The Cataclysm that Killed the Dinosaurs.

• Position: CHICXULUB IMPACTOR

It was tens of miles wide and forever changed history when it crashed into Earth about 66,000,000 years ago. The Chicxulub impactor, as it is known, was a plummeting asteroid or comet that left behind a crater off the coast of Mexico that spans 93 miles and goes 12 miles deep. Its devastating impact brought the reign of the dinosaurs to an abrupt end, scientists say, by triggering their sudden mass extinction, along with the end of almost three-quarters of the plant and animal species then living on Earth.

The enduring puzzle always has been where the asteroid or comet originated, and how it came to strike the Earth--and now a pair of researchers at Harvard University, Cambridge, Mass., believe they have the answer.

In a study published in Scientific Reports, Avi Loeb, professor of science, along with astrophysics major Amir Siraj, put forth a theory that could explain the origin and journey of this catastrophic object and others like it.

Using statistical analysis and gravitational simulations, Loeb and Siraj maintain that a significant fraction of a type of comet originating from the Oort cloud, a sphere of debris at the edge of the solar system, was bumped off-course by Jupiter's gravitational field during its orbit and sent close to the sun, whose tidal force broke apart pieces of the rock. That increases the rate of comets like Chicxulub (pronounced Chicks-uh-lub) because these fragments cross the Earth's orbit and hit the planet once every 250,000.000 to 730,000,000 years or so.

"Basically, Jupiter acts as a kind of pinball machine," says Siraj, co-president of Harvard Students for the Exploration and Development of Space. "Jupiter kicks these incoming long-period comets into orbits that bring them very close to the sun."

It is because of this that long-period comets, which take more than 200 years to orbit the sun, are called sun grazers, explains Siraj. "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.

You get what's called a tidal disruption event and 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.

The calculations from Loeb and...