Buckle up for a trip to the asteroid belt—though it's not nearly as dangerous out there as you might think. But there's a LOT waiting to be discovered, including some crucial clues about the formation of the solar system itself.
NARRATOR: It's one of the most fascinating areas of our solar system, and it's also probably the one that's most often portrayed inaccurately in media. The asteroid belt is usually depicted as a perilous obstacle course of flying rocks. Even in the Cosmos reboot, Neil deGrasse Tyson is shown darting around it in the Spaceship of the Imagination like he's steering the Millennium Falcon through the Hoth asteroid fields. But it turns out that if you're in the asteroid belt, it's easier to fly through safely than it is to actually find an asteroid.
Located between the orbits of Mars and Jupiter, the belt covers an enormous region, about 2 to 3 times the size of the Earth's distance from the sun. That's around 50 trillion cubic kilometres. And while most of that is empty space, it does contain trillions of space rocks, ranging in sizes from bits of dust to the quarter the size of our moon. And together, they have perplexed astronomers for centuries.
So where did the asteroid belt come from? Astronomers used to think that it held the remnants of a planet that was destroyed by a comet or a collision with some other protoplanet in the earliest days of the solar system's formation. But it turns out there's not enough mass in the asteroid belt to account for even a small terrestrial planet.
So far we've only found 4 objects in it that are more than 400 kilometres in diameter: the asteroids Vesta, Pallas and Hygiea, plus the dwarf planet Ceres. Together, those four objects contain more than half of the total mass of the asteroid belt, with Ceres accounting for most of that. There are also too many chemical differences among the asteroids for them to have originated from a single object. Most of them are composed of rocky minerals, for example, while a few contain mostly metals like iron and nickel. So, more than likely, the asteroids are remnants of the solar system's formation 4.5 billion years ago. As our system's giant disc of gas and dust slowly accreted into larger and larger particles eventually forming planets, some swathes of the material weren't able to collect into anything.
In the asteroid belt's case, the enormous gravitation of Jupiter likely prevented those particles from coalescing into even a small planet. And they're not the only leftovers we've found from those early days. The asteroid belt is sometimes referred to as the main belt, to distinguish it from the Kuiper Belt. That's the big band of debris out past Neptune, where Pluto and other dwarf planets hang out, and it's thought to have similar, primordial origins.
To learn more about the vestiges of the solar system's birth still floating around in the main belt, NASA has sent the Dawn spacecraft to investigate. In 2011, Dawn orbited Vesta, where it mapped the giant asteroid's entire surface and found that it's more geologically complex than we thought originally, with a crust, mantle, and iron core, more like a protoplanet than just a chunk of rock. And now, Dawn is en route to Ceres—the largest object between Mars and Jupiter, which has already been found to have water ice on its surface and even a thin atmosphere. It'll take up orbit there in 2015. And by the way, Dawn joins more than a dozen other spacecraft that have navigated safely through the asteroid belt, in case you needed any more proof that it wasn't some celestial mine field.
Pioneer 10 became the first to make its way through in 1972—with no laser cannons needed!