Astronomie - Fireball from Solar System’s edge isn’t what astronomers expected


Rocky object suggests distant cloud of comets also contains asteroids


Just before dawn on 22 February 2021, a fireball lit up the skies across Canada’s Alberta province when a 2-kilogram space rock vaporized as it plunged through Earth’s atmosphere. Although the object hailed from the Oort Cloud—a conglomeration of comets at the edge of the Solar System—it wasn’t a comet, researchers now say. Data collected during its fall suggest the object was made of rock rather than ice and behaved more like an asteroid.

Independent observers of the new work say the find sheds light on the processes that formed our Solar System and challenges the conventional wisdom that the Oort Cloud only holds icy comets. “It’s telling us that there was scattering and depositing of material from all over the Solar System into the Oort Cloud,” says Karen Meech, a planetary scientist at the University of Hawaii’s Institute for Astronomy.

The discovery could provide support for models that suggest objects from the asteroid belt were dispersed into the Oort Cloud soon after the Solar System’s birth 4.6 billion years ago, says Bill Bottke, a Solar System dynamicist at the Southwest Research Institute. “This is very exciting,” he says. “Now, we have to see what we can do to explain it.”

First proposed by Dutch astronomer Jan Oort in 1950, the Oort Cloud is a spherical halo of comets that stretches out halfway to Proxima Centauri, the Sun’s nearest neighbor, well beyond the view of even the largest telescopes. “Everything we know about it is indirect,” says Denis Vida, a meteor astronomer at Western University who led the new study.

Scientists presume the Oort Cloud became populated with comets when the gravitational muscle of Jupiter and the other giant planets scattered far and wide the icy objects that were leftover from the formation of the outer Solar System. Occasionally, a passing star will gravitationally nudge an Oort Cloud object and send it plummeting into the inner Solar System. These objects are known as long-period comets, defined by their eccentric paths that take hundreds or even thousands of years to orbit the Sun.

In 2016, Meech and colleagues reported the discovery of an unusual long-period cometthat was dark and lacked a bright tail of vaporized ice. In fact, the object seemed much more like an asteroid—a clue that the Oort Cloud’s composition might not be so homogenous. Meech called it a Manx comet, after a breed of cat without a tail. Although astronomers have since detected dozens more of these comets, they have yet to definitively demonstrate that the objects are asteroids because they are so faint and fast-moving.

Now, with the Canadian fireball, researchers believe they have caught one of these rare objects crashing into Earth’s atmosphere. “It was very bright, very fast, and it left a luminous train for several seconds,” says Vida, who presented the work today at a meeting of the Division for Planetary Sciences of the American Astronomical Society.

In addition to hundreds of reports from witnesses who caught the fireball on dashboard and security cameras, Vida and his colleagues also worked with images from the Global Fireball Observatory, a network of high-precision sky cameras. Even a lightning monitor on an orbiting satellite caught the flare of the fireball. By combining these observations, the team calculated the object’s trajectory and found it had a roughly 1000-year orbit—proof it came from the Oort Cloud.

Despite its provenance, the object was distinctly uncometlike. Most cometary fireballs are fragile; they fragment and burn up high in Earth’s atmosphere. But this object, plunging at 62 kilometers per second, penetrated much deeper, Vida says, suggesting it was tough and rocky rather than icy. It also broke up in two phases at two discrete pressures–mirroring the breakup of a common kind of asteroid that drops meteorites to Earth.

Vida and colleagues turned to historical data to see whether other objects like this had been overlooked. They found that in 1979, a network of fireball cameras in Canada had tracked the demise of a 20-gram object that, like the Alberta object, was on a long-period orbit characteristic of an Oort Cloud object. It, too, fell through the atmosphere like rock, not ice. After comparing the 2021 and 1979 events with the total number of long-period comets caught by the two fireball networks, they calculated that between 1% and 20% of the Oort Cloud must be rocky.

Bottke, however, is skeptical about extrapolating from such a small number of events. He also thinks there might be a “survivability bias” toward detecting rocky objects because they’re tougher than comets, skewing the true proportion of rocky objects in the Oort Cloud toward the low end of the estimate.

But even if the Oort Cloud is just 1% rocky, it will challenge theorists to explain how these objects got there from the asteroid belt, says Alan Jackson, a planetary astronomer at Arizona State University, Tempe. He says the finding could lend support to one hypothesis called the Grand Tack, which suggests that just 3 million years after the Solar System’s birth, Jupiter swooped inward toward the Sun, nearly to Earth’s orbit, before moving back out to near to its contemporary position. “As you can imagine, in the process of doing that, it stirs stuff up”—including flinging many of the rocky objects in its path out to the Oort Cloud, Jackson says.

Like Bottke, Meech is also worried about making too much of two fireball events. But she’s eager for Vida and his colleagues to capture more of these unusual streaks on the sky. “It’s very interesting,” she says. “I’m hoping they’ll get more of them.”

Quelle: AAAS

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