Look Up
Asteroids, M’s, and comets
by Mitchell Tester, College Student
At the center of our solar system sits the arbiter of life, our Sun. Eight planets orbit that Sun, although, what else is out there in our solar system? To answer that question, I will be discussing asteroids, meteoroids, meteors, meteorites, and comets. While next month, I will discuss our planets’ closest relatives, the dwarf planets.
For you to understand what meteoroids are and their counterparts, we must first establish what asteroids are. Between Mars and Jupiter lies what is called the Asteroid Belt, home to over one million asteroids. The abundance of asteroids in this location is largely due to Jupiter and its gravity not allowing the forming of planetesimals (think of them as pre-planets) to collect enough material to become one of the eight planets, instead, left to live their lives as asteroids. Some asteroids got large enough to differentiate, which largely indicates that asteroids were never large fully formed planets that were broken up millions of years ago like people may think, but simply planetesimals that never got large enough to become planets due to the likes of Jupiter and Mars.
Asteroids are classified into three types. C-type consists of clay and rock and are the most common. The next type is the S-type, which are made up of rock and nickel-iron. The last type is the M-type, which due to them experiencing high temperatures, they partly melt, resulting in iron sinking to the bottom. This is what we call differentiation. When a piece of the asteroid—whatever the reason—breaks off and goes on its own way, that piece becomes a meteoroid. Think of the meteoroid as the object that travels through the void of space. What changes a meteoroid is if it makes contact with the Earth’s atmosphere; they are then referred to as meteors. The contact with the atmosphere causes friction, which in return causes the meteor to heat up, giving way to a bright streak in the sky—not to be confused with a comet, which will be discussed later. Most meteors are small enough that they burn up in the atmosphere, never making contact with the ground. Meteors large enough make it to the ground, which were once meteoroids before making contact with Earth’s atmosphere and turning into meteors, turn into meteorites once they have made contact with Earth’s surface. Think of meteoroids, meteors, and meteorites as the same object, and the way you refer to that object is dependent upon where it is in its journey. In outer space means it is a meteoroid, in the Earth’s atmosphere means it’s a meteor, and only the ones that make contact with the Earth’s surface are meteorites.
Meteorites can be of three different compositions. These include stony, stony-iron (being the rarest), and iron. Within stony type of meteorites lie two additional types: chondrites and, the less common, achondrites.
Chondrites come from some of the oldest asteroids in our solar system, chondrules (millimeter-sized celestial bodies), through accretion, formed asteroids. The much rarer counterpart is known as the achondrites, which, like chondrites, are mostly rock, but still experienced some differentiation during the birth of the solar system long ago.
Differentiation occurs when the heavier elements, such as iron, sink to the core. No differentiation of the asteroid means that the rocky piece is quite lumpy, and mixed in; no clear indication of layers. The next is stony-iron, which is from larger asteroids. Asteroids, that either heated up enough through impact or size, results in the iron to sink to the core because it is the heavier element. The “stony” part in the name comes from the rocky part of the meteoroid, indicating that it came from the transition layer of silicate to iron due to iron also being present in the stony-iron. Strictly coming from the transition layer of an asteroid, as you would expect, makes for the stony-iron type to be quite rare. The last composition that meteoroids may come in is iron. This, like the stony-iron, is from an asteroid that differentiated. The iron meteoroid comes from the core of an asteroid. Although most meteoroids come from asteroids as mentioned above, some can even come from larger celestial bodies such as the Moon or Mars.
Next, a much colder and more distant object is the comet. Comets are objects very far out, even past the roughly 2.7 billion miles of Neptune’s distance from the Sun. A simpler way astronomers have to think of these distances to and from objects in space is a form of measurement called AU, or astronomical unit. One AU is the average distance from the Earth to the Sun, which is 93 million miles. So, Neptune is 30 AU from the Sun; therefore, 30 AU is roughly 2.7 billion miles. The vastness of space and distances to objects makes the AU necessary.
Now back to the comets. Astronomers predict that these comets come primarily from the Oort cloud. A spherical cloud of comets that encompasses our solar system some 10,000 AU to 100,000 AU away from the Sun (100,000 times the distance from our home, Earth, to the Sun—very far!). The nucleus (center) of the comet is where the ice, dust, and some gas is located, loosely held together by its gravity. Many of these comets that come from the Oort cloud are then caught by the Sun, whose orbits are very eccentric, meaning that they are much more oval shaped than circular. When these comets pass so that we can see them, they are being melted by the Sun. They have a tail, that tail being part ion and the other part dust. The ion tail, which is always facing directly away from the Sun, has a blue tint and is due to UV rays. The other part of the tail is simply the blow from the Sun’s solar radiation, blowing the dust off from the heating of the nucleus and its ice and gases.
This two-part tail can make some very pretty scenery for observers on Earth. Comets do primarily come from the Oort cloud, although they don’t all come from it. Some come from the Kuiper belt, similar to the asteroid belt, but instead of being in between Mars and Jupiter, it sits right outside of Neptune’s orbit. Extending some 10 AU. Along with its position in our solar system being different, it is also the home of comets and not asteroids. Along with comets and asteroids, there also exists something else in our solar system, called our dwarf planets.
Tune in next month, as I discuss some of the most interesting and strange dwarf planets our solar system has to offer.
