March 1, 2013 1 Comment
In my last post, “It seems the sky is falling…”, I talked about the Russian meteor event and flyby of asteroid 2012 DA14, both of which occurred on Friday, February 15, 2013. In that post I talked a lot about the various terms of things in space that can enter the Earth’s atmosphere and ultimately cause an “impact”. But there are a lot of terms and some of them have very minute differences, so I figured I’d devote a post just to explaining these terms. Specifically, I’d like to look at a few differences.
“Meteoroid” vs. “Meteor” vs. “Meteorite”
These words all share the same root, the Greek word meteōros, meaning “suspended in the air”, and look very similar, but they do mean different things. To start off, let’s think of a small piece of rock in space. We don’t care what kind of rock it is or where it comes from, let’s just call it a small rock. Now, let’s say that small rock is happily zipping around the solar system, obeying the law of gravity as it orbits the Sun, when suddenly it gets too close to Earth and the gravitational pull of the planet sends tugs it out of its original orbit and towards our planet. Now, that small piece of rock that’s on it’s way into the Earth’s atmosphere, that’s a “meteoroid”. Once that “meteoroid” hits the Earth’s atmosphere travelling at high speed it’s going to heat up and leave a trail in the sky. That heating up and the resulting streak in the sky is a “meteor”- commonly referred as a “shooting star”. If you get a whole bunch of associated “meteoroids”, say a whole bunch of little pieces of rock left over from an asteroid or comet that passed by, that enter the atmosphere at the same time, creating meteors, that’s called a “meteor shower”. So, the “meteoroid” is the small rock that causes streak of light and the “meteor” is the actual visible streak we see. Now as that “meteoroid” is hurtling through the atmosphere and heating up, it can literally blow up. That huge flash that’s caused by the disintegration of the “meteoroid” is known as a “fireball” and really bright “fireballs” are known as “bolides”. That huge flash of light is usually associated with a large deposit of energy into the atmosphere that causes a pressure wave like to ones seen in Tunguska and Chelyabinsk.
The solar system is full of little pieces of debris moving really fast and without the atmosphere that debris would constantly be pummeling the surface of the planet…and us. So the atmosphere protects us. Things are constantly entering the atmosphere and burning up, creating “meteors”. Most of these “meteoroids” are about the size of a pebble- much to small to reach the Earth’s surface. But it does happen occasionally. When large objects enter the atmosphere and make it down to Earth, that remaining piece of rock that reaches the ground is known as a “meteorite”. So yeah, if you’re one your way to work in the morning and see that there’s a huge piece of rock sitting on your car, that’s probably a “meteorite”…or there’s someone who really doesn’t like you. Don’t worry though, as Bad Astronomer Phil Plait writes, only one person has ever been hit by a meteorite and that occurred in Alabama in 1954.
Now, not all “meteors” and “meteorites” are caused by natural objects in space. Think about all of the satellites and “space junk” orbiting the Earth. If any of that space junk were to re-enter the atmosphere it would burn up, just like a space rock, and cause a meteor. And another, less appealing example is astronaut waste. On the now-retired Space Shuttles, the urine was expelled out into the upper atmosphere to burn up/evaporate– this actually created a visible glow. Solid waste on the Space Shuttles was collected and removed once the Shuttle returned to the ground, unfortunately that’s not really an option on the International Space Station (ISS), where astronaut waste is stored, then loaded into a disposable space probe and ejected out to burn up in the atmosphere. So yeah, next time you wish on a shooting star, just think that it might actually be astronaut poop.
“Comet” vs. “Asteroid”
Okay so now we’ve talked about the differences between the things that enter the atmosphere. But beyond man-made sources, where do those “meteoroids” come from? Many of them are from rocky, metallic objects in the solar system known as “asteroids”. What are asteroids? According to NASA:
“Most asteroids are made of rock, but some are composed of metal, mostly nickel and iron. They range in size from small boulders to objects that are hundreds of miles in diameter. A small portion of the asteroid population may be burned-out comets whose ices have evaporated away and been blown off into space. Almost all asteroids are part of the Main Asteroid Belt, with orbits in the vast region of space between Mars and Jupiter.”
Most asteroids are actually leftover bits and pieces of planets that weren’t able to coalesce under gravity. As the NASA page describes, most asteroids live in the Asteroid Belt that orbits between Mars and Jupiter. However, as those asteroids travel around the Sun, they can bump into each other, causing a rogue asteroid to leave the Asteroid Belt and traverse the solar system. Sometimes those asteroids fall into the Sun, sometimes they collide with Earth or other planets.
So what’s the difference between an asteroid and a comet? A “comet” is an icy body that lives out in the farthest regions of the solar system. There is belief by scientists that many comets primarily live in a region at the edge of the solar system known as the Oort Cloud. As these icy bodies come into the inner solar system and approach the Sun, they increase in brightness as the heat from the Sun causes the ice to melt and reflect sunlight. Comets are generally much easier to view than asteroids due to the high reflectivity of the water vapor it releases as they travel through the inner solar system. Generally comets that pass by the orbit of the Earth leave a debris trail in their wake. When the Earth’s orbit takes it through one of those debris trails, it causes a meteor shower.
So comets are mostly icy bodies that live out at the very edge of the solar system and asteroids are rocky, metallic bodies that generally live in the Asteroid Belt in the region between Mars and Jupiter.
So what did we learn?
So let’s review in this handy table made by the great folks at NASA’s Near-Earth Object Program:
|Asteroid||A relatively small, inactive, rocky body orbiting the Sun.|
|Comet||A relatively small, at times active, object whose ices can vaporize in sunlight forming an atmosphere (coma) of dust and gas and, sometimes, a tail of dust and/or gas.|
|Meteoroid||A small particle from a comet or asteroid orbiting the Sun.|
|Meteor||The light phenomena which results when a meteoroid enters the Earth’s atmosphere and vaporizes; a shooting star.|
|Meteorite||A meteoroid that survives its passage through the Earth’s atmosphere and lands upon the Earth’s surface.|