Scientists examining the amount of rare metals released by the impact that caused the Chicxulub crater in Mexico (which scientists believe is where the object that hit the Earth to spark the mass extinction event at the end of the Cretaceous took place) now think that the Earth was hit by a smaller object than an asteroid but one traveling at a higher velocity, a comet in other words.
This is because the amount of debris deposited is less than would be expected if a larger asteroid struck the Earth. The object that hit the Earth is calculated to would need to be 5km in diameter to result in the rare metals like iridium detected however the size of the crater would indicate a much larger object, unless it was traveling fast. Other scientists say that it might not be possible to say conclusively from the metals detected and their displacement may not have been uniform.
However the strength of the impact was sufficient to send huge amounts of dust into the atmosphere and cause the "Nuclear Winter" that killed off the Dinosaurs and many other creatures, 70% of all species.