There are many and lots asteroids in our solar system with many sizes. Most of them are small. Everyday there are asteroids that goes into our earth, but it burns before it landing at the ground because its small. How if there is a very very big asteroids strike the earth? What will happen if it near us?
We cannot predict if there is an asteroid that goes to the earth without unaided tools, but we can see it if we use a telescope. A very very big asteroid like the asteroid that hitted the earth 65 million years ago that made dinosaurs extinct is dangerous. It can prevent the light from the sun and the earth couldn't get enough light for living things in it.
We should be careful and alert