A streak of light lights up the dark skies above you. You just spotted a meteor! Or was it a meteorite? Why do astronomers have different names for space rocks?
Perseid Meteor Shower over six hours: image credit Fred Bruenjes/NASA
As the Earth orbits around the Sun, it passes through clouds of dust and rocky debris. Some of these rocks can be as large as a soccer ball or as small as a grain of sand. When these rocks get pulled down to Earth by our gravity, they may burn up harmlessly in our atmosphere or crash into the ground or into the ocean. Astronomers give different names to these space rocks depending on their size and location.
Asteroids vs. Meteoroids – It’s All About Size
Our Solar System began to take shape around 4.6 billion years ago when immense amounts of dust and gas were pulled together under the force of gravity. Over millions of years, dust balls grew into rocks and those rock eventually grew into planets. Any material from the formation of the Solar System that did not become a planet is now floating freely in space.
Most of these free-floating objects are made of stone, similar to the rocks you would find while digging in your garden. A few of these objects are made entirely of metals such as nickel and iron. Any non-planetary object floating in space that is larger than 10 meters in diameter is called an asteroid.
The largest asteroid in our Solar System is named ‘Ceres’. It is a gigantic rock 950 kilometers in diameter that is floating between Mars and Jupiter. The largest known asteroid that orbits closer to our planet is a 34 km long asteroid named ‘433 Eros’. Luckily at its closest orbit to Earth it is still 22 million kilometers away and has no chance of ever hitting our planet.
Rocks in space that are smaller than 10 meters in diameter are called meteoroids. Astronomers estimate that there are trillions upon trillions of meteoroids floating within our Solar System. There are so many of these small meteoroids in space that on average the Earth collides into 50 to 200 tons of dust and rock every day!
Meteors vs. Meteorites – It’s All About Location
When a meteoroid gets caught up in Earth’s gravity and rockets down to the planet, it can generate a tremendous amount of heat. Falling at an average of 40,000 km/h to 250,000 km/h, the rapid collision of air particles against the outer layer of the meteoroid heats up both the rock and the surrounding atmosphere.
As the meteoroid plunges through the sky it creates a bright streak that resembles a falling star – hence the expression Shooting Star. When a meteoroid lights up the sky in this manner, it is now classified as a meteor.
Did you know that most meteors occur at an average height of 80 to 120 km above the ground?
Most space rocks are too tiny to heat up into meteors. These dusty gains of rocks get buoyed by air currents and float slowly down to Earth. At the other end of the scale are large meteors that do survive the fiery trip through our atmosphere. Some of these larger objects end up smashing into the sea or land or may violently explode high in the air and rain down pieces of rocks across a large area.
Meteoroids that are found on the ground after an impact or air-burst are called meteorites. The last major meteorite event occurred over Russia on February 15th, 2013. A 20-meter 10,000 ton meteor exploded 30 kilometers above the region of Chelyabinsk Oblast, damaging buildings and blowing out windows across a huge area. Dozens of meteorite fragments were found in the snow; the largest piece being a 650 kg hunk of stone recovered by divers in nearby lake.
Comets – One of These Things is Not Like the Other
Not all small objects in space are made of stone and metal. Some consist of frozen liquids and gases such as water, carbon dioxide, methane and ammonia. These snowballs in space are called comets.
Comets look and behave much differently in our night skies. A meteor creates a brief streak of light only when it burns through our atmosphere. Comets can be viewed hundreds of millions of miles away from Earth and can light up our night skies for weeks or even months at a time.
A icy comet usually starts its trip from past Pluto’s orbit. We don’t tend to detect comets until they reach Saturn’s distance from the Sun. At this point there is just enough heat from the Sun to start melting the comet’s outer layer of frozen gas.
As it melts, tiny particles of ice crystals and dust leave a dusty path behind the comet to form a long tail that can stretch for millions of kilometers. The immense tail brightly reflects sunlight, allowing observers on Earth to see the comet for many weeks or even months as completes its trip in and out of our Solar System.
Comets, asteroids and meteoroids are all located in space. Meteors only exist in our skies, meteorites are found on the Earth. So the next time you wish upon a falling star, you can wish that you find the meteorite that started as the meteor flying overhead and be grateful that is was just a meteoroid and not an asteroid!