Scientists in the US and the Czech Republic have discovered the origins of the meteorite that helped to bring about the demise of the dinosaurs. Bill Bottke from the Southwest Research Institute in Boulder Colorado, together with David Vokrouhlicky and David Nesvorny from Charles University in Prague, used numerical simulations to wind back the cosmic clock over 160 million years to piece back together the puzzle of what plunged the planet into the equivalent of a nuclear winter and wiped out the dinosaurs. The team used computer simulations to track the orbits of these fragments back to the time when they first formed. Their calculations show that a large 170km diameter asteroid, known as the Baptistina parent body, collided with a smaller 70km object between Mars and Jupiter about 160 million years ago. The result was the creation of more than a thousand smaller asteroid pieces each measuring 1 kilometre or more in diameter. This occurred in a part of the solar system where the combined gravitational effects of Mars and Jupiter create what Bill Bottke terms a "celestial escape hatch". Here warming by the sun's rays triggers a process called the YORP effect, which can alter the path of small objects and nudge them into Earth-crossing orbits. The result was a 100 million year long meteor shower, which, say the researchers, included the 10km diameter "Chicxulub" impactor that struck Mexico's Yucatan Peninsula about 65 million years ago. And a recently-discovered meteorite fragment recovered from the Pacific in sediments dating back to the same era is a chemical match for the Baptista fragments still in orbit out beyond Mars. "So we can say with more than 90% probability that this breakup event 160 million years ago looks like the origin of the impactor that produced the mass extinction event 65 million years ago," says Bottke.