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The rock that hit the Earth was probably on the order of 10 km across. The crater it formed is more than 100 miles across. That is a fact. The crater still exists buried beneath the Yucatan peninsula and Gulf of Mexico. Experiments done with smaller impactors indicate debris is ejected from the impact (it has to go somewhere). Extrapolating from the empirical data and the known size of the crater researchers can estimate the amount of dust dumped into the atmosphere. Likewise, only 25 percent of the species found on the Cretacious side of the K-T bounary are found on the Tertiary side. Assuming the number of discovered species to be a statistical sampling of species extant at the time, researchers can deduce with a high level of confidence that 75 percent of the species extant at the time were killed off by the impact and subsequent ecological disaster. If you have a better method of determining these figures, please feel free to post them here.