Asteroids and the Asteroid Belt: What They Are... and Aren't

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Asteroids: What Are They?

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A schematic of how asteroids are distributed throughout the solar system. NASA

Understanding Asteroids

Asteroids are rocky chunks of solar system material that can be found orbiting the Sun throughout nearly the entire solar system. Most of them lie in the Asteroid Belt, which is an area of the solar system that stretches between the orbits of Mars and Jupiter. They occupy a huge volume of space out there, and if you were to travel through the Asteroid Belt, it would seem quite empty to you. That's because the asteroids are spread out, not crowded together in swarms (like you often see in movies or some pieces of space art). Asteroids also orbit in near-Earth space. Those are called "Near-Earth Objects". Some asteroids also orbit near and beyond Jupiter as well. 

Asteroids are in a class of objects called "small solar system bodies" (SSBs). Other SSBs include comets, and a group of worldlets that exist in the outer solar system called "Trans-Neptunian objects (or TNOs)". These include worlds such as Pluto, although Pluto and many TNOS are not necessarily asteroids. 

The Story of Asteroid Discovery and Understanding

Back when asteroids were first discovered in the early 1800s—Ceres was the first one found. It's now considered a dwarf planet. However, at the time, astronomers had an idea there was a planet missing from the solar system. One theory was that it existed between Mars and Jupiter and was somehow broken apart to form the Asteroid Belt. That story isn't even remotely what happened, but it also turns out that the Asteroid Belt IS made up of material similar to objects that formed other planets. IThey just never got it together to actually MAKE a planet.

Another idea is that the asteroids are the rocky leftovers from the solar system's formation. That idea is partially correct. It's true they formed in the early solar nebula, just as chunks of cometary ice did. But, over billions of years, they have been changed by internal heating, impacts, surface melting, bombardment by tiny micrometeorites, and radiation weathering. They've also migrated in the solar system, settling mostly in the Asteroid Belt and near the orbit of Jupiter. Smaller collections also exist in the inner solar system, and some shed debris that eventually falls to earth as meteors

Just four large objects in the belt contain half the mass of the whole belt. These are dwarf planet Ceres and asteroids Vesta, Pallas, and Hygeia

What Are Asteroids Made Of?

Asteroids come in several "flavors": carbonaceous C-types (containing carbon), silicate (S-types that contain silicon), and metal-rich (or M-types). There likely millions of asteroids, ranging in size from small specks of rock to worldlets more than 100 kilometers (about 62 miles) across. They are grouped into "families", whose members show the same types of physical characteristics and chemical composition. Some of the compositions are roughly similar to the compositions of planets such as Earth. 

This huge chemical difference between the types of asteroids is a big clue that a planet (that broke apart) never did exist in the Asteroid Belt. Instead, it looks more and more like the belt region became the gathering place for planetesimals left over from the formation of the other planets, and through gravitational influences, made their way to the belt. 

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A Short History of Asteroids

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An artist's concept showing how families of asteroids are created, through collision. This process and others changes asteroids by heating and impact processes. NASA/JPL-CalTech

The Early History of Asteroids

The early solar nebula was a cloud of dust, rock, and gases that provided the seeds of the planets. Astronomers have seen similar disks of material around other stars, too.

These seeds grew from bits of dust to eventually form Earth, and other "terrestrial-type" planets such as Venus,  Mars, and Mercury, and the rocky interiors of the gas giants. Those seeds—often referred to as "planetesimals"—accreted together to form protoplanets, which then grew to become the planets. 

It's possible if conditions had been different in the solar system, a planet MIGHT have formed where the Asteroid Belt is today—but nearby giant planet Jupiter and its formation may have caused the existing planetesimals to collide too violently with each other to accrete into a world. As infant Jupiter traveled from its formation area nearer the Sun, its gravitational influence sent them scattering out. Many collected in the Asteroid Belt, others—called Near-Earth Objects—still exist. Occasionally they cross Earth's orbit but usually pose no threat to us. However, there are many of these small objects out there, and it is entirely possible that one COULD wander too close to Earth and possibly crash into our planet.  

Groups of astronomers DO keep an eye out on the Near-Earth asteroids, and there is a concerted effort to find and predict the orbits of those that might come to close to us. There's also a great deal of interest in the Asteroid Belt, and the Dawn spacecraft's main mission has studied dwarf planet Ceres, which was once thought to be an asteroid. It previously visited asteroid Vesta and returned valuable information about that object. Astronomers want to know more about these old rocks that date back to the earliest epochs of solar system history, and learn about the events and processes that have changed them throughout time.