As you probably already know, the shooting stars that you sometimes see on beautiful summer nights are not really "stars”: they are simply dust or small rocks that burn up in our atmosphere and leave a trail of light behind. But what happens when a larger object enters our atmosphere and does not have time to burn up completely before reaching the ground? This is what today’s article is about.

A meteorite is a rock fragment that comes from an asteroid, a planet, a comet or even a satellite (like our Moon). It becomes a "meteorite" when it lands on our planet.

Most meteorites come from the asteroid belt located between Mars and Jupiter. Some also come from the Kuiper belt (beyond Pluto), and sometimes from the Oort cloud located at the border of the Solar System. There are also Martian, lunar and dwarf planets meteorites.

Why are there more meteorites coming from asteroids than planets? How is a meteorite born?

First of all, it is necessary to understand how these rock fragments are formed and find themselves on Earth’s trajectory. Although the Solar System is big and made up mostly of empty space, many large-scale impacts occurred during the last few billion years. This is often referred to as the "cosmic billiards". When an asteroid impacts another object, it may fragment. But do these fragments automatically become meteorites? No, because the gravitational forces of the parent body lock them in orbit or make them crash back down. In order to reach Earth, the fragments must be freed from the gravitational field. To free itself, the fragment must gain a speed higher than the speed of liberation, which is proportional to   (with  being the density of the parent body and R its radius). Therefore, it becomes harder for a rock fragment to free itself when the parent body is larger and denser. For example, the liberation speed of Mars is about 5000 km/s. However, Earth is larger, and its liberation speed is 11 000 km/s. This is why meteorites come mostly from asteroids, much smaller bodies.

Once released, a rock fragment can fly around the universe for millions of years, until one day, it gets close enough to another celestial body to be attracted by its gravitational field and fall to its surface. This rock fragment then becomes a meteorite.

We’ve seen meteorites come mainly from asteroids. Let’s learn more about them! What are the different types of asteroids?

Asteroids can be roughly classified in two categories: deformed asteroids and spherical asteroids. A spherical asteroid is massive enough for the heaviest elements to gather in the center under the effect of gravity. The asteroid is then qualified as differentiated. Such an asteroid has an internal structure similar to a telluric planet (like Earth and Mars). It is composed of a metallic core in its center. Spherical asteroids give rise to three main types of meteorites. First, the differentiated achondrites that come from the equivalent of the Earth's crust. Second, iron meteorites that come from the core of the asteroid. And finally, mixed meteorites that are a combination of both previously mentioned types, i.e silicate and metal. Thanks to these meteorites, we have a good understanding of the internal structure of Earth, telluric planets in general and asteroids.

As for the deformed asteroids, they give birth to meteorites called chondrites. A large majority of the meteorites we receive come from these asteroids (more than 80%). As they have not evolved since the birth of the Solar System, these meteorites can therefore teach us a lot about that period.

How do you recognize a meteorite? And above all, where to find them?

On Earth, a meteorite is characterized by its fusion crust. Indeed, the rock fragment burns when it enters the Earth's atmosphere, and therefore has a blackened crust. Moreover, meteorites are very compact and dense objects. They are differentiated from terrestrial rocks by the presence of metal in their composition. It is however not recommended to use a magnet to recognize them: it degrades their magnetic properties and thus, impacts their scientific value. It is also not recommended to put them in water to measure their density. Indeed, they usually contain iron and are therefore very vulnerable to rust.

Meteorites found on Earth are often discovered in places that have not evolved for thousands or even millions of years, such as hot and icy deserts. For example, many meteorites are found in Antarctica or in the Sahara. Sometimes, they can also be picked up after impact, near their craters.

Historical anecdote: the first man-made iron tools were made with iron meteorites!

What about meteorites from other planets of the solar system?

Scientists have already identified with certainty more than a hundred meteorites from Mars. Regarding the other terrestrial planets (Mercury and Venus), there are no referenced meteorites for the moment. It is very unlikely that we will find a meteorite coming from Venus. Indeed, because of its large size (about the same as Earth's) and its very dense atmosphere, Venus has a very high release speed. On the other hand, it has been proven that it is possible for a meteorite to come from Mercury. Its small size and lack of atmosphere would allow fragments to break free from its gravitational attraction. Nevertheless, no meteorite has yet been classified for sure as Mercurial. Moreover, the position of this planet close to the Sun and its mass (99% of the Solar System’s mass), means that possible fragments from Mercury would most likely fall into the Sun.

Finally, Mars is not spared from impacts from outer space objects either! Some rovers have found meteorites on its surface, such as Opportunity in 2005 with Meridiani Planum, an iron-nickel meteorite located in the region of the same name. This meteorite is the size of a rugby ball!