Dossiers scientifiques

Une météorite détectée au large de la Californie 23 avril 2001


  Pour en savoir plus
  L'article du New York Times


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

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

Pour en savoir plus
Listening for Nukes: a Meteor Detection Project
NASA Near-Earth Objects Program
Meteorits and their properties
Meteorite Central
Timbres et pièces de monnaie à l'effigie des météorites

L'article du New York Times

May 29, 2001
Military Warning System Also Tracks Bomb-Size Meteors

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.