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What's Inside the Asteroid Belt?

VO: Ashley Bowman WRITTEN BY: Nathan Sharp
Way out in the Solar System, between Mars and Jupiter, lies the Asteroid Belt - an orbiting ring of space rock that so often crops up in science fiction. But, what exactly is the Asteroid Belt? How did it get there? What's the science behind it? And will it ever change? In this video, we find out!
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What’s Inside the Asteroid Belt?


Out in the cosmos, between Mars and Jupiter, lies the Asteroid Belt, where various, large chunks of rock are floating in a disc and orbiting the sun. It’s a stand-out feature in our Solar System, but is there more to it than simply miles and miles of random, well, asteroids? You bet there is!

Back in the mid 18th century, the German astronomer Johann Titius worked on the Titius-Bode law hypothesis – in which it’s suggested that each planet in the Solar System was twice as far away from the sun as the preceding planet, making Venus twice as far away as Mercury, Earth twice as far as Venus, and so on. Regardless of the accuracies (or inaccuracies) of this theory, in 1766 Titius correctly applied it to assume that there must be a celestial object 2.8 astronomical units away from the sun, between Mars and Jupiter.

Fast forward to 1801, and Giuseppe Piazzi, a Catholic priest, discovers such an object moving against the distant stars. Piazzi originally thought (and hoped) it was a new planet, but he officially announced it as a comet, not wanting to disappoint the world should he be wrong. Mathematician and physicist Carl Friedrich Gauss then studied the mysterious object’s orbit, and ultimately proved Piazzi correct – he had indeed discovered a small planet, and it was exactly where the Titius-Bode law predicted that it should be. This planet was swiftly named Ceres Ferdinandea after the Roman goddess (and King Ferdinand IV of Naples and Sicily). Of course, the Ferdinandea part of its name was eventually ditched, and plain ol’ Ceres is today classified as a ‘dwarf planet’ – but the discovery sparked a slew of similar finds.

The following year, a second body was identified; 2 Pallas. This massive asteroid actually accounts for 7% of the total mass of the entire Asteroid Belt… So, as far as space rocks go, 2 Pallas is a stonker! Then, by 1807, two further small planets were discovered. So, that’s four new celestial neighbours showing up in our skies within six years… Naturally, scientists at the time were seriously excited.

Next came a brief pause for the Napoleonic Wars, but afterwards even more space-based bodies were found… so much so that by 1850 our knowledge had expanded enough to understand that not all of these sightings so close to one another could be genuine planets. In fact, none of them were. Struck off the official ‘Planets List’, they became known as asteroids instead, and were grouped within the ‘Asteroid Belt’.

By 1868, 67 years after the first was discovered, over one hundred asteroids had been found. Almost a century and a half later, by the year 2000 we’d officially charted 100,000 of them. And, while there’s no official and conclusive number today, there are potentially millions of asteroids floating between Mars and Jupiter. Ironically, the Titius-Bode law that started it all off has since been discredited as being total nonsense, and Piazzi’s initial discovery of Ceres has been labelled as little more than ‘lucky’. But, both developments clearly prompted a significant change in how we understood the Solar System.

So, we know there are millions of these things out there, but what exactly IS an asteroid? Simplistically speaking, it’s a space rock. Some of these rocks, like Ceres - which has a diameter of 945 kilometers and accounts for an incredible 33% of the belt’s total mass – are massive. Others are tiny, reaching less than 10 meters across. It's a common misconception that all asteroids are huge, but it’s just not the case. In fact, if you could somehow collect every asteroid in the asteroid belt and fuse them together by gravity, forming one massive asteroid planet, that planet would still weigh less than the Earth’s moon.

Similarly, for years Hollywood movies have had us picturing the asteroid belt as a massive and intimidating wall of impenetrable rocks destined to destroy any passing spaceship. But, that's just Hollywood being Hollywood. In reality, the chances of actually hitting an asteroid while passing through the belt are less than one in a billion. Yes, there are millions of asteroids, but space is so unbelievably vast that they’re all extremely far away from each other.

As for their composition, asteroids are made up of various materials, and are separated – for the most part – into three classifications. The most common type of asteroid is a C-type, or carbonaceous asteroid. These account for 75% of all them, consist primarily of silicate rock and clay, and appear gray in color. The second most common is the S-type, or silicaceous asteroid. These are a greenish red and are made of nickel-iron and silicate. Finally, the rarest major type of asteroid is the M-type, or metallic asteroid. These are fully red, and are composed primarily of nickel-iron. There are other, even rarer, types out there – like the V-type, which is made of volcanic crust – but C, S and M are the major players.

But, what’s with the ‘belt’ itself? What brings all of these asteroids together? And why haven’t they relocated yet? As with many things, it’s all down to Jupiter.

When the solar system was forming over four-and-a-half billion years ago, in a well-known creation story, massive clouds of dust and rock were brought together under the force of gravity to form the planets. But, Jupiter grew to be so massive that its gravity directly influenced the way in which nearby rocks moved too, preventing those close by from coalescing into fully-fledged worlds. While some collided every now and then, and some were snagged to eventually become one of Jupiter’s many moons, the majority were kept at a constant distance away from the gas giant – and so they formed the belt.

Were it not for Jupiter, it’s entirely possible that the rocks in the asteroid belt could have created another new planet, or else attached themselves to existing planets like Mars. But Jupiter’s gravity overpowered their own and prevented them from uniting. This way, Jupiter also protects us (here on Earth) from a constant asteroid bombardment. Given that Jupiter’s gravitational pull keeps all of the asteroids in the same place (most of the time), the space rocks careening through the Solar System are few and far between. But of course, even with Jupiter, there are asteroids outside of the belt. Scientists believe that Mars’ moons, Phobos and Deimos, started life this way, and there have been at least 18,000 near-Earth asteroids throughout history – some coming closer than others…

But, back to the belt, and it’s true that some of the rocks that reside there – while affected by Jupiter – are massive enough to exert their own gravitational pull. In the case of Ceres, it’s actually the only asteroid to be completely rounded by its own gravity – which only increases its planet-like properties. Elsewhere, though, over 150 asteroid belt asteroids boast a companion moon… and scientists have even discovered binary or triple asteroids, where two or three are locked in a gravitational orbit around each other.

So, all things considered, the asteroid belt is very far from just a random assortment of space rock. It’s more like a well-balanced, carefully-shaped and continuously working machine – with asteroids orbiting other asteroids (orbiting the sun). There are a few dwarf planets in there, amongst millions of other bodies, each as unique as the last. But, the belt could perhaps be best described as a place where another planet might’ve formed, had the Solar System been structured differently. As it is, it’s the world that wasn’t (deconstructed into a fantastic disc of debris). Our only hope is that one of its inhabitants doesn’t veer off our way. Because, then we’re in big trouble.
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