Thank you for sending in such a terrific question! The very same question has stumped some of the world's smartest scientists for a very long time. This should come as no surprise. After all, it's been almost 65 million years since the dinosaurs last roamed the Earth and there certainly were no people on the planet at the time to see what really happened. Thank goodness: could you imagine a world like the one in Jurassic Park where dinosaurs and people lived together? I'm afraid we wouldn't make terribly good neighbors!

Truthfully speaking, scientists don't know exactly why the mighty t-Rex went extinct, but we have some really good ideas. Scientists call these ideas "theories" and a really big clue to the newest theory glows brightly in the night sky. If you were to use a telescope to get an up-close look at the moon, you certainly wouldn't see any cows jumping over it (unless, of course, you used a really strange telescope)! On the other hand, you'd probably quickly notice all of the holes and craters on its surface (one reason why people with very big imaginations suppose that the moon resembles Swiss cheese). Now we're onto something!

Would you be surprised to learn that the Earth also has holes and craters on its surface? Scientists know about these holes and craters because they sent very powerful cameras into space to take pictures of the Earth's surface. Not a bad idea, huh? Some of these holes are gigantic. Ask your teacher for a yardstick and try to image what a hole that measures 3000 yardsticks across would look like. That's how big some of these craters are! The interesting thing about some of these holes, besides their enormous size, is their age. Curiously enough, some seem to be about the same age as the oldest dinosaurs that roamed the planet. Where did these holes come from -- and what do they have to do with dinosaurs, especially our beloved T-Rex?

Let's leave this question for a moment and gaze back into the night sky. We already talked about the moon, but what else is up there in outer space? Well, the tiniest specks of twinkling light are the stars spinning around in space. You might be interested to know that these tiny points of light are smaller versions of a much bigger star that we call the "sun". Besides our home planet of Earth and the stars and the moon and the sun, there are also other planets, like Mars and Venus and Jupiter, which are so far away that we can scarcely see them without a telescope. Can you name the other planets? Is there anything else that we can add to our list?

If you guessed meteoroids and comets, you'd be right on the money! You might have even been lucky enough to see their long, bright tails streaming behind them as they race around in the blackness of space. Scientists now believe that about 65 million years ago, as the dinosaurs were going about their business on a much younger planet, an enormous meteoroid crashed into Earth. The huge amount of dust that flew up into the sky blocked out the sunlight for a very long time, which made the planet very cold and killed most of the plants that use the sun's rays to make food for themselves. Then all the dinosaurs that ate plants -- the herbivores -- had nothing to eat or couldn't handle the cold, so they died. This isn't sounding so good, is it? As you may have already guessed, the dinosaurs that ate other dinosaurs -- the carnivores -- also eventually died, and so the age of the dinosaurs came to a very sudden end. All that remained was a giant crater and a few kinds of plants and animals.

So you see, T. rex wasn't the only dinosaur to go extinct -- most of the other big reptiles did as well. You should look on the bright side though, because their extinction made room for a very different kind of animal to roam the Earth. Unlike the big reptiles, these animals had fur and their mothers' made milk to feed their young. These animals -- the mammals -- took their place as the new rulers of the Earth and people like you and me are among them. A lucky break, I suppose!

Nathaniel, I want you to know that scientists who study space tell us not to worry about any big meteoroids crashing into the Earth anytime soon. Some really small meteoroids race toward the Earth every single day, but burn away before they ever hit the ground. Most people call these tiny meteoroids "shooting stars" and make fond wishes when they see them flashing in the night sky.

I got nice information about this question on the next link:

T-rex_extinction

T. rex went extinct during the K-T mass extinction, about 65 million years ago. This extinction killed the remaining dinosaurs (not just T. rex) and many other animal and plant groups.

There are several theories about the dinosaurs extinction, but one is that this extinction was probably caused by a catastrophic asteroid colliding with Earth. It is thought that an asteroid 4-9 miles (6-15 km) in diameter hit the Earth off the coast of Mexico. The impact probably penetrated the Earth's crust, scattering dust and debris into the atmosphere, and causing huge fires, volcanic activity , tsunamis , and severe storms with high winds and highly acidic rain. The impact could have caused chemical changes in the Earth's atmosphere, increasing concentrations of sulfuric acid, nitric acid, and fluoride compounds. The heat from the impact's blast wave would have incinerated all the life forms in its path.

The dust and debris thrust into the atmosphere would have blocked most of the sunlight for months, and lowered the temperature globally.

Those organisms that could not adapt to the temperature and light changes would die out. Since plants' energy is derived from the sun, they would likely be the first to be affected by changes in climate. Many families of phytoplankton and plants would die out, and the Earth's oxygen levels may well have dramatically decreased, both on land and in the oceans, suffocating those organisms which were unable to cope with the lower oxygen levels.

Major changes in the food chain would result from all of these environmental upheavals. The herbivores (plant eaters) who ate those plants that died out would starve soon after these plants died. Then, at the top of the food chain, the carnivores (meat eaters) like T-Rex, having lost their prey, would have to eat each other, and eventually die out. Their large carcasses must have provided smaller animals with food for quite a while.