While many people are awed by the galaxy, filled with planets and stars and mysteries such as black holes, many forget about the space debris currently floating in the hold of Jupiter’s gravity: the Asteroid Belt. Or perhaps people simply push them out of their thoughts in an attempt to deny the possibilities of destruction that come with the asteroids. Often confused for comets, asteroids are too small to earn the classification of a planet, they are made up of chucky rocks and icy mud balls. The distinction isn’t always clear, but comets tend to more chemical compounds when heated, appear fuzzier by telescope and tend to have orbits shaped much more elliptically than asteroids.
Asteroids, also known as planetoids, and comets are thought to be a result of the giant cloud of gas that condensed in the Big Bang to form the planets, moons, and suns nearly 4.5 billions years ago. Nowadays, the majority of the asteroids in our solar system sit between Mars and Jupiter in a tightly packed area, which is called the Asteroid Belt. It is widely accepted that the reason these asteroids did not form into the planet was because they got trapped by Jupiter’s gravitational pull. Other scientists believe all the asteroids in the belt was once a planet, that was broken apart in a collision So why are asteroids something to be feared?
Well, nearly 65 million years ago, it is widely believed that an asteroid collided with the earth, causing the mass extinction of the dinosaurs as orbital collisions, gravitational tugs and jostling sent an asteroid off its normal orbit and hurtling towards the Earth. And there’s nothing to stop it from doing so: in fact, in November of last year, an asteroid bigger than an aircraft carrier darted between the Earth and the moon, the closest encounter we’ve had with an asteroid in over thirty five years. Put the fright of that close encounter with the knowledge that there are over 5,000 asteroids (most of which reside in the Asteroid Belt, while others live off in other strange orbits) that cross paths with the Earth’s orbit, it’s amazing that these catastrophic collision only occur once every millions years or so. In fact, many “collisions’ with the Earth do occur, but because most are so small, they burn up in the Earth’s atmosphere without being any serious threat to the planet.
Asteroids can range from a few feet in diameter to miles, and due to their smaller size, are unable to pull themselves into a ball shape because they lack the gravity pull. Because of this, their shapes can be very different, and scientists group them by how they reflect sunlight. The Asteroid Belt is split into two sections: the inner and outer belt. The inner belt is made up of asteroids that are within 250 miles of the sun, and are mostly made out of metals. The outer belt includes all asteroids beyond 250 miles from the sun, and is rocky in appearance, with a darker color due to their richness in carbon.
Despite the large amount of asteroids, if we were to squish all the materials together, the mass would still be smaller than our moon. There have been 26 discoveries of exceedingly large asteroids, but it is expected that there are millions of smaller asteroids that are too small to be detected by our telescopes (under one mile in diameter). Ceres, which was believed to be the largest asteroid for almost two hundred years, is six hundred miles wide, and accounts for about one third percent of the entire asteroid mass; it’s about a quarter of the mass of the moon. However, in 2002, Ceres was beat out by the newly discovered 2001 KX76, which is even bigger than Ceres. Despite Ceres getting pushed out of the top spot, it was added to a list of dwarf planets, along with Pluto. Gaspra is another well known asteroids, believed to be over 200 million years old judging by the number of craters found on its surface. Vesta is best known for it’s diverse terrain; the third largest asteroid in the Belt, scientists believe that ancient lava flows, meteorite impacts, and radioactive shrapnel shaped this asteroid. Eros is known for it’s odd shape, looking almost like a mustache cut out; it lies right outside the main belt and is twice the size of Manhattan.
Asteroids can be categorized in many different ways. Apollo asteroids are asteroids that come within the orbit of Earth at their perihelions (closest approach to the Earth). There are currently 240 known Apollo asteroids. Asteroids that are always closer to the Sun than the Earth are called Atem; the first Atem asteroid was discovered in 1918, and there have been 30 discovered since. The reason there are so few is because they are so difficult to see, since they lie between the Earth and the Sun. NEO’s, or Near Earth Objects (sometimes known as NEA, or Near Earth Asteroids), are asteroids that whose orbits get changed because of gravitational pulls of other planets, that cause them to enter the area around Earth.
While scientists do not consider asteroids to be planets, some do have their own moons if their gravitational pull is large enough to snap one of the smaller asteroids, called meteoroids. In 1993, the small moon, Dactyl, was discovered to be orbiting around the asteroid Ida; Dactyl is only one mile in diameter, compared with Ida’s nineteen mile diameter, explaining this occurrence. Since then, over two dozen of these “binary asteroids”, as their called, have been discovered.
The Asteroid Belt is often portrayed as dense and impassible, as the density appears large compared to the space around it, the spacing between the asteroids is actually quite immense. It is so thinly spaced, that many spacecrafts have been able to pass through it. The first space mission to make it through the Belt was Pioneer 10 in 1972, and since then, there have been 12 missions that made it safely through. Many scientists hope that these missions will allow us to learn more about the asteroids, since most of what has been learned came from asteroids that fell to Earth. Some scientists hope to eventually mine on the asteroids for minerals and compounds.
New information regarding asteroids has been discovered that supports the theory that Mars and Jupiter migrated to their current orbits more than four million years ago, plowing through the Asteroid Belt as they went. While the Asteroid Belt has been known to hold gaps for many years now, known as Kirkwood Gaps, caused by unstable zones where Jupiter and Mars pull asteroids away forming a gap, recent studies show that some of these gaps don’t add up. Scientists found that some gaps were formed in relevance with others, which would not hold true if the gaps were Kirkwood Gaps. One scientist described it as taking a snow plough through the asteroid belt, and shoving the others to the side. With this new theory, it is widely accepted that that giant plants—consisting of Jupiter, Uranus, Saturn and Neptune—formed twice as close to the sun as present day, and slowly moved outwards, pushing a path through the Asteroid Belt. This is believed to have caused a chain reaction, destroying much of the Belt population, and causing a late, but heavy, bombardment of asteroids on the inner solar system, which may explain many of the crater’s on our moon, which would place the event at 3.9 billions years ago, or 600 million years after the Solar System was created.
In a discovery in 2005, scientists believe they have found an asteroid belt circling around a sun very similar to our own. They are hoping this will give them a significant look into distant star systems that resemble our own, and may help them learn where and how planets like Earth form. While they can’t see other terrestrial plants, they believe the asteroid belts to be old planets, and by studying their “fossils” they can discover much about both asteroids and solar systems like ours. While this belt is very similar to our own, it varies in some ways too: it is much closer to the sun it orbits, located in an orbit similar to that of Venus, rather than Jupiter like ours, and is much thicker, with about twenty five times as much material as our own. To give a mental image, if our asteroid belt was this thick, the dust would like up the night sky in a bright band.
Some scientists argue that this is not an asteroid field but a supercomet, the size of Pluto, that got knocked into the inner solar system and is slowly burning up, leaving behind a trail of dust as it goes, using the evidence of the small silicate crystals like those discovered in the comet Hale-Bopp. Most scientists consider this theory to be a long shot however, and believe future missions will make it clear that this belt is in fact an asteroid belt.
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Sources:
http://science.nationalgeographic.com/science/space/solar-system/asteroids-comets-article/http://www.kidsastronomy.com/asteroid.htm http://www.universetoday.com/32856/asteroid-belt/Class:
Asteroids
Subject: Space's Junkyard: The Asteroids 
