"We have not searched for them all [asteroids], we only know a small fraction of the ones that exist, and...
those particular ones are not going to hit any time in the next 100 years or so, which is about as far as we can reliably calculate the orbits, but the other 90 percent that we haven't found yet could hit any time." "A comet can have the hiccups, and those hiccups can change its course in space.
From the amount and distribution of iridium present in the 65 million year old "iridium layer", the Alvarez team estimated that an asteroid about 5 milies wide (10-14 kilometers) must have collided with the Earth.
This iridium layer has been found worldwide at 100 different sites.
On June 1, 2006 scientists, using satellite data, discovered the 300-mile-wide crater buried more than a mile beneath the East Antarctic Ice Sheet in what's known as the Wilkes Land region of Antarctica.
It utterly dwarfs Meteor Crater in Arizona, which is about three-fourths of a mile (1.2 kilometers) in diameter.'Because of the nature of the orbit, we cannot predict thousands of years into the future for this object,' said Alan Harris, a senior research scientist at the Space Science Institute some rumors have suggested the asteroid's forecasted course might be off by enough to cause a collision with Earth 'The actual path of it has indeed varied a bit from the original calculated,' Harris said." "Orbits change with time because of gravitational tugs by the Sun and planets, among other factors...the 323-day orbit of asteroid 2004 MN4 lies mostly within the orbit of Earth.indicating that the body that collided with Earth came in in chunks - bang, bang, bang..." The impact disaster wiped out 70 percent of land vertebrates and 90 percent of marine animals.The research performed by NASA, the National Science Foundation, and three leading universities, revealed a type of carbon structures called fullerenes in 251 million year old geological samples taken from Japan and China.In 1981, exploring for oil reserves in Yucatan, Mexico, Penfield and Camargo recognized that the gravity and magnetic anomalies centered on the village of Chicxulub, at the tip of the Yucatan peninsula in Mexico, resemble those identified at large impact structures.