Asteroids are large pieces of rocky iron material. They mostly orbit the sun between Mars and Jupiter called the asteroid belt. We think that they are pieces of planets that never formed because of the big pull from Jupiter's gravity. With telescopes, astronomers can see thousands of objects in space. The Galileo Spacecraft took pictures of two of these asteroids. We are now studying asteroids using radar from Earth and The Space Telescope ( Hubble Space Telescope) The asteroid is called Gaspra. On its travel to Jupiter, The Galileo spacecraft passed Gaspra. This was the first close look at an asteroid. This is shaped like a potato. The craters on it even small objects like asteriods bump into other objects in space. Ida, another strangley shaped object, shows many of its craters on its surface. We can now look at some of the bigger asteriods with space telescopes. Not all asteriods stay in orbit near Mars and Jupiter. Some come closer to Earth. When they do, astronomers can take pictures using radar.

    There are many theories on where asteroids come from. One theory suggests that they are the remains of a planet that was destroyed in a massive collision long ago. More likely, asteroids are material that never formed into a planet. In fact, if all asteroids were put together, they would be less than 1,500 kilometers (932 miles) across which is  less than half the diameter of the Moon.
Thousands of asteroids have been identified from the Earth. By estimation, there are around 100,000 asteroids that are bright enough to eventually be photographed through Earth based telescopes. Asteroids that are near Earth are called meteoroids. When a meteoroid strikes our atmosphere, it forms a streak of light called a meteor. If the meteoroid does not burn up completely, what's left strikes Earth's surface and is called a meteorite. The best place to look for meteorites is at the ice cap of Antarctica.
Of all the meteorites examined, 92.8 % are composed of silicate (stone), and 5.7 %  are composed of iron and nickel; the rest is a mixture of the three materials. Stony meteorites are the hardest to identify because they are closely related to terrestrial rocks.
Since asteroids are material from the early solar system, scientists are interested in their composition. Spacecraft that has flown through the asteroid belt have found that the belt is really empty and that asteroids are separated by huge distances.
The impact of a large asteroid can be very catastrophic. For example, some astronomers believe that the object that smashed into the Yukatan Peninsula 65 million years ago was the event that wiped out the dinosaurs. The Gulf of Mexico marks the crater that was left behind. The asteroid that would have hit there would have been between 10-20 miles across. The hole it created was approximately 20 miles deep. It was instantly converted into a white hot vapor.
A large asteroid hitting the earth would create debris that would crash down into neighboring areas. It would throw dust into Earth's atmosphere, circling the globe, dimming the sunlight, ruining agriculture year round, and end civilization as we know it.
At this point in time NORAD has a small amount of people monitoring the sky for asteroids. An asteroid would enter quickly. Asteroids must be monitored.
Space Guard is an international network of telescopes where people work together to track asteroids and keep them from hitting the Earth. Blowing up an asteroid is not the answer. Large pieces still would impact on the earth and do lots of damage also.
The Earth is always encountering inner planetary debris in various sizes. Although the rarity increases with size, we know that there are asteroids big enough to cause a catastrophe if they collided with the Earth. Because of the influence of the major planets, the asteroid orbits tend to "wander", and whether (or when) a particular object might impact the Earth may require accurate knowledge of the orbit. Such as can be provided by radar observations.
The solar system has a large number of rocky and metallic objects that orbit the Sun but are too small to be considered planets. These objects are known as asteroids or minor planets. Most asteroids, but not all, are found in a band or belt between the orbits of Mars and Jupiter. Some have orbits that cross Earth's path. There is evidence that Earth has been hit by asteroids in the past. One of the least eroded and best preserved examples are the Barringer Meteor Crater near Winslow, Arizona.