Asteroids: What Are They And Where Do They Come From?

Asteroids are rocky bodies orbiting around the Sun. They range from specks of dust to large chunks of rock. Some of them are even bigger than our Moon.

Asteroids are named after the Greek word ‘asteros’ meaning star. The term was coined by Italian astronomer Giuseppe Piazzi in 1801. Asteroid is also the name given to the majority of the smallest class of celestial objects.

Asteroids What Are They and Where Do They Come From

Where Do Asteroids Come From?

There are many theories regarding the origin of asteroids.

One suggestion is that they originate from the destruction of a planetesimal—a lumpy mass of leftover material from the formation of the solar system, which may have had enough mass to eventually become a planet, but instead fragmented into countless smaller celestial objects.

Yet another explanation is that they were once part of a planetary system like ours, but got ejected from it, possibly due to impacts from other asteroids.

This theory has been used to explain the formation of the moon, but whether or not it has been proven for asteroids or other celestial bodies is a different matter.

Meteorite impacts tend to generate massive amounts of debris when they hit a planetary body, some of which is ejected back into space, so this isn’t an impossible explanation.

What Causes An Asteroid To Change Orbit?

An asteroid can change its orbit around the sun if it gets closer to the sun or farther away from the sun. If it gets closer to the Sun, then the gravitational pull of the Sun will make the asteroid fall towards the Sun.

In return, the asteroid will gain speed and get closer to the Sun.

However, if the asteroid moves further away from the Sun, then it will lose speed. Eventually, the asteroid will move so far away from the Sun that it will no longer feel any gravitational force from the Sun (or very little anyway).

If an asteroid gets too close to the Sun, it will melt and disintegrate completely due to the intense heat.

Where Is The Asteroid Belt?

The asteroid belt lies between approximately 3.8 and 4.5 astronomical units from the Sun. It is one of the four major divisions of the Solar System.

Its existence was discovered in 1610 by Galileo Galilei when he noticed a gap between the orbits of Mars, Jupiter, and Saturn.

The asteroid belt is a ring of leftover material from the formation of the solar system, between Mars and Jupiter. It was formed by collisions that happened early in the life of our Solar System.

The most famous asteroid is Ceres, which contains the largest amount of mass in a singular celestial body in the asteroid belt and is technically considered a ‘dwarf planet’ it is so large!

How Did We Find Out About The Asteroid Belt?

Asteroids have always been part of astronomy, even before telescopes were invented. However, until the invention of the telescope, finding them was difficult.

Telescopes weren’t able to detect anything smaller than a few hundred kilometers across at that time. So how did we find out about asteroids?

Well, part of what led to its discovery wasn’t just how it was found, but how it affected other planets and celestial bodies.

Astronomers from the 14th and 15th centuries, such as Johannes Kepler (from which we gave the name to the Kepler space telescope), noticed that there were irregularities in the orbits of the planets Mars and Jupiter.

Kepler, who is the first recorded person to have noticed this phenomenon, speculated that there was another planet whose orbit existed between these two planets that was causing this irregularity.

Whilst he wasn’t exactly right about there being a true, full-sized planet there, his theories allowed for later astronomers to eventually discover the asteroid belt.

Astronomers later understood that, although the individual asteroid of the asteroid belt was rarely larger than a few kilometers, the collective mass of this space debris, including some noticeably larger objects, such as the dwarf planet Ceres, allow for their collective gravitational pull to disturb the orbits of both Mars, and even Jupiter, the largest planet in the solar system!

This is the disturbance that Kepler noticed in his studies.

Size Of Asteroids

Asteroids may be classified into two types: rocky asteroids with a diameter less than about 1 km (0.6 miles) and icy asteroids with a diameter greater than about 1 km. Most of the asteroids in our solar system have diameters between 10 and 100 kilometers (6.2 to 62 miles).

Some larger objects are called dwarf planets, such as Ceres as we have already mentioned, because they are much smaller than the planets but still large enough to classify them as a planet thanks to their rotational and gravitational mass pulling them into a round object.

The Solar System consists of an insane amount of rock and ice that make up all the planets, moons, comets, and asteroids. There are many different kinds of asteroids, each having its unique characteristics.

For example, small asteroids tend to be made of silicate rocks whereas large ones can be composed of iron or nickel.

Categories Of Asteroids

Asteroids What Are They and Where Do They Come From

There are three main categories of asteroids, based on size: Main Belt Asteroid, Transneptunian Object, and Centaurs.

Main Belt Asteroids

These are the smallest asteroids. They range in size from about 100 meters to several thousand kilometers in diameter.

The vast majority of asteroids fall within this category. These include most of the asteroids seen by amateur astronomers through backyard telescopes.


These are objects that lie between Jupiter’s orbit and Saturn’s. A centaur has the same general orbital path as Uranus, though not quite as far out as celestial bodies such as Pluto.

It’s also located at one end of the Kuiper Belt, which lies beyond the orbit of Neptune.

Transneptunian Objects

These are objects that orbit around Neptune. They vary in size from just over 100 meters to about 50,000 kilometers in diameter.

Because they’re farther away from the Sun than the Earth, only very faint light reaches them. Their surface temperatures are extremely cold too, so they can’t possibly support life as we know it.

The most famous centaur is Eris, discovered in 2005, which is roughly 3 times as massive as Pluto. However, some other notable transneptunian objects are Sedna, Quaoar, Varuna, Makemake, Haumea, and Orcus.

Are Asteroids A Danger To Earth?

Despite being incredibly small, asteroids pose a serious threat to earth. Many asteroids are potentially hazardous asteroids, meaning that they could hit us someday.

If these do strike, they would cause devastating environmental damage. An impact of this magnitude would destroy pretty much any surface object in its immediate vicinity above ground.

Potentially Hazardous Asteroids (PHAs)

We know of eight PHAs, ranging in size from 11 to 17 kilometers in diameter. All are near the inner edge of the main asteroid belt, where there are fewer collisions. These are the most likely to strike us.

Near-Earth Asteroids (NEAs)

NEAs are those asteroids that come closer to us than 0.05 AU. They can be anywhere in the sky, and you can sometimes even see them without binoculars if you look carefully.

You’ll notice something strange about them though – they move more slowly than expected for their distance from the sun. That’s because they’re moving faster than the speed of light relative to the sun.

However, whilst the likelihood of a large asteroid hitting the Earth and causing widespread destruction is relatively high (as the dinosaurs would surely tell us if they were here today), they only really pose much of a threat on an astronomical level, which could be hundreds, thousands, even millions of years in the future.

So there’s very little to worry about for tomorrow or next weekend


So that was it. Now hopefully you understand a bit better what asteroids are, how they got here, and why they might affect our planet. Hopefully we’ve been able to clear up any misconceptions you may have had!

Gordon Watts