What is an Asteroid?
Asteroids are boulders orbiting the Sun, with sizes ranging from some hundred metres to several kilometres. An asteroid is called a meteorite if it hits the Earth. If it completely evaporates in the Earth’s atmosphere before crashing on the surface, it’s called a meteor. People usually refer to meteors as ‘shooting stars’. Most meteorites are composed of silicates or a mixture of iron and nickel. In the past, some huge meteorites have struck Earth. Sixty-five million years ago, almost 90% of animal species were eradicated (including dinosaurs) when a meteorite hit Yucatan, Mexico. Luckily, this happens very rarely! We owe this to Jupiter, which attracts many asteroids with its gravitational pull.
Asteroid Ida
How are asteroids formed?
Dust particles in the early Solar System collided, forming larger clumps, known as planetesimals. These could grow by attracting more dust with their gravitational fields; some grew large enough to form the planets. Others remained, becoming the asteroids. Some of these asteroids collided with each other (and the early planets), fragmenting into smaller asteroids. Some of the collisions were slow enough that the asteroids merged, producing oddly shaped asteroids.
Where are asteroids located?
Many asteroids form large rings or belts around the Sun. There are two asteroid belts in our Solar System: the main belt (or simply called the asteroid belt) between Mars and Jupiter, with thousands of asteroids (see picture below), and the Kuiper belt, named after its discoverer, a disk-shaped region that extends outside of Neptune’s orbit and contains countless asteroids and many dwarf planets, of which Pluto is the most famous.
Most asteroids are found in the asteroid belt; however, there are asteroids that are not in that orbit, and they are called Near Earth Objects (NEO). Sometimes these NEOs can reach our planet Earth.
Why are asteroids important to study?
Asteroids are part of a group known as minor bodies. Asteroids, comets and meteoroids can provide valuable information about the evolution of our Solar System
Additionally, there are small bodies called meteoroids, remnants of the formation of the Solar System. These meteoroids can be as small as a grain of rice. Meteoroids constantly meet Earth, producing what we see on Earth as ‘shooting stars’.
Is it true that an asteroid can hit Earth?
An asteroid 10 kilometres in diameter hit the Earth 65 million years ago. This asteroid impact on the Earth is assumed to be one of the reasons why the dinosaurs became extinct.
Comets
You can also tell the students about comets. Comets are like dirty snowballs or icy lumps of mud. They consist of a mixture of ice (from water as well as from frozen gases) and dust. Like asteroids, comets revolve around the Sun. However, their orbits are strongly elongated compared to planets. That is, they occasionally get very close to the Sun, and at times they get very far away. When they cross a planet’s orbit, they could collide with it. This happened, for example, in 1994, when the comet Shoemaker-Levy collided with planet Jupiter and broke into pieces. When comets come close to the Sun in their orbit, the ice in their core melts and evaporates. This results in a beautiful tail, which can be clearly seen in the night sky if the comet passes close enough to the Earth.
In 2061, Halley’s Comet will once again come close to the Earth. It orbits our Sun once every 76 years. Remember to mark its arrival on your calendar!
Halleys Comet