons. This wasn't
your average meteoroid -- it was basically a C-class asteroid
detonating in the atmosphere over the Arctic!"
Above:Duane
Hilton's rendition of a brilliant fireball streaking above
a snowy Canadian landscape.
The first fragments of the object were discovered in January
by a local resident near the spot where the meteorite hit.
"The
fragments have been positively identified as carbonaceous chondrites,"
says Brown. "This is very important. Carbonaceous chondrites
are the most pristine, organically-rich meteorites known. The
ones that we find soon after a fall are even better than Antarctic
meteorites, which have been sitting out for a long, long time
-- in some cases 10,000 years or more. This is the first time
a meteorite has fallen in a cold arctic area and been quickly
recovered."Above: This sequence of pictures was captured by Ewald Lemke (Atlin Realty, Atlin, British Columbia) on January 18, 2000. It shows the expanding smoke train of the Yukon meteor over a 14-minute period. The first frame shows a smoky red vapor trail just 1 minute and 30 seconds after the initial flash.
In April, 2000, Brown and a team of scientists returned to the icy lake to look for more fragments that might have been uncovered as the snow began to melt with the coming of Spring.
 meteorite -- a rock from space that hit the ground
because it did not burn up entirely in the atmosphere (see also
meteor and meteoroid).C-class asteroid --these comprise about 75% of all objects in the asteroid belt. They are extremely dark and have chemical compositions similar to the Sun, minus hydrogen and helium (see also S-type and M-type asteroids). carbonaceous chondrite -- a dark, crumbly carbon-rich meteorite, similar in appearance to a charcoal briquette. |
"The outer layers were hot [due to friction with the
atmosphere], but carbonaceous chondrites are very porous and
don't conduct heat very well," he explained. The inside
of the object was still frozen by the icy cold of space when
the pieces reached the ground.
These are the only freshly fallen meteorite fragments ever recovered
and transferred to a laboratory without thawing. Keeping
the fragments continuously frozen minimized the potential loss
of organic materials and other volatile compounds in the fragments.
"This is the first carbonaceous chondrite found just after
landfall since the Murchison meteorite in 1969. This will be
the first time ever that we can use modern techniques to study
one of these. People are going to want to look for fullerenes
and amino acids. This meteorite was 6% carbon, by weight; other
carbonaceous chondrites are only 2%. It's very rich in carbon
compounds."
Right:
Samples of three carbonaceous chondrite meteorites. Left: Allende
meteorite fell on Mexico in 1969. Center: Sealed in a nitrogen
bag, a sample of the Yukon meteorite that exploded over western
Canada earlier this year. Right: Murchison meteorite sample,
which fell to Earth in Australia in 1969. [more
information from the NASA/Johnson Space Flight Center]
Carbonaceous chondrites, which comprise only about 2 percent
of meteorites known to have fallen to Earth, are typically difficult
to recover because they easily break down during entry into Earth's
atmosphere and during weathering on the ground.
"They are rare because they are so very fragile," continued
Brown. "You need an incoming meteorite that's huge -- something
that can afford to lose hundreds of metric tons as it blazes
through the atmosphere and still deposit many kilograms on the
ground."
The fragments -- lumps of crumbly rock with scorched, pitted
surfaces -- resembled partly used charcoal briquettes: black,
porous, fairly light -- about the same density as lightweight
pumice.
Brown and colleagues are trying to compute an accurate orbit
for the meteorite to discover where it came from.
"The
data we have now indicates the object followed a low-inclination
orbit coming from the asteroid belt," says Brown. "Its
incoming velocity was 15-16 km/s -- if we can pinpoint the velocity
with a precision a few hundred m/s, we might be able to tell
which asteroid family this object came from. So far we know that
the object has a typically asteroidal orbit, though it is remotely
possible that it might be related to short-period comets."
Although the Yukon meteor was spectacular, Brown notes that it
didn't add much to the amount of extraterrestrial material that
falls to Earth every day.
"Daily the Earth is bombarded by 80 to 100 metric tons
of microscopic space dust (in the form of 10-5 gm particles),"
said Brown. "Thus, the Yukon meteorite was only 2 or 3 days
worth of space dust."
Editor's note: The Nomenclature Committee of the Meteoritical
Society has officially designated the name Tagish Lake Meteorite
for the Yukon fall specimens.
Yukon Meteor Blast -January 25, 2000, Space Science News
Huge Fireball Dazzles Midwest -Thousands of people across the Eastern US saw a brilliant fireball streak across the night sky just 24 hours before the 1999 Leonids. (November 17, 1999)
Frequently Asked Questions about Fireballs -from the American Meteor
Society
YUKON
METEORITE MAY PROVIDE NEW WINDOW INTO THE UNIVERSE
-- Johnson Space Center press release H00-41.