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Dossiers scientifiques |
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Une météorite
détectée au large de la Californie
23 avril 2001 |
 Sommaire
L'événement
Le 23 avril 2001 à 06:12:35 (TU), les satellites militaires
américains détectent l'entrée d'un bolide
dans l'atmosphère au-dessus de l'Océan Pacifique.
L'objet est observé à une altitude de 28,5 km
au point (29,90 N - 133,89 W). L'énergie totale de l'événement
est de 4,6.1012 joules, soit une énergie équivalent
à 1 kT.
Des signaux infrasons sont observés sur quatre stations
TICE (IS10-Canada, IS26-Allemagne, IS57-Pinon Flat, IS59-Hawaï),
sur les stations expérimentales de Los Alamos (DLIAR,
SGAR) et sur les stations prototypes UAF (Alaska) et Flers (France).
Pour la première fois, la localisation d'une météorite
a pu être réalisée par croisement des azimuts
et des temps de propagation à partir de détections
à l'échelle planétaire. En France,
les signaux infrasons ont été détectés
à 15h45 TU après 10 300 km de propagation. La
fréquence centrale des signaux est comprise entre 0,05
et 0,5 Hz. La vitesse horizontale mesurée à la
station est proche de la vitesse du son. La vitesse apparente
de propagation depuis la source est de 291 m/s. Celle-ci correspond
a des temps théoriques de propagation de phases thermosphériques.
 |
Localisation par intersection
des azimuts |
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Traitement interactif
PMCC pour le calcul des paramètres de propagation
du signal détecté à Flers |
|
Signal détecté
à Flers remis en phase |
|
Diagramme polaire vitesse
horizontale / azimut du signal observé à
Flers |
L'article
du New York Times
May
29, 2001
Military Warning System Also Tracks Bomb-Size Meteors
By WILLIAM J. BROAD
In the early darkness of April 23, as Washington
was beginning to relax after the spy plane crisis
in China, alarm bells began to go off on the military
system that monitors the globe for nuclear blasts.
Orbiting satellites that keep watch for nuclear
attack had detected a blinding flash of light over
the Pacific several hundred miles southwest of Los
Angeles. On the ground, shock waves were strong
enough to register halfway around the world.
Tension reignited until the Pentagon could reassure
official Washington that the flash was not a nuclear
blast. It was a speeding meteoroid from outer space
that had crashed into the earth's atmosphere, where
it exploded in an intense fireball.
"There
was a big flurry of activity," recalled Dr.
Douglas O. ReVelle, a federal scientist who helps
run the military detectors. "Events like this
don't happen all the time."
Preliminary estimates, Dr. ReVelle said, are that
the cosmic intruder was the third largest since
the Pentagon began making global satellite observations
a quarter century ago. Its explosion in the atmosphere
had nearly the force of the atomic bomb dropped
on Hiroshima.
The episode shows how the system that warns of missile
attack and clandestine nuclear blasts is fast evolving
to detect bomb-size meteors as well. Now, it finds
them about once a month, on average. But Dr. ReVelle,
a scientist at the Los Alamos National Laboratory
in New Mexico, said in an interview that the developing
system was likely to find many more of the natural
blasts in the years ahead.
"The real
number is probably bigger," he said. "There's
no doubt about that. But we don't know how much
bigger."
Already, the system has shown that the planet is
being continually struck by large speeding rocks,
and that the rate of bombardment is higher than
previously thought. The blasts light the sky with
brilliant fireballs but people seldom see the blasts
because they usually occur over the sea or uninhabited
lands.
The rocky objects are anywhere from a few feet to
about 80 feet wide. They vanish in titanic explosions
high in the atmosphere, their enormous energy of
motion converted almost instantly into vast amounts
of heat and light.
The Air Force did not publicly disclose its imaging
of the recent blast until late May, more than a
month afterward. In a terse release on May 25, its
Technical Applications Center, at Patrick Air Force
Base in Florida, said the flash was "non- nuclear"
and consistent with past observed meteor explosions.
A Defense Department satellite, the Air Force said,
detected bright flashes over a period of more than
two seconds.
After that disclosure, Los Alamos got the military's
permission to reveal its own detection of the April
event. Its ground-based sensors are even more sensitive
than orbiting satellites to the repercussions of
meteor blasts. The ground-based sensors work like
sensitive ears to detect very low-frequency sound
waves, which radiate outward from an exploding rock
over hundreds and thousands of miles.
The sensors record sounds well below the range of
human hearing, including those from underground
nuclear tests as well as atmospheric blasts.
Dr. ReVelle said four arrays of the lab's sound
sensors had picked up the April blast. In addition,
he said, sound detectors in Los Angeles, Hawaii,
Alaska, Canada and Germany had picked up its shock
waves. Two sensors in South America made tentative
detections, he added.
"It was a big
event," he said. "There are people worrying
about impacts on the earth, and these things are
giving us a better understanding of the impact rate.
That's the real byproduct scientifically."
The speeding boulder was perhaps 12 feet wide, he
added.
An even more sensitive global ear is emerging as
the world's nations try to monitor the Comprehensive
Test Ban Treaty, a tentative accord that seeks to
end the exploding of nuclear arms and to police
compliance. When finished in the next year or so,
the global acoustic system is to consist of 60 arrays
that give complete global coverage, increasing the
odds that even more large meteor impacts will be
detected.
The disclosure of such intruders is seen as bolstering
the idea that the earth is periodically subjected
to strikes by even larger objects, including doomsday
rocks a few miles wide. Objects this size are predicted
to hit once every 10 million years or so, causing
mayhem and death on a planetary scale. |
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