What are asteroids?
An asteroid is a small body that orbits the sun. The size range from 10m across and up. The upper end isn’t well defined yet. Anything smaller than 10m across is considered to be a meteoroid. The upper end question is where do they go from being an asteroid to being dwarf planets.
When was the first one discovered? What is the largest asteroid?
The first asteroid to be discovered was Ceres in 1801. It is also the biggest with a diameter a 952km. When it was discovered they considered it to be a planet. As they found more and more of them they stopped calling them planets and renamed them asteroids. In 2006 scientist need to define what a planet is. In doing so they created a category called dwarf planet. Ceres meets the criteria for dwarf planet and is the only one in the inner solar system. It is also still considered to be the largest asteroid.
Where are they located?
Most asteroids are found in the asteroid belt between Mars and Jupiter. Some are found in Trojan points of a planet with Jupiter having the most. Trojan points are 60 degree in front of or trailing the planet. Thus these asteroids travel around the sun with their planet but are not orbiting the planet. The third type and the ones we want to keep an eye on are called Near Earth Asteroids. These have orbits that pass close to Earth.
How many are there?
Millions. They estimate that the belt has between 1.1 and 1.9 million asteroid over 1km in diameter. There are less Near Earth asteroid, about 7000 and only 500 to 1000 of those are over 1 km. Most of the Trojan asteroid are travelling with Jupiter. We think there are a lot of them and maybe as many as the asteroid belt. It is an area we don’t know a lot about yet.
What are they made of?
Another area that we don’t know a lot about. Various forms of rocks for most although Ceres is thought to have an icy crust. Some may be a solid single rock. A lot seem to be rubble piles of smaller rocks held together by gravity.
How do we find them?
The first ones were found by people looking through telescopes and keeping very detailed records. Things that moved compared to the background stars were planets and as stated earlier Ceres. They found a number this way. Later people used photographs and digital cameras through telescopes. A computer can watch an area of space and compare multiple photos for things moving.
How to make Potato Asteroids. Cool Recipe on the NASA.
Most of this came from my own head and reading through the Wikipedia article on asteroids.