Asteroids are small, rocky objects in the solar system that travels around the Sun. There are millions of asteroids and they are often grouped by their composition. As they are like planets but smaller in size, they are referred to as minor planets or planetoids. They vary in size and shape and range from less than 1 km to 600 miles across.
In the ancient Greek language, the name ‘asteroid’ means ‘like a star’, but they are actually different from stars. Asteroids may look like small stars in the sky, but they do move around the Sun, while stars only seem to move because the Earth spins. Like planets, asteroids do not make their own light.
The first asteroid was found by Giuseppe Piazzi in 1801. It was named Ceres and is considered to be the biggest object in the asteroid belt. The most famous asteroids are dwarf planets – Ceres; Pallas (a huge asteroid); and Vesta (a very large, bright object).
The asteroids are irregularly shaped because they are smaller in size and have a small gravitational field.
Objects with a large mass have a large gravitational field which exerts a greater gravitational force that pulls the materials inwards, bringing the larger objects such as planets and moons into a spherical space. Asteroids are not able to do this, as they are smaller in size and have a smaller gravitational field which is just enough to hold the materials together, but not adequate to make this into a round/spherical shape.
NASA launched a first of its kind discovery mission called the Near-Earth Asteroid Rendezvous (NEAR) mission to go into orbit around an asteroid. On Feb 12, 2001, NEAR Shoemaker became the first spacecraft to orbit and then land on an asteroid named Eros, a near-Earth asteroid. Eros is the second-largest known asteroid in an Earth-crossing orbit; the asteroid is roughly the size of the Caribbean island country of Barbados.
Where do asteroids come from?
It is thought that asteroids are the leftover debris from the formation of the inner planets. Asteroids are the leftover rock and other material from the formation of the Solar System. These rocks were too small to make a planet and there are a lot of leftovers out there. The objects that we see today are leftover from a time when the solar system formed 4.5 billion years ago.
Asteroids are rich in precious metals and other metals, as well as water. Some asteroids are actually blown-out comets. The ices are gone, and all that’s left is the rocky material. Some asteroids have moons of their own.
Types and Composition
Based on their composition, asteroids are grouped into different categories:
Where are asteroids located?
Many asteroids form large rings or belts around the Sun. There are two asteroid belts in our solar system
While many asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles termed ‘ices’ such as methane, ammonia, and water. The Kuiper belt is home to three officially recognized dwarf planets: Pluto, Haumea and Makemake.
The Asteroid Belt is not just a small strip with a dense asteroid population. Asteroids in the Asteroid Belt are actually pretty far from each other. The average distance between two asteroids is around 600,000 miles – more than the distance from Earth to the Moon. Still, many asteroids lie outside of the main belt. The distribution of asteroids in the main asteroid belt is not uniform. In particular, there are regions which contain very few asteroids – these are known as the 'Kirkwood Gaps.
Asteroid belt can be divided into an inner and outer belt. The inner belt is within 250 million miles of the Sun. The outer belt lies beyond the 250 million mile boundary. It is made of more stony and carbon-based asteroids.
Near Earth Objects (NEOs)
Most asteroids are found in the Asteroid Belt; however, there are some asteroids that are not in that orbit and are called Near-Earth Objects (NEO) because they come close to our planet Earth.
A Near-Earth object (NEO) is any small solar system body whose orbit brings it to proximity with Earth. By convention, a solar system body is a NEO if its closest approach to the Sun (perihelion) is less than 1.3 astronomical unit (AU).
If a NEO’s orbit crosses the Earth’s orbit and the object is larger than 140 meters it is considered a potentially hazardous object (PHO). Most known PHOs and NEOs are asteroids, but a small fraction of PHOs and NEOs are comets.
Perihelion distance (q) is when the Earth is closest to the Sun
Aphelion distance (Q) is when the Earth is farthest away from the Sun.
The vast majority of NEOs are asteroids, referred to as Near-Earth Asteroids (NEAs). The Near-Earth Asteroids are divided into four groups – Atira, Aten, Apollo, and Amor, according to their perihelion distance (q), aphelion distance (Q), and their semi-major axes (a)
NEAs = q < 1.3 au
Atira = a < 1.0 au, Q < 0.983 au
Aten = a < 1.0 au, Q > 0.983 au
Apollo = a > 1.0 au,
Amor = a > 1.0 au, q < 1.017 au
Asteroids are not comets or meteoroids
Asteroids are often confused with comets and meteoroids. They may look the same but in reality, they are not.
Asteroids and comets: Asteroids and comets are both large, floating space objects but asteroids are rocky objects and comets are icy objects.
Asteroids and meteoroids: Both asteroids and meteoroids are rocky but the difference lies in their size. Meteoroids are much smaller than asteroids and when they fall to Earth they’re called meteors. Lots of meteors fall down to Earth but they aren’t nearly as dangerous as asteroids.
Earth Trojan Asteroids
There is actually an additional kind of category of asteroids, known as Trojan, due to a process called libration. Simply put, Earth Trojan asteroids are already captured by Earth and orbit the Sun synchronized with Earth. These asteroids are normally far from Earth, not in any circular or elliptical orbit close to Earth. They are better described as orbiting the Sun in Earth’s orbit.
Is it true that an asteroid can hit Earth?
An asteroid with 10 kilometers diameter hit the Earth, off the Yucatan Peninsula, around 65 million years ago. This asteroid’s impact on Earth is assumed to be one of the reasons why dinosaurs went extinct.
There is a very small chance of an asteroid colliding with our planet. Earth and other planets in the solar system were created through the process of objects colliding into one another to form larger objects. These collisions are still happening, but luckily most of the larger items are gone, now making up parts of the planets we know. Luckily smaller objects would be destroyed by the Earth’s atmosphere if they come close.
Roughly once every 2000 years an object about the size of a football field collides with Earth. The asteroid that hit Earth 65 million years ago would have been much larger than this.
If all the known asteroids are grouped, their mass would still be smaller than that of our Moon.
Also, there are many astronomers keeping a watch on the positions of any asteroids coming close to Earth. They map out the orbit path of these objects and can predict impacts well in advance.
