Just before dawn on February 22, 2021, a fireball lit up the skies across the Canadian province of Alberta when a 2-kg space rock vaporized as it sank into Earth’s atmosphere. Although the object descended from the Oort cloud – a group of comets at the edge of the solar system – it wasn’t a comet, the researchers now say. Data collected during the fall indicated that the object was made of rock rather than ice and was behaving like an asteroid.
Independent observers of the new work say the discovery sheds light on the processes that shaped our solar system and challenges the conventional wisdom that the Oort Cloud contains only icy comets. “It tells us that there has been scattering and deposition of material from all over the Solar System in the Oort cloud,” says Karen Meek, a planetary scientist at the University of Hawaii’s Institute of Astronomy.
The discovery could provide support for models indicating that objects from the asteroid belt scattered in the Oort cloud shortly after the solar system’s birth 4.6 billion years ago, says Bill Bottke, a solar system dynamics scientist 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 extending halfway to Proxima Centauri, the Sun’s closest neighbor, out of view of even the largest telescopes. “Everything we know about it is indirect,” says Dennis Vida, a meteorite astronomer at Western University who led the new study.
Scientists hypothesize that the Oort cloud became filled with comets when the gravitational muscles of Jupiter and other giant planets scattered widely over the icy bodies left by the formation of the outer solar system. Occasionally, a passing star gravitationally thrusts the body of the Oort cloud and sends it into the inner solar system. These objects are known as long-period comets, which are defined by their eccentric paths that take hundreds or even thousands of years to orbit the sun.
In 2016, Meech and his colleagues reported the discovery of an unusual long-period comet that was dark and lacked a bright tail of evaporating ice. In fact, the object looked more like an asteroid – evidence that the composition of the Oort cloud may not be very homogeneous. Meech called it the Manx comet, after a breed of cat without a tail. Although astronomers have since discovered dozens of these comets, they have not definitively proven that the objects are asteroids because they are so faint and fast-moving.
Now, with the Canadian Fireball, researchers believe they’ve caught one of these rare objects crashing into Earth’s atmosphere. “It was so bright and fast, it left a luminous train for several seconds,” says Vida, who presented the work today at the American Astronomical Society’s Department of Planetary Sciences meeting.
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-resolution sky cameras. Even spotting lightning on a spinning satellite caught fireball. By combining these observations, the team calculated the object’s trajectory and found that its orbit is about 1,000 years old – evidence that it came from the Oort cloud.
Despite its source, the body was clearly not similar. Most comet fireballs are fragile; They fragment and burn high in the Earth’s atmosphere. Veda says that this object, which was diving at a speed of 62 kilometers per second, penetrated much deeper, indicating that it was hard and rocky rather than icy. It also collapsed in two phases at two separate pressures – reflecting the disintegration of a common type of asteroid that drops meteorites to Earth.
Vida and his colleagues turned to historical data to see if other things 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, such as that of Alberta, that had been in orbit for a long time characteristic of an Oort cloud object. It also fell into the atmosphere like rocks, not ice. After comparing the events of 2021 and 1979 with the total number of long-range comets captured by two fireball nets, they calculated that between 1% and 20% of the Oort cloud should be rocky.
However, Bottke doubts extrapolation from such a small number of events. It is also believed that there may be a ‘survival bias’ towards the detection of rocky bodies because they are stronger than comets, skewing the true proportion of rocky bodies in the Oort cloud toward the lower end of the estimate.
But even if the Oort cloud is only 1% rocky, that would 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 discovery could support one hypothesis called the Grand Tack, which suggests that just 3 million years after the birth of the solar system, Jupiter sped inward toward the sun, roughly into Earth’s orbit, before returning to somewhere close to its contemporary position. . “As you can imagine, while doing this, it stirs things up” — including the fling of several rocky objects on its way into the Oort cloud, says Jackson.
Like Putki, Meech is also concerned about doing too many two fireball events. But she is keen that Vida and his colleagues capture more of these unusual streaks in the sky. “It’s so much fun,” she says. “I hope they get more of them.”