Tens of thousands of meteorites have been found on Earth, but most remain mysterious. These rocks obviously came from space, but without knowing their flight path, it's hard to pinpoint their exact origin, within or beyond our solar system.
But now researchers think they've linked a meteorite found in the Austrian Alps decades ago to a bright flash of light that emanated from a space rock speeding through Earth's atmosphere. Linking a meteorite to its parent “fireball” is rare, and the team suggests that these results show the utility of putting old data sets together. Their findings were published in May in the journal Meteoritics & Planetary Science.
In 1976, forest ranger Josef Pfefferle was clearing avalanche debris near the Austrian village of Ischgl when he noticed an oddly shaped rock. He brought the fist-sized black stone home and put it in a box.
Thirty-two years later, when Pfefferle heard about a meteorite discovered in Austria, he wondered if his strange rock might also have come from outer space, so he decided to take his rock to a university for analysis.
Pfeffere's find turned out to be a meteorite, and was relatively large, weighing over two pounds, and its unweathered appearance suggested it had fallen to Earth shortly before Pfeffere picked it up.
“It was a very new meteorite,” said planetary scientist Maria Gricevic of the University of Helsinki in Finland, who led the recent study. “It was very well preserved.”
Dr. Gricevic and his colleagues speculated that if the Ischgl meteorite had fallen to Earth relatively recently, its arrival might have been recorded on film. A network of 25 astronomical cameras across southern Germany has been collecting long-exposure images of the night sky since 1966. By the time the network ceases operation in 2022, it will have recorded more than 2,000 fireballs.
“The most logical thing to do was to trace it back to recent fireball sightings in the region,” Dr Grycevic said.
She and her team located negatives of images containing fireballs stored at the German Aerospace Center in Augsburg. After digitizing the images, the researchers estimated various parameters about the incoming meteorite, including its mass, shape, speed, and angle of entry. Using that data, the researchers zeroed in on 12 events that likely produced sizable meteorites. Only three occurred before 1976.
The research team reconstructed the trajectories of these three fireballs and calculated where the meteorite would most likely be found. Only one matched the location where the Ischgl meteorite was recovered. This led the researchers to conclude that the Ischgl meteorite was created by a fireball that arced low over the horizon in the early morning hours of November 24, 1970.
“This was a perfect match,” Dr. Grycevic said.
Dr Grytsevich and his colleagues calculated that the meteorite struck Earth at a speed of about 45,000 miles per hour. Dr Grytsevich said that while that's fast, it's within the range of speeds for meteorites that originate in our solar system. Meteorites that came from outside the solar system would have traveled much faster, he added.
The team believes the meteoroid that created the 1970 fireball once orbited the Sun relatively close to Earth, and Dr Grycevic said the meteoroid likely did not come from the main asteroid belt between Mars and Jupiter, where most meteoroids originate.
Linking meteorites to where they came from is important, says Mark Freese, a planetary scientist at NASA's Johnson Space Center in Houston, who was not involved in the study. “It changes meteorites from just rocks that you find on the ground to rocks that come from specific places in the solar system,” he says. To date, the orbits of about 50 meteorites have been determined, and Ischgl is the third oldest of them.
But the Ischgl case remains unresolved, says planetary scientist Peter Brown of Ontario's Western University, who was also not involved in the study. After all, he says, it's always possible the meteorite stayed on Earth's surface for more than six years. The Alpine environment where the meteorite fell may have preserved the rocks quite well.
“It could have been there for decades or even centuries,” Dr Brown said.
Still, he says, there's a great story here: “It's great to show that there's value in this old data.”
