Dinosaur killing space rock was a comet

The space rock that hit Earth 65 million years ago and is widely implicated in the end of the dinosaurs was likely a speeding comet. That is the conclusion of research which suggests the 180km-wide Chicxulub crater in Mexico was carved out by a smaller object than previously thought.
Many scientists consider a large and relatively slow moving asteroid to have been the likely culprit. Details were outlined at the 44th Lunar and Planetary Science Conference. But other researchers were more cautious about the results.
"The overall aim of our project is to better characterise the impactor that produced the crater in the Yucatan peninsula [in Mexico]," Jason Moore, from Dartmouth College in New Hampshire, told BBC News.
The space rock gave rise to a global layer of sediments enriched in the chemical element iridium, in concentrations much higher than naturally occurs; it must have come from outer space.
However, in the first part of their work, the team suggests that frequently quoted iridium values are incorrect. Using a comparison with another extraterrestrial element deposited in the impact – osmium – they were able to deduce that the collision deposited less debris than has previously been supposed.
The recalculated iridium value suggests a smaller body hit the Earth. So for the second part of their work, the researchers took the new figure and attempted to reconcile it with the known physical properties of the Chicxulub impact.