What Are Comets, Asteroids, Meteoroids, Meteors and Meteorites?
Small Solar System Bodies
Comets, asteroids and meteoroids are different small solar system bodies. Some of these can come closer to us and may even impact Earth. Meteoroids are the smallest of the three bodies and they often impact Earth but without much damage. Meteoroids become meteors or shooting stars when they enter Earth's atmosphere and become meteorites if one or few fragments manage to reach the ground.
I used to wonder earlier if scientists have their own whims and fancies when classifying the solar system objects. My doubts cleared once I learnt the logic behind classifying these objects. The composition and the nature of the orbits determine if an object is a comet or an asteroid. The differences between meteoroid, meteor and meteorite may be confusing because of the phonetic similarity.
There is a certain line of thinking in classifying these objects as meteoroids, asteroids or comets. Once we know this logic, we can easily understand how the solar system objects are classified. Size and appearance are also factors in classifying these objects. Most comets sport a tail but some may not have a long tail and they may appear like fuzzy balls.
Composition of Asteroids and Comets
Asteroids are basically the rocky bodies that orbit the Sun well within the orbits of planets. Asteroids are like planets since they also orbit the Sun in a elliptical orbits. They are considerably smaller in size compared to planets. Asteroids are sometimes called minor planets because of this reason. So we are clear that asteroids are similar to planets the way they orbit the Sun but are much smaller.
The biggest near Earth asteroid is close to thirty kilometers in diameter. Most other asteroids have a diameter of just under a few kilometers. Any object that is larger than 500 meters could cause severe damage to properties and lives. Thankfully, no known asteroid has been found to have any chance to impact Earth in the near future.
Comets are bodies primarily made of ice and dust. It is the presence of ice and dust that makes some comets a treat to watch. Comets are known to originate from the outer reaches of the solar system far beyond the orbit of Neptune. Comets are nearly identical in size to an asteroid. But some of them have an orbit that takes them near the outer edge of the solar system. Comets have highly eccentric orbits compared to planets or even asteroids.
Some comets have well defined orbits making them periodic comets. Halley's comet is the most famous periodic comet with a periodicity of around 75 years. Edmond Halley was the first astronomer to predict the periodicity of this short period comet. He rightly predicted that it would appear on 1759 based on historic data available. Sadly, he did not survive long enough to witness its appearance but it appeared as per his estimate.
Comets also differ from the asteroids in appearance. Comets are brilliant objects at a given size when asteroids hardly reach naked eye visibility. Comets sport a central coma that includes the nucleus and the atmosphere. Comets may have a tail made of ice and dust at times. Contrary to public perception, a comet can be without a long tail too! Majority of comets are invisible to naked eye. We may possibly have a short period comet by name "Wirtanen" reaching naked eye visibility in December 2018
Meteoroid, Meteor and Meteorite
I would give a simple definition for a meteoroid, meteor and meteorite that is easier to understand. Meteoroid is basically a small asteroid with a diameter of less than 10 meters. Some meteoroids can be so small that these could be just a few microns in size. So we are clear that the asteroids are smaller versions of planets and meteoroids are smaller version of asteroids. So what are meteors and meteorites?
Some of the meteoroids can be near earth objects and impact Earth very often. These meteoroids pose no serious threat to Earth as they get burnt up while entering the Earth's atmosphere and would not reach ground. These meteoroids appear like shooting stars when they burn up in the Earth's atmosphere and rightly called so.
If hundreds and thousands of shooting stars are spotted in a night, it is called meteor shower. Meteor showers are usually caused by earlier comet passes. Each meteor shower happen in a specific period every year when Earth crosses the path taken by the comet. The debris left behind by the comet bombards our planet causing the meteor showers.
The smaller space rocks usually do not survive the trip through the atmosphere and vanish into thin air, quite literally. Slightly bigger rocks could survive and a small portion of it could hit the Earth. The space rocks thus identified on the ground are called meteorites. So a meteoroid would become a meteor when it enters the Earth's atmosphere as a shooting star and would finally become a meteorite if some part of it manages to reach the ground.
So the three terms refer to the same space rock as it passes through various stages in its life cycle. These space rocks are odd shaped unlike the near spherical planets.
So a meteoroid would become a meteor when it enters the Earth's atmosphere as a shooting star and would finally become a meteorite if some part of it manages to reach the ground.
Two Asteroid Impact Events in Recent Times
Now let us understand what caused the Chelyabinsk meteor few years back in Russia. The space rock that exploded in the Russian skies in the year 2013 was estimated to be around 20 meters in size and had an estimated mass of over 12,000 tons. Since the size is more than 10 meters, this space rock comes under the category of asteroids.
The Chelyabinsk Meteor never reached the ground and it disintegrated on its atmospheric entry resulting in a deafening explosion in the sky. The explosion had sent shock waves down causing damage to buildings and injuring people.
So meteors (shooting stars) are caused not just by meteoroids but also are caused when an asteroid impacts Earth. Events such as the Chelyabinsk meteor caused by an asteroid are extremely rare. Another large asteroid is believed to have struck the largely uninhabited region of Tunguska in Russia in the year 1908 flattening a large forest area and destroying millions of trees.