An asteroid being named after you or discovering a Comet so bright that it outshines the moon in the night sky or owning a rock worth millions- all these sound adventurous, thrilling, and blow your minds off, right?
Not just you, even I was wonderstruck when I first came to know about this. Don’t let your excitement levels go down as I will take you through an adventure. But before that, it is necessary to learn about the space rocks and their types and a few properties. We shall learn about space rocks in the first part of the article.
Just try to understand what the word suggests- Space + Rocks. Any solid chunk that is out there in empty space, whether revolving around a body or not, is a space rock. Even the rocky planets are the rocks in space but ironically they are not space rocks. If I talk about the scale of the solar system, all those chunks which are the leftovers of the star and planet formation are the space rocks in our Solar System and they usually orbit the Sun.
They are majorly categorized as:
Now, we shall learn about each one of them briefly.
The word ‘Comet’ has originated from a Greek word ‘komētēs’ which means ‘a long haired star’. But comets are not stars, they are dirty snowballs. Dirty because the dust is a key ingredient which makes them and snowballs because they come from the places in the Solar system which are so cold that these balls are frozen. To talk about their composition, they are made up of rock, dust, water ice and frozen gases like carbon dioxide, methane, ammonia, and carbon monoxide.
The structure of a comet includes a rocky nucleus, a coma, and two tails. The nucleus comprises chunks of rock, ice, and frozen gases embedded with dust. The size of the nucleus can range from a few hundred meters to tens of kilometers. When this nucleus comes in the proximity of the Sun, the heat from the Sun causes the ice and frozen gases to sublimate (change directly from solid to gaseous form). Now, when these frozen gases sublimate, the gravity of the rocky nucleus holds them around the surface and this gives a comet its atmosphere- the coma.
As the comet’s proximity to the Sun increases, the solar radiation pressure from the Sun pushes the coma away from it and thus making a beautiful gas or ion tail. It is the gas tail which has a color, characteristic to the gases present in the coma. Now, as I have mentioned above, a comet has two tails, out of which one is a gas tail. So, what about the other one? If you have read carefully, then, you might guess the answer right. The nucleus had embedded dust particles too. If the ice and frozen gases sublimate, the dust no longer remains embedded and becomes free, forming another tail- a dust tail. Both these tails are in a direction opposite to the Sun.
The coma can be bigger than planets and the ion tail can extend upto distances as large as the distance between the Earth and the Sun.
The famous Halley’s Comet returns to the neck of the Solar system after every 76 years but the recent comet NEOWISE will return after 6800 years. This clearly states that comets come from different places in the Solar System because if they had been visiting from the same part of the Solar System then the time period of their orbits would have been the same.
Comets, on the basis of their orbital periods, are classified as short and long period comets. A short period comet takes nearly 200 years or less to orbit the Sun once. They come from a belt of icy objects beyond the orbit of Neptune called the Kuiper Belt. On the other hand, if a comet takes more than 200 years then it is classified as a long period comet and they visit us from the outermost region of the Solar system- the Oort Cloud. It is a giant spherical shell of icy bodies that surround the Solar system.
So, now you know that you can see Halley’s comet in your lifetime once but won’t be able to witness the return of the comet NEOWISE.
The comets, when in proximity to the Sun, keep shedding bits and pieces of its rocky nucleus and dust in its orbit nearer to the Sun. These leftover pieces are called meteoroids and the term used to define them collectively is ‘Meteoroid Stream’. Now, the Earth also orbits the Sun. It happens that the Earth, in its year long orbit around the Sun, passes through this stream. These small chunks when attracted by the Earth’s gravity burn up in the atmosphere at high temperatures and leave streaks of light behind them. These streaks of light are called ‘Meteors’, popularly known as ‘Shooting Stars’. They burn up showing a color characteristic to the element present in them.
You might have witnessed or at least heard about Meteor Showers. These comet leftovers or the meteoroid stream is responsible for the meteor showers. The meteoroid stream leftover from comet Halley produces Eta-Aquarids and Orionids meteor showers each year. This is because the path of comet Halley around the Sun intersects with the Earth’s orbit twice.
As noticeable, the meteor showers are named after the constellations. The reason for this is the region in the sky, on the Earth, from where these meteors appear to originate. The point from where they originate is called the ‘radiant’. The brightest and most observed meteor showers are the Perseids, associated with the comet Swift-Tuttle and the Geminids. However, Geminids, unlike other meteor showers, are not associated with a comet. Instead, they are associated with an asteroid 3200 Phaethon which orbits the Sun every 1.4 years.
Here is a table which lists all the major meteor showers of the year.
Now, if the chunk is big enough to survive the burning in the atmosphere, then a part of it might reach the ground which you can find. It is then called a ‘Meteorite’. Irrespective of their major composition, all meteorites have some amount of iron in them. Since iron has a very high melting point, it successfully survives the high temperatures while burning in the atmosphere. All the other elements having a low melting point, as compared to iron, mostly vaporize in the atmosphere.
We will see in the next article, how to distinguish meteorites from the meteorwrongs.
Asteroids are the chunk of rocks in space leftover from the formation of the Solar system. In terms of size, they can range from hundreds of miles to a few meters in diameter. An asteroid, as big as a mountain, could have wiped out all life including dinosaurs that lived on the Earth 66 million years ago. That is one of the reasons why we hunt asteroids- to search for asteroids that can pose a threat to our species and take suitable measures to protect humankind. The asteroid that wiped out the dinosaurs caused an impact crater in Mexico and is called the Chicxulub crater (shown in the image below). That’s the power of a space rock.
Just like the comets, asteroids can also be classified on the basis of their positions in the Solar system. Most of the asteroids, including the biggest asteroid and a dwarf planet- Ceres, reside in the Asteroid Belt which is between the orbits of Mars and Jupiter. They are called Main Belt Asteroids. Many asteroids which are found within or around the orbit of the Earth are classified as the Near Earth Asteroids, which can further be classified as Atiras, Atens, Apollos, and Amors based on their orbits. Some asteroids that share their orbit with Jupiter and are called Trojans and Greeks. The asteroids which are found outside the main belt and before the orbit of Jupiter are called Hildas. More than 994,992 asteroids have been discovered and many others are awaiting discovery.
Not just based on their place in space, asteroids are also classified based on their composition. The C- type (chondrite) asteroids are composed mainly of clay and silicate rocks, the S- type (stony) are made up of silicates and nickel iron whereas the M- type (metallic) are mostly made up of nickel iron.
Since we have learned about space rocks, we shall next learn how to hunt for them in the continuation of this article.
References and Further Readings
 Dijl, Photograph by Martin Van. “Dino-Killing Asteroid Hit Just the Right Spot to Trigger Extinction.” National Geographic, 9 Nov. 2017, www.nationalgeographic.com/news/2017/11/dinosaurs-extinction-asteroid-chicxulub-soot-earth-science/.
 “In Depth.” NASA, NASA, solarsystem.nasa.gov/asteroids-comets-and-meteors/asteroids/in-depth/.
“In Depth.” NASA, NASA, solarsystem.nasa.gov/asteroids-comets-and-meteors/comets/in-depth/.
 Old Farmer’s Almanac. “Meteor Shower Calendar 2020: When Is the Next Meteor Shower?” Old Farmer’s Almanac, www.almanac.com/content/meteor-shower-calendar.