What killed off the dinosaurs: Asteroid or comet?

BBC : In an effort to better understand the extraterrestrial object that struck Earth 65 million years ago and may have been responsible for killing off the dinosaurs, Jason Moore of Dartmouth College and his colleagues have proposed that the object was probably smaller and faster than previously thought. By studying the amounts of extraterrestrial elements deposited by the impact, they determined that the object left less debris than previously supposed. But for a smaller object to have produced the 180-km-wide crater found in Mexico, it must have been traveling much faster than earlier estimates indicated. According to the researchers, the object was more likely a comet than an asteroid. Their findings are not definitive, however. Gareth Collins of Imperial College London does not believe that it’s possible to determine impactor size from geochemistry alone. Although he called the recent findings “thought-provoking,” he does not rule out a bigger, slower-moving object, such as an asteroid.