They trace it back to the Oort cloud:
The fact that a huge piece of extraterrestrial rock struck what is now the Yucatan Peninsula 66 million years ago is not controversial. And, year by year, scientists working in different disciplines keep amassing more evidence that this unprecedented event caused our planet’s fifth mass extinction. The incredible heat of impact debris returning to the atmosphere, global wildfires and a dust cloud that blocked the sun for years all played a role. In the end, almost three quarters of known species went extinct during the cataclysm…
Published in Scientific Reports today, the new study by astronomers Amir Siraj and Avi Loeb of the Harvard-Smithsonian Center for Astrophysics, propose that a series of break-ups and chance events sent the huge chunk of space rock our way.Riley Black, “Astrophysicists Chart Source of Asteroid That Killed Dinosaurs” at Smithsonian Magazine
Can’t seem to find the paper but anyway, at ArsTechnica we learn:
Siraj and co-author Avi Loeb concluded from their analysis that Jupiter’s gravitational field was strong enough to bump many such long-period comets from the Oort cloud off course, bringing them very close to the Sun. Such comets are known as “sun grazers”; about 20 percent of long-period comets become sun grazers, per the authors. And the Sun’s powerful tidal force in turn can break them into fragments.
A Hubble Space Telescope image of the comet Shoemaker-Levy 9, taken on May 17, 1994. Enlarge / A Hubble Space Telescope image of the comet Shoemaker-Levy 9, taken on May 17, 1994.
Siraj likened the effect to a pinball machine. “When you have these sun grazers, it’s not so much the melting that goes on, which is a pretty small fraction relative to the total mass, but the comet is so close to the Sun that the part that’s closer to the Sun feels a stronger gravitational pull than the part that is farther from the Sun, causing a tidal force,” he said. “You get what’s called a tidal disruption event, so these large comets that come really close to the Sun break up into smaller comets. And basically, on their way out, there’s a statistical chance that these smaller comets hit the Earth.” Siraj and Loeb’s calculations showed that there was an increase in the likelihood of long-period comets impacting Earth by a factor of 10, and that new rate jibes with the age of the Chicxulub impactor, making this a viable theory of its origin.Jennifer Ouellette, “Astronomers: A comet fragment, not an asteroid, killed off the dinosaurs” at Ars Technica
Their comet theory is not without its critics but it keeps life interesting.
Worried? Riley Black at the Smithsonian advises,
If all of this has you a little nervous looking at the night sky, though, don’t worry. The new model predicts that a comet or asteroid the size of the one that struck at the end of the Cretaceous is only going to strike Earth every 250 to 730 million years or so. What happened 66 million years ago was a truly exceptional and rare event, underscored by the fact that it is the only mass extinction in the history of life on Earth to be caused by an impact rather than Earth-bound causes like intense volcanic activity.Riley Black, “Astrophysicists Chart Source of Asteroid That Killed Dinosaurs” at Smithsonian Magazine
Here’s a simulation of what happened to the dinos: