About 4.6 billion years ago, a giant cloud of gas and dust collapsed. Most of this material fell in the center of this cloud, forming the Sun and originating our solar system. The rest of the particles circled the young Sun. Some of these particles of dust collided with each other condensing, and the process repeated until they reached the size of massive rocks and originating the planets.

Collision is a natural process in the solar system and happened even more often in the young solar system. The impact can end life but can also play an essential role in evolution. However, billions of small rocks, such as the asteroid belt objects, were never incorporated into planets and have never evolved. They have altered very little since their formation, different from the planets.  These small space bodies, leftovers from our solar system’s formation, can help us understand our origins, the early solar system conditions, and the processes and events that originated our world.  

Scientists believe that fragments from earlier collisions crashed into Earth, delivering water and raw materials necessary for life to begin on Earth. For example, the amino acid glycine used by organisms in the production of proteins was discovered by the Stardust mission in comet dust.  

More than 100 tons of dust particles crushes into Earth daily. Scientists often discover asteroids and comets with orbits passing near Earth and the Sun. However, only a few of these bodies have potential hazards to Earth, but the more we understand about them, the better prepared we will be to mitigate if necessary.  

Comets

Comets are made of frozen gases, some rock, and dust, like cosmic dark snowballs, due to their dark organic material. Their sizes can go from few kilometers to tens of kilometers wide, and they orbit the Sun like planets and asteroids. However, their orbit usually is elongated, lop-sided, instead of circular-like planets.  

Comets have a small frozen part, the nucleus, composed of gases and few particles of dust. When their orbit is near the Sun, they heat up, boiling and blowing gases and dust, forming a glowing cloud that can be larger than a planet, called a coma. The sunlight lit the coma and, together with the solar wind, blow the dust and gas away, forming two bright tails (a dust tail and an ion/gas tail) that stretch millions of miles. 

According to Kuiper’s theory (1951), beyond Neptune, there is a disc-like belt of icy bodies, and its population of dark comets orbits the Sun in Pluto’s realm. When pushed by gravity, these objects go closer to the Sun, becoming short-period comets, taking less than 200 years to orbit the Sun. In many cases, their appearance is predictable. Less predictable are long-period comets, often coming from the Oort Cloud, and their orbit around the Sun can take 30 million years. It is estimated that billions of comets are orbiting our Sun in the Kuiper Belt and the Oort Cloud.  

However, most comets travel a safe distance; they sometimes crash into the Sun or get too close (sungrazers), evaporating or breaking up, releasing dust and rock particles. The comet debris gets tossed along the comet’s path, generating more debris, mainly in the inner solar system where we live. Sometimes Earth’s orbit crosses paths with a comet. 

Comets are sometimes confused with meteors. However, you can see a comet even when it is far away from Earth; you can only see a meteor when it’s in our atmosphere.  

Asteroid  

Asteroids are small rocky, airless objects that orbit the Sun, sometimes called minor planets or planetoids. Most are made of different kinds of rocks, but some have clays or metals, such as nickel and iron, and they mostly are black-brown and rich in carbon. Asteroids’ diverse elements depend on how far from the Sun they were formed. Asteroids are classified into three broad groups according to their composition, C-(chondrite), S- (“stony”), and M-types (nickel-iron). C-types are the most common and likely to be composed of clay silicate rock. They are part of the most ancient objects in the solar system and have a dark color. S-types are made of silicate materials and nickel-iron. M-types are metallic.  

Asteroids can be as big as a car or as big as a dwarf planet, and they have an irregular shape and are often pitted or cratered. The total mass of all the asteroids together is less than the mass of Earth’s Moon. The asteroids generally have families of the same color and made of the same materials, traveling close together in different parts of the asteroid belt, a region between Mars and Jupiter. They can also be found in other locations, and there are discussions among astronomers about the existence of a large number of asteroids in the Kuiper Belt and Oort Cloud.  

Besides the elliptical orbit around the Sun, asteroids can rotate erratically. Some asteroids orbits pass near the Earth; they are called Near-Earth Objects (NEOs). Earth and other planets have asteroids following the same orbital path. Some asteroids pass in front and behind Jupiter, called Trojans. 

They can also be thrown out of the main belt in all directions in space by Jupiter’s gravity or encounters with Mars and other objects. Besides crushing among themselves, asteroids and their debris can crush into Earth and other planets altering these places’ geological history and the evolution of life on Earth. Some even become moons of planets. They can also have a moon themselves, called binary (double) asteroids, and sometimes they can even have two moons.  

According to NASA, hundreds of thousands of asteroids fly close to Earth. About once a year, a car-sized asteroid crashes into Earth’s atmosphere, creating an intense fireball, but it burns up before reaching the surface. Scientists believe that space debris more extensive than one to two kilometers (little more than one-half mile) could have worldwide effects. And only once every few million years, an object large enough to threaten civilization comes in the direction of Earth’s orbit.  

Meteor, meteoroid and meteorite?  

They are all the same object and are related to what we call “shooting stars,” their definitions depend on their locations. 

Meteoroids   

Meteoroids are space debris formed by stone or metal-like material orbiting the Sun. They are smaller than asteroids, originated by an asteroid or comet collision. Their sizes range from dust grains to small asteroids. Think of them as “space rocks.” orbiting the Sun. Large meteoroids may come from the asteroid belts, while some of the smaller ones may come from the Moon or Mars. 

A rocky meteoroid larger than 25 meters but smaller than one kilometer might cause local damage if it crashes into Earth. Every about 2000 years, a meteoroid with about 92 meters hits Earth, causing significant damage to the area. 

Meteors    

When the meteoroid enters a planet’s atmosphere, it burns up, vaporizing into the meteor, and it burst into light in the sky. These lights are what we call “shooting stars,” even though they aren’t real stars. These streaks of light make meteors be confused with comets. A very bright and large meteor explosion in the atmosphere is called bolide or fireball, and they can create shockwaves that can cause problems disturbing life on Earth. 

About 48.5 tons of meteoritic material falls on the Earth daily, and almost everything is vaporized in Earth’s atmosphere. It is possible to see several meteors per hour during the night, and this number can increase considerably during meteor showers, which occurs at regular intervals. Meteors showers generally happen when Earth passes through a dusty trail left by a comet and are often named a star or constellation near the meteors’ position in the sky.  

Meteorites  

Sometimes meteoroids don’t entirely vaporize in the atmosphere landing on Earth’s surface, and they are then called meteorites. Depending on their sizes, they can make a crater in the ground, and they are divided into three broad groups, depending on their structure, mineralogy, and chemical and isotopic composition. The stony meteorites, mainly composed of silicate minerals, the iron meteorites, formed primarily of metallic iron-nickel. And the stony-iron meteorites, which contain a considerable amount of both metallic and rocky material. 

The difference among those bodies depends on the location they are being observed. You can only find asteroids and meteoroids in space; they become meteors when they enter the atmosphere, and on the ground, they become meteorites.

Asteroids and comets were formed concurrently but under different conditions. While asteroids formed near the dense solar nebula center under extreme temperatures, where only rock or metal remained solid, comets formed beyond the frost line, an area cold enough for water and gases, such as carbon dioxide, to freeze. 

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This article was written by Juliane Verissímo - Marketing Department of VisionSpace.