The truth behind the demise of the dinosaurs may never be fully resolved, but a growing body of evidence has convinced many scientists that at least one of the culprits was a seven-mile-wide asteroid that hit Earth 65 million years ago near what is now the Yucatan peninsula. Exploding on impact, the monster rock produced a crater more than 180 miles wide, continent-drowning tsunamis, and winds far more powerful than any hurricane. In addition, dust kicked up by the explosion would have completely darkened daytime skies for months, devastating plant and animal life across the globe.

The prime piece of evidence supporting the theory is a thin layer of 65 million-year-old iridium-rich clay found in dozens of locations across the globe. (Iridium is extremely rare on Earth, but common in meteorites.) Furthermore, other rock samples from that era indicate an exposure to extreme heat and pressure, as would occur in an asteroid impact -- and there is that enormous 65 million-year-old crater in Mexico...

Still, the argument is far from over. Many paleontologists point out that the dinosaurs -- and many other life forms -- were already dying out before the massive extinctions at 65 million years ago. Global temperatures and sea level had been dropping for millions of years. The larger animal life of the time must have been feeling the effects. The rock layers from that era also show evidence of extensive volcanic activity, which could account for a deadly worldwide dust cloud as well as the elevated iridium levels.

So we may never know what really killed the dinosaurs, but research into the possibility that an asteroid caused such a major "extinction event" has at least opened our eyes to the real threat of such an impact -- and that puts us one up on the dinosaurs.

Not much in our lifetimes -- perhaps 1 in 10,000 -- but over thousands or millions of years, major impacts become pretty likely. Ancient craters on Earth's surface prove that large objects have hit Earth in the past, and there's no reason to think this won't continue in the future.

The chance of an impact depends on the size of the object: the bigger the comet or asteroid, the smaller the chance, since there are many more small objects out there than large ones. Tons of debris -- much of it in pieces smaller than grains of sand -- strike Earth's atmosphere and burn up every day. These are the "shooting stars" commonly seen at night. Some larger rocks survive their fiery descent to the surface; you can see some of these "meteorites" displayed in museums. The truly dangerous objects, those large enough to cause regional or global catastrophe when they hit, may appear once every few hundred thousand years. Therefore, the chance that such an object will hit us in any given year is roughly 1 in 300,000 -- nothing to lose sleep over.

Many scientists believe an extremely large asteroid (about six miles in diameter) struck Earth 65 million years ago near the present-day Yucatan peninsula of Mexico. The impact caused catastrophic conditions across the entire planet, including thick clouds of dust and ash that caused global temperatures to plummet, causing the extinction of the dinosaurs and much of the rest of the life on Earth.

The path Earth follows in its orbit around the Sun is littered with untold pieces of debris. Unlike the dinosaurs, we have the means to find the largest of these "Near-Earth Objects" (NEOs) and calculate their orbits, to see if they might ever come close to us. Currently, several different telescopes routinely and automatically scan the sky for them.

Perhaps a better question might be why isn't Earth covered with craters? Both Earth and Moon -- and the other inner planets -- were heavily bombarded by meteors and comets in the tumultuous days of the early solar system. Mercury and our Moon still bear the scars of the terrific pounding, while Earth, Venus and Mars show few signs of damage at all.

The relatively crater-free surfaces of Earth, Venus, and Mars can be explained by the existence on these three worlds of powerful surface-changing mechanisms, namely plate tectonics (Earth), the eroding effects of wind and water (Earth and Mars), and extensive volcanic activity (all three). These forces have helped smooth out the cratered landscapes.

While the Moon is known to have once been quite volcanically active -- great lava flows produced the dark "seas" on the surface -- it has long since quieted, and the extraordinarily tenuous atmosphere of the Moon is incapable of producing any erosion effects. Many of the craters we now see on the Moon remain almost exactly as they must have appeared hundreds of millions of years ago.