Non-Planetary Bodies


Asteroids



Definition of an Asteroid:
Asteroids are metallic, rocky bodies that do not have atmospheres. Asteroids orbit the sun much like planets do, but they are too small to be considered even a terrestrial planet. However, asteroids are often referred to as minor planets. (1)

Who First Discovered the Asteroid?
In 1801 Giuseppe Piazza discovered what he thought was a new planet, but as it turned out, Giuseppe actually was credited with discovering the first asteroid. This first asteroid was Ceres, and following its discovery, many more were identified. It was difficult at that time to discover all the asteroids that we know today because of limitations with technology, but because of technological advancements, we are now able to discover and research, in depth, new asteroids very frequently. (2)

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A picture showing the locations of asteroids in our solar system. (6)

Where are Asteroids Located?
The Asteroid Belt
This is where the majority of asteroids are congregated. It is located between the orbits of Mars and Jupiter, approximately 2 to 4 AU away from the sun. The asteroids in this area are most likely remnants of the protoplanetary disk. Most likely, Jupiter's strong gravitational force in this area prevented the material from accreting into anything resembling a planet, as they were only able to form into what they currently are; asteroids. (2) (3)
Trojans
Trojans, indicated by the green colors on the image to the right, are asteroids that share an orbit with planets. The asteroids and respective planets never collide primarily because the asteroids orbit in the Lagrangian points of stability, which are ultimately 60 degrees ahead and behind the planet. Jupiter is the planet with the most asteroids orbiting with it. Mars also has a few notable trojan asteroids associated with it, as well as the Earth. (2) (3)
Earthly Asteroids
These asteroids actually orbit, or come very close to, Earth. They are referred to as "Near-Earth Asteroids". Related to those are "Earth-Crossers", which are asteroids that have actually crossed Earth's orbit. (2) (3)

What do Asteroids Look Like?
Asteroids are obviously a large percent of all space debris in the universe, and it is no surprise since asteroids are thought to be "primordial material prevented by Jupiter's strong gravity from accreting into a planet-sized body when the solar system was born." This would prove to indicate that asteroids had the capacity to become small-sized planets, if they were not affected by Jupiter's gravity. For example, if the asteroids were, in fact, allowed to accrete into a planet shaped object, such an object would be comparable to almost half the size of Earth's moon. (1)
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Known near-Earth asteroids from 1980-2011. (7)


How Big or Small can Asteroids be?
The sizes of asteroids can vary in size from minuscule objects just inches in diameter, all the way to objects with diameters reaching up to 600 miles, such was the case with Ceres, an asteroid first discovered in 1801. (2)

How do Asteroids move in our Solar System?
The motion of asteroids is very systematic, as they orbit the sun in much the same way that the planets do. Asteroids orbit in an almost elliptical orbit in the same direction as the Earth. A full revolution of asteroids around the sun would
take approximately 3 to 6 years to complete. Asteroids not only orbit, or revolve around the sun, but they also rotate on their own axis, much like the planets do. Larger asteroids are able to rotate faster, but because of this, often times they disperse a lot of loose material into the universe. (1)

Basic Classifications of Asteroids:
Orbital Classification:
Asteroids can be classified according to their specific orbits that they take as they revolve around the sun. Related to this, asteroids can be similarly classified together if they are of the same family, which means that they were discovered at the same time. Therefore, closely related and named asteroids would have been discovered around the same time. Other asteroids, with orbits not common to most asteroids, orbit with horseshoe shaped orbits. These asteroids often orbit in horseshoe shapes, allowing them to, for a period of time, become a satellite of a planet. Such planets that have such temporary satellites are Earth and Venus. (2)
Reflectance Spectrum:
Asteroids can also be classified according to their reflectance spectrum. This would be based upon it's color, albedo, and shape. This directly ties into the three possible asteroid types; C, S, and M, all discussed below. Two models were developed; the Tholen and the SMASS classification. Both have built off one another and have helped to classify asteroids into numerous categories, always coming back to the C, S, and M types. (2)

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Various asteroids and sizes in our solar system. (8)

Types and Composition of Asteroids:
There are three primary categories of asteroids found in our universe:
1. C-type: comprises up to 75% of all asteroids, very dark, albedo of 0.03-0.09, composition similar to that of the sun, depleted in H, He, and other elements, found primarily in the main belt's outer regions. C-type asteroids are mainly carbon rich, hence, the C type. (2) (3)
2. S-type: comprises about 17% of all asteroids, very bright, albedo of 0.10-0.22, metallic iron composition with some magnesium-silicates, located in the inner asteroid belt. S-type asteroids are of stony materials, hence, the S type. (2) (3)
3. M-type: comprises the remaining population of asteroids, bright, albedo of 0.10-0.18, composition of metallic iron, located in the belt's middle region. M-type asteroids are of metallic compositions, hence, the M type. (2) (3)

What is the Surface of an Asteroid like?
The surface of an asteroid is almost similar to that of Earth's moon, or a geologically dead world, such as Mercury. This is true because asteroids are littered with craters and impact features on their surfaces. Some asteroids even have compression fractures and basins, which could all be caused by impacts of objects slamming into the asteroids, altering their surfaces, much in the same way that it happens on the geologically dead planets. (2)

How are Asteroids Formed?
Asteroids are formed much in the same way that the planets were formed--through the Nebular Theory. The Nebular Theory explains how the planets of the solar system formed, and from this, the planetesimals in the asteroid belt can be credited with being created. Once Jupiter, the biggest of the planets, was formed according to the Nebular Theory, it began ejecting the small planetesimals. These small ejections soon began to congregate around Jupiter's orbit, and also in the current asteroid belt, which is located between the orbits or Mars and Jupiter. With enough time passing, the planetesimals were able to form into what we now know as asteroids. (1)

Current Events Related to Asteroids
Asteroids, often times, have the ability to come in close contact or proximity with Earth, Earth's atmosphere, and Earth's orbit. In some instances, a form of an asteroid can even collide with Earth, if it was not first burned up by the atmosphere. In one recent event, Asteroid 2005 YU55 came very close to Earth at the beginning of November 2011. This asteroid was extremely large at over 400 meters in diameter. It is the biggest asteroid to pass this close to Earth since 1976. When astronomers say the asteroid came very close to Earth, they are still talking about 200,000 miles away, which puts Earth in no danger at all. Astronomers actually referred to 2005 YU55 as looking like a giant rock just floating in space, rather than what we would think, which would be an object burning up, and flying by our planet, such as seen in the movies. Astronomers actually believe that this asteroid has been coming in close contact with Earth for thousand of years, after it was nudged out of the asteroid belt by Jupiter's gravity. Asteroid 2005 YU55 poses no threat to Earth, at least for the next 100 years, according to scientists. In fact, this occurrence allows for scientists and astronomers to be able to further explore the universe without having to embark on a space mission. (4)
To get a better, first-hand idea of what this asteroid is like, utilize this link, provided by NASA, which illustrates how YU55 functions and moves. (5)



Sources of Information Utilized:
1. "Asteroids." Welcome to the NSSDC! NASA / Near Press Kit. Web. 29 Nov. 2011. <http://nssdc.gsfc.nasa.gov/planetary/text/asteroids.txt>.

2. "Asteroid." Wikipedia, the Free Encyclopedia. Web. 29 Nov. 2011. <http://en.wikipedia.org/wiki/Asteroid>.

3. "Asteroids L Asteroid Facts, Pictures and Information." The Nine Planets Solar System Tour. Nine Planets. Web. 29 Nov. 2011. <http://nineplanets.org/asteroids.html>.

4. "Asteroid 2005 YU55 'looks like a Giant Rock Floating through Space,' Reports Astronomer - CSMonitor.com." The Christian Science Monitor - CSMonitor.com. Christian Science Monitor, 9 Nov. 2011. Web. 29 Nov. 2011. <http://www.csmonitor.com/Science/2011/1109/Asteroid-2005-YU55-looks-like-a-giant-rock-floating-through-space-reports-astronomer>.

5. Greicius, Tony. "NASA - Asteroid and Comet Watch." NASA - Home. NASA / Near Press Kit, 7 Nov. 2011. Web. 29 Nov. 2011. <http://www.nasa.gov/mission_pages/asteroids/multimedia/index.html>.

6. "File:InnerSolarSystem-en.png." Wikipedia, the Free Encyclopedia. Web. 29 Nov. 2011. <http://en.wikipedia.org/wiki/File:InnerSolarSystem-en.png>.

7. Chamberlain, Alan B. "Known Near-Earth Asteroids." Asteroids. 7 Oct. 2011. Web. 29 Nov. 2011. <http://neo.jpl.nasa.gov/stats/images/web_total.png>.

8. "Asteroids." Alumnus.alumni.caltech.edu. UIUC Edu, Aug. 1998. Web. 29 Nov. 2011. <http://alumnus.caltech.edu/~marcsulf/asteroid/asteroid.html>.