Since the first object in what is now commonly known as the ‘Asteroid Belt’ was observed in 1801, over 100,000 asteroids have been found in the same region. More than 90 per cent of these have been discovered in the last decade and there are suspected millions more too small to be of note. So what are they all doing there and why aren’t these bands of space rock more common?
When the dwarf planet Ceres was first spotted, quickly followed by the large asteroids Juno and Vesta, it was proposed that they were the remains of a larger planet that had succumbed to a catastrophic event such as a massive impact with another celestial body. The theory stuck for decades but today most scientists consider this origin of the Asteroid Belt an unlikely scenario. The energy required to shatter a progenitor planet even of this small size would have been enormous and, besides, the range of elements and chemicals present in the various asteroids suggest they originated from something other than a single body.
It’s generally accepted that the Asteroid Belt is a vestige of our early Solar System. Though we still don’t know exactly how our planetary system evolved, it’s believed that a collapsing nebula created the Sun and then each planet -both rocky and gaseous – developed out of accreting particles orbiting in discs similar to today’s Asteroid Belt. However, the millions of particles between Mars and Jupiter were unable to form a planet in this way because they were perturbed by the mighty influence of Jupiter’s gravity. So instead they have remained as a disc of orbiting material to this day.
While viewing the known Solar System in 1766, German astronomer Johann Daniel Titius made a very insightful observation. He noticed a mathematical pattern in the layout of the planets: if you applied a certain regular numerical sequence to the order of the planets, it very closely approximated the orbital radii of each. The formula was substantiated when William Herschel discovered Uranus in 1781, matching its orbital radius almost exactly. There was a problem, however: by what would later be known as the Titius-Bode Law, there should have been a planet between Mars and Jupiter, but 18th-century astronomers could see nothing there. Suddenly, that conspicuously empty region of space became an observational hotspot. Titius-Bode has since been proven to be a mere coincidence, but it prompted us to point our telescopes towards a region of space much earlier than we might have otherwise.
Dawn in the Asteroid Belt
In 2007, NASA launched the Dawn spacecraft with a mission not to visit any of the main planets in the Solar System, but the Asteroid Belt. It arrived at Vesta – one of the first objects ever observed in the region – in July 2011 and was inserted into its orbit. The main mission of Dawn is to gather data from the largest bodies in the Asteroid Belt to give scientists on Earth a clearer picture of how the Solar System formed.
Dawn left Vesta in September 2012 and is due to reach the biggest object in the Asteroid Belt – the dwarf planet Ceres -in February 2015. It’s the first exploratory probe to be powered by ion propulsion, using three xenon ion thrusters, meaning it’s easily capable of making multiple insertions. In contrast, standard chemical propulsion limits spacecraft like the Voyager probes to only flybys on secondary targets.