What we know about the asteroid belt, the hundreds of thousands of minor planets orbiting between Mars and Jupiter.
For thousands of years, our ancestors believed that the Earth was the center of the universe and that everything of consequence orbited around it, mostly because the revered ancient philosopher Aristotle said it was so. Copernicus, Galileo, Kepler, and their contemporaries dispelled that myth during the Renaissance, and it's since been realized that our world is instead part of a rich, intricately-connected system of worlds, all orbiting the Sun.
Several of those worlds had been known since ancient times, but had been interpreted as odd, wandering stars. New theories and better optics allowed astronomers to identify Mercury, Venus, Mars, Jupiter and Saturn for what they were: sovereign worlds in their own right, with Jupiter and Saturn boasting their own retinues of moons. It wasn't long before several stargazers concluded that the gap between Mars and Jupiter was unusually wide. Both theory and religion argued that there should be a planet there -- after all, the Creator wouldn't waste so magnificent a space -- but nothing had been found, despite repeated attempts. A new planet was indeed discovered in 1781, but Uranus, as it was named, lay well beyond the orbit of Saturn.
Still, the mysterious gap beckoned. It seemed logical that any planet orbiting there should lie along the plane of the ecliptic, the thin band in which all known planets orbited around the Sun. In September 1800, a group of European scientists calling themselves the "Celestial Police" divided the plane of the ecliptic into 24 parts, each of which was to be carefully examined by one of the members. By January 1, 1801, they'd had their first hit: Italian monk Giussepe Piazzi sighted a new wanderer eventually named Ceres, after the Roman harvest goddess. Ceres was immediately hailed as the missing fifth planet. But on March 28 of the following year, another planet was discovered by the German Heinrich Olbers, who had been looking for Ceres. He named it Pallas, after the Greek goddess of wisdom (also known as Athena). Amazingly, Pallas seemed to share the same gap between Mars and Jupiter that held Ceres. Sir William Herschel, the discover of Uranus, cleared up the confusion when he realized that the two "planets" were quite small, just a few hundred miles in diameter (Ceres is about 580 miles in diameter and Pallas about 370, as opposed to Earth's 7,900 miles). He named them asteroids ("starlike") and suggested that they were fragments of a destroyed planet that had once existed in the gap. If this was true, then there should be many more to find -- and find them they did.
More than five hundred asteroids (a.k.a. "planetoids") were known by 1900, and after the advent of astrophotography many thousands of others were discovered. Several hundred thousand are currently known, and an estimated million asteroids larger than a half-mile across have so far escaped our detection. Today, it's generally thought that the asteroids are the remains of a planet that should have formed but never did, thanks to Jupiter's gravitational interference. Most of the resulting debris was ejected from the solar system early in its history, leaving the asteroids as the meager remnants.
Asteroid Composition
Most asteroids are rocky bodies, and can be characterized as "flying mountains." Astronomers have identified a variety of different types, but the classification basically boils down to this: they can consist of pure rock, mixtures of rocks and elemental metals such as iron and nickel, mixtures of rocks and organic (but nonliving) materials, or mixtures of rocks and ices. In addition to iron and its relatives, metallic asteroids probably also contain less common metals, through in proportions smaller than might be found on Earth. However, the smaller sizes of these bodies might make the metals easier to access, when we finally begin mining the asteroids for raw materials. In other words, there's gold in those flying mountains -- and aluminum, silver, palladium, iron, nickel, and just about everything else we'll need to make a comfortable living in space. However, it may well be that those asteroids containing elements important to sustaining life itself -- such as carbon, hydrogen, oxygen, nitrogen and phosphorous -- will prove far more important to our space efforts than those containing useful and/or pretty metals.
Where the Asteroids Are
Most asteroids lie between Jupiter and Mars in the plane of the ecliptic, but this is by no means a diehard rule. Some lie outside the ecliptic, even perpendicular to it, and others follow eccentric orbits that take them much closer to the sun. Most of these bodies were perturbed out of their original orbits by collisions or by Jupiter's gravity, usually millions or billions of years ago. Some even cross the Earth's orbit -- and have, in the distant past, actually collided with the Earth. Sixty-five million years ago, a comet or asteroid six miles across struck what is now the Yucatan peninsula in Mexico; the resulting firestorm and subsequent nuclear winter is blamed for the extinction of the dinosaurs. Just 50,000 years ago, a house-sized asteroid smashed into northern Arizona, gouging out Meteor Crater, an impact formation nearly a mile across and several hundred feet deep. In 1908, something exploded in the air above Tunguska, Siberia, flattening trees in an 800 square mile region and generating a blast of heat that could be felt 50 miles away; the tremors were felt 500 miles away. The most likely culprit was a small comet or asteroid. If it had hit a city instead of wilderness, that city would have been destroyed as thoroughly as if nuked. And in the early 1970s, a small asteroid skipped through the Earth's atmosphere before bouncing back out into space. The fireball was highly visible, and was even caught on film. If it had struck the earth, a new Meteor Crater would have formed, possibly somewhere in the Rocky Mountains. An asteroid this size could easily have destroyed a mid-sized city.
In addition to these exciting Earth-crossing asteroids and their relatives, in recent years scientists have become aware of a second belt of minor bodies, located well beyond the orbit of Neptune. This has been dubbed the Kuiper Belt, after a scientist who theorized its existence in the 1950s. Because this part of the solar system has always been far from the central fire, most of these bodies consist of various mixtures of rock, ice and other volatiles that couldn't survive any nearer the sun. Some of these bodies are significantly larger than Ceres. Several known "transneptunians" are much larger; one is Neptune's moon Triton. Its composition, as well as the fact that it which orbits Neptune backward in relation to the planet's other moons, suggests that it was captured from the Kuiper Belt millions of years ago.
Two other massive transneptunians exist. They orbit each other, with the smaller about half the size of the larger, forming the only true double world in the Solar system. Their identities might surprise you. The smaller body wasn't discovered until 1978, but the larger was known as early as 1929. It was discovered by a farmboy-turned-astronomer named Clyde Tombaugh, who identified it (though some say he misidentified it) as the ninth planet he'd been looking for.
The larger body, which is slightly larger than Triton but much smaller than our Moon, he named Pluto. Its slightly smaller sister world, discovered almost fifty years later, is its moon, Charon.
