Asteroids, Comets and Earth

From Wikipedia:

The Alvarez hypothesis posits that the mass extinction of the dinosaurs and many other living things during the Cretaceous–Paleogene extinction event was caused by the impact of a large asteroid on the Earth. Prior to 2013, it was commonly cited as having happened about 65 million years ago, but a 2013 paper by Renne et al. gave an updated value of 66 million years.^[1] Evidence indicates that the asteroid fell in the Yucatán Peninsula, at Chicxulub, Mexico. The hypothesis is named after the father-and-son team of scientists Luis and Walter Alvarez, who first suggested it in 1980.

In March 2010, an international panel of scientists endorsed the asteroid hypothesis, specifically the Chicxulub impact, as being the cause of the extinction. A team of 41 scientists reviewed 20 years of scientific literature and in so doing also ruled out other theories such as massive volcanism. They had determined that a 10–15 km (6–9 mi) space rock hurtled into earth at Chicxulub. For comparison, the Martian moon Deimos is 12 km (7 mi) and Mount Everest is almost 9 km (5.6 mi). The collision would have released the same energy as 100,000,000 megatonnes of TNT (4.2×10²³ J), over a billion times the energy of the atomic bombs dropped on Hiroshima and Nagasaki.

So if you assume a density, ρ, of 5 (similar to the Earth's average density), and since the diameter is 10 to 15 Km, you can calculate the mass, M, from the density and volume - picking a volume of a sphere with a nominal radius of 6 Km. The volume, V, of a sphere is 4/3*pi*R³. Then M = ρ*V

Once you have the mass, the velocity comes from the kinetic energy equation, E = 1/2 M*V^2, where E is 4.2×10²³ J

Solve for V and you'll have the velocity.

You can do the math.

And just btw, I still prefer the earlier term, Cretaceous-Tertiary, or K-T. The 'K-T Event' is just a cooler sounding name for the extinction.