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The History of Asteroids
Back on January 1, 1801, good old Giuseppe Piazzi, a Sicilian astronomer, discovered the planet Ceres. It's a planet located between Mars and Jupiter.
OK, so I lied. Piazzi thought it was a planet. But, what he discovered was the very first asteroid that was later reclassified as a dwarf planet. An asteroid (aka minor planet) is a small, planet-like rocky world.
In a few years' time after Piazzi's discovery, astronomers had found so many of these so-called planets between Mars and Jupiter that they realized they were a new type of celestial object altogether. They called them asteroids. Their properties and origins will be discussed in due course.
The Origins and Types of Asteroids
Asteroids are likely the remnants of early solar system material that could not form a planet due to Jupiter's gravitational influence. While the largest remaining asteroids might be essentially planetesimals, early solar system bodies that helped form planets, smaller asteroids formed from many collisions between the original objects in the asteroid belt over billions of years.
The asteroid belt (aka main belt) is a region of the solar system found between 2.0 and 3.3 AU from the sun, where the vast majority of the asteroids in our solar system have their orbits. There are many different types of asteroids, and they are classified according to their reflectance spectra. That's basically saying that they're classified based on how well they will reflect different wavelengths of light.
- Roughly, 75% of asteroids are C-type asteroids. They are very dark asteroids with no strong absorption features. 'C'-type asteroids are the most 'c'ommon type of asteroids and are as dark as a lump of 'c'oal.
- The majority of the remaining asteroids are S-type asteroids, which show an absorption feature thanks to a mineral called olivine. 'S'-type asteroids are 's'lightly reddish in color.
- M-type asteroids have reflectance spectra like that of metallic iron and nickel. Think 'M'-type = 'm'etallic.
There are many other types of asteroids, but they're not important for our current discussion.
The Orbits of Asteroids
Just like there are many different types of asteroids, their orbits also vary quite a bit in size and shape. When compared to the planets in our solar system, they usually have more elliptical orbits that are also more inclined to the ecliptic plane. The ecliptic is the plane out in space where Earth's orbit lies around the sun.
Despite these differences, almost all of them orbit the sun in the same direction as our planet. That is to say, counterclockwise, when viewed from the north.
As described before, the large majority of asteroids orbit in the asteroid belt, which is located between Mars and Jupiter. As the image on the screen may imply, it looks like this region of space is crowded with asteroids, and lots of collisions occur all of the time as a result. In fact, I bet you saw at least one movie or TV program that made it seem to be so.
I hate to burst your collision- and explosion-seeking bubble, but most of the asteroid belt is actually empty space. If you managed to hitch a ride on one of these asteroids, it could be months or even years before you see another asteroid. In fact, you may even die of old age before you ever come close to another asteroid.
Non-Main Belt Asteroids
Not all asteroids orbit in the main belt. Some have orbits crossing planets in our solar system. Some even come close to Earth.
Near-Earth objects, or NEOs, are asteroids that have orbits that cross Earth's orbit. The most dangerous kind of NEOs are called Apollo objects. Like any NEOs, they can hit Earth and cause utter devastation.
The now famous Chelyabinsk meteor was one such Apollo asteroid and served to remind everyone on Earth how dangerous they can be. Just check out the cool video on screen to experience its awesome powers if you haven't seen it already.
But not all asteroids orbit near Earth. For example, there are the Trojan asteroids, which are asteroids that share Jupiter's orbit, and centaurs, which are outer solar system bodies having an orbit within the region of space of the four outer planets. They are called centaurs because they have characteristics pertaining to both asteroids and comets, like the mythical centaurs had characteristics of both humans and horses.
An asteroid (aka minor planet) is a small, planet-like rocky world. The asteroid belt (aka main belt) is a region of the solar system found between 2.0 and 3.3 AU from the sun, where the vast majority of the asteroids in our solar system have their orbits. Roughly 75% of asteroids are C-type asteroids, which are as dark as coal.
Asteroids have more elliptical orbits that are also more inclined to the ecliptic plane when compared to the planets in the solar system. The ecliptic is the plane out in space where Earth's orbit lies around the sun. However, almost all of them orbit the sun in the same direction as our planet.
While most asteroids are located in the asteroid belt, not all are. There are the Trojan asteroids, which are asteroids that share Jupiter's orbit, and centaurs, which are outer solar system bodies having an orbit within the region of space of the four outer planets.
And, let's not forget the near-Earth objects, or NEOs, which are asteroids that have orbits that cross Earth's orbit. The most dangerous kind of NEOs are called Apollo objects.
By the time you are done with this lesson, you might have accumulated the knowledge required to:
- Summarize the way in which asteroids were discovered
- Recall the location of most asteroids
- Write down the classification system for asteroids
- Explain the orbits of asteroids
- Characterize NEOs and centaur asteroids
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