There's the asteroid belt between Mars and Jupiter, and then there's the Kuiper belt beyond Neptune. Both are known for bring rings of space debris, loose rocks and bits of planets and moons separated from the main body by a meteor. Asteroids.
Only, not every item is of such a small statue. There are planets in those belts too.
Meet Ceres, named for the Roman goddess of plants and motherly love, this hunk of rock and ice is 950 km in diameter and weighs 9.5x1020 kg. It's massive size categorizes it as a dwarf planet. Ceres is a special dwarf planet though, more so than the others. Its size, like mentioned above is one reason; it's the largest and most massive body in the asteroid belt, making up a third of the belt's total mass. Sharp-eyed humans in exceptional viewing areas can actually see it with the naked eye. Ceres is also round, spherical like a planet, and not oblong or irregular like smaller asteroids. This means it has pretty decent gravity, enough to shape it's surface. It's also suspected to have an atmosphere. Plus, it contains so much ice, an estimated 200 million cubic km, that it surpasses Earth in terms of amount of fresh water. And as water is necessary for life...the potential for Ceres to have some evidence of it is much more likely than on other planets in the solar system. Curiosity is currently on Mars looking for evidence of the possibility of life, it might have better luck on Ceres.Credit: http://en.wikipedia.org/wiki/Ceres_%28dwarf_planet%29
That doesn't mean NASA is ignoring pretty little Ceres. No siree. In 2007 the space agency launch Dawn, an ion propelled space craft to take up the work of the Voyager missions. That is, to explore the solar system. More specifically, it's tasked to missions that will help answer questions about how our solar system was formed.
Dawn first visited Vesta, an asteroid in the asteroid belt, and in September 2012 left it behind to start on it's way to Ceres. It's a two and a half year journey between the two asteroid belt bodies. Dawn runs an ion propulsion system, using electricity to ionize xenon to power it's engines. It's the not most powerful system, hence the long time to travel between observation points, but it is fuel efficient. As Marc Rayman told Geek magazine, it wouldn't be surprising if Dawn's fuel source lasts beyond expectations and it can do additional missions around Ceres not fit into the original itinerary.
The dwarf planet was discovered back in 1801 by an Italian named Giuseppe Piazzi. Piazzi had been looking for a star listed as Mayer 87 by Francis Wollaston, but it wasn't were Wollaston said it would be. And then Piazzi noticed a star-like object that moved. He thought it was a comet at first, but it was too slow moving and had a uniform orbit. The Titius-Bode Law, which came about in the late 1760s is the result of the mathematical relationship between the planets. It predicted two missing planets, one where Uranus's orbit is (which was discovered in 1781) and another between Mars And Jupiter. Piazzi had discovered this planet.
But then another planet in the area was discovered in 1802, Pallas. And there started a debate – were they really planets? They didn't fit the conventional idea of one, they could only be seen through a telescope while farther planets, like Mars could be seen with a naked eye. Sir William Herschel called them 'asteroids' instead, meaning star like, for that was how they appeared. Perhaps they were the remnants of a destroyed planet.
Most astronomers still thought of them as planets though. And then Juno as discovered in 1804, and Vesta in 1807. Both were also considered planets. Eventually though it became apparent that the four of them weren't alone. Between 1845 and 1851 an additional eleven objects in the area between Mars and Jupiter had been found. And thus, all those new planets were demoted to asteroids and the region was called the asteroid belt. Today there are 100,000 known objects in the region.
The same thing happened to Pluto actually. As time went on, more objects in the neighborhood was discovered. Most notably Eris which is actually bigger than the ex-ninth planet. This area is now known as the Kuiper belt.
These new discoveries were causing a bit of trouble for classification, so the definition of a planet was changed in 2006 by the International Astronomical Union. It is now “a celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a ….nearly round shape, and has cleared the neighborhood around it's orbit.” Thus, since Pluto didn't have a clear orbit was put into the new category of dwarf planet. Ceres, and Eris, due to their round shapes were also added to the category. There are only three dwarf planets today.
It was Ceres's size, and it's spherical nature that led to it being promoted to dwarf planet statues; it's the only asteroid to gain such a promotion. The new 2006 definition of planet
For being almost 3 AUs away from Earth, we know quite a bit about Ceres. But there are still some questions. Satellite images from 1995 onward don't always match up. There is a white spot in photos taken by the Hubble Space Telescope, though what causes it is unknown. Hopefully the close up views Dawn will provide will help sort out what exactly are the features of Ceres's surface. Additionally, it's believed that the composition of Ceres is different of that from other asteroids, and the question of 'why' is no where near to being answered at this time. All it does is show that Ceres developed differently form the asteroids that surround it.
Dawn returned stunning views of Vesta, allowing scientists to figure out Vesta in the past had been completely melted forming layers and an iron core. It also shows evidence of Vesta having survived two huge impacts in the past two billion years, it's a survivor from the turbulent times of the early day of our solar system and could provide more clues as to what occurred during that time.
Hopefully, Dawn's mission to Ceres will be just as fruitful.