Presenting their findings at the 44th Lunar and Planetary Science Conference in Texas this week, Jason Moore explained two separate findings that lead to the team's conclusions. First, Moore and colleagues re-evaluated the iridium and osmium deposits at the K-Pa boundary. New data on those deposits indicate that the overall amount of space-derived iridium in the layer is lower than previously measured. As such, Moore and his colleagues concluded that the bolide -- the impactor -- was not 13 kilometers wide, because an asteroid that size would have left more iridium in the K-Pa boundary than what has been found. This by itself presents a problem, though: most asteroids are traveling too slowly for a smaller rock to generate the 3x10^23 J-giant boom that created Chicxulub crater. Comets, on the other hand, travel a lot faster than asteroids. A comet of 7 kilometers across traveling at typical comet velocities could release enough energy upon impact to create the crater and extinction event.