Asteroids are odd-shaped rock
Asteroids are odd-shaped rocks made of metal and other elements.
While the small ones are minor planets, the bigger ones can be considered planetesimals.
There are at least 100,000 known asteroids and we’ve named thousands of them.
Most of them are in the asteroid belt between Mars and Jupiter.
Why are there so many asteroids between Jupiter and Mars?
Think of the asteroids as being smack dab in the middle of a gravity tug-of-war. Neither side is winning so asteroids are stuck in between.
The asteroid belt is kilometers in size. If you were to take all the asteroids and put them together, they would make up just 4% of the mass of the moon.
This means that there’s really not a lot of material in the asteroid belt orbiting the sun. The largest asteroid is Ceres and is considered dwarf planet because of its size.
Asteroids didn’t mould into planets
The most common way of thinking is that asteroids are remnants of early solar system. During the creation of our solar system, dust coalesced to form planets like Earth.
Asteroids were loose chunks of metal and rock. When they were large enough in size, gravity would start taking over. But the smaller ones never formed a planet.
In the early solar system, more and more debris would form baby planetesimals. Then, there were millions of planetesimals whizzing around the solar system. Some collided and grew larger.
But asteroids and planetesimals were never allowed to become a planet because they never had the opportunity to do so.
How often do asteroids strike the Earth?
Asteroids collide into Earth about 30 times a year. Some are larger than others. We know this because NASA tracks them with their bolide events map.
We monitor asteroids because they swing into Earth’s orbit causing changes on Earth. There is evidence of large asteroid collision events in Earth’s history such as in the Cenozoic Era.
For example, there is evidence that an asteroid or comet crashed into the Gulf of Mexico. This impact was first revealed from the Chicxulub crater which ultimately triggered the extinction of the dinosaurs 65 million years ago.
Also in Siberia, the Tunguska event is the largest recorded impact in history. This impact was either caused by an asteroid or comet.