Unlike comets that are made up of mostly ices and gas, asteroids are small objects orbiting the Sun that consist of mostly rocks and metals.
Asteroids were once mistaken for small planets. Giuseppe Piazzi thought an asteroid was a planet in 1801.
He even gave the object the name “Ceres” after the Roman harvest goddess. However, as Piazzi continued to watch, he found out that these were not planets but instead, some other form of celestial body.
Looking through their telescopes he noted that they shared a characteristic that was similar to viewing stars.
It is this reason that the name asteroid was given, because it means “star-shaped.” Astronomer William Herschel came up with the name “asteroid” in 1802.
A larger percentage of the asteroids in our Solar System orbit the Sun in an area referred to as a “band” between Jupiter and Mars.
This band is called the Asteroid Belt. There are hundreds of thousands of asteroids with the Asteroid Belt and it is believed that the gravity from Jupiter has kept most of them away from our Earth.
Jupiter acts like a kind of “protector”. There are around 200 asteroids that are known to be bigger than 100 km in diameter, although most asteroids are considered to be small objects.
Scientists have a theory that the asteroids that we currently have in the Asteroid Belt are the remaining leftovers from the Solar System development 4.8 billion years ago, and might be parts of planets that never succeeded in forming.
The additional idea is that once Jupiter was formed, its gravitational pull and mass caused problems in the Asteroid Belt objects so that they didn’t fuse together to create planets.
If you added up the mass of all of the asteroids, it would be an object with a diameter of nearing half the size of our Moon.
Some of the more recent theories of asteroid development include the Grand Tack and the Nice Model that suggest that our gas giant planets did a lot of moving around prior to settling to their current positions.
This movement might have been responsible for throwing asteroids throughout the growing Solar System, causing them to hit the various terrestrial planets.
It’s thought that this process not only emptied the asteroids but replenished the originating Asteroid Belt with new ones.
There are a lot of asteroids that exist outside of the main belt. These asteroids, known as “Trojan asteroids,” orbit the larger planets in areas called the “Lagrange points.”
The orbit is caused by the balance of the gravitational pull of the planet that they orbit and the Sun.
Jupiter has the most Trojan asteroids, with almost as many as exist in the main asteroid belt. Other planets that have Trojan asteroids are Neptune, Mars, and even Earth.
Those asteroids known as “near-Earth” (NEAs) are closer to our Earth than the Sun. Amor asteroids have orbits that come really close to Earth, but never cross the path of Earth.
Apollow asteroids are those that do cross Earth’s path, but most of their time is spent outside of the path. Aten asteroids are those that are near-Earth, whose orbits are within the orbit of the Earth.
The ESA (European Space Agency) estimates that there are around 10,000 NEA asteroids.
Earth has had asteroid impacts throughout its history. Unlike other planets whose impact craters are visible, our craters are constantly being camouflaged and changed due to our atmosphere, tectonic plate movement, erosion, and plant growth.
It’s estimated that an asteroid around the size of a car enters earth’s atmosphere about once per year.
It creates a fireball effect to see and usually burns up in the atmosphere before hitting the ground.
Asteroids are also classified by what they are made of: