Between the orbits of Mars and Jupiter circle millions of bodies of the asteroid belt, being debris that was left over after the planets of our solar-system were formed 4.6 billion years ago. The most massive of these is Ceres, which alone contains around a third of the asteroid belt’s mass; Vesta comes next.
These and the other big bodies are actually failed terrestrial planets, who have retained within them records of the processes that led them to their present states. In 1994 when I started my investigations into the early solar system, I had hoped to extract this ancient information. I soon realized that  these bodies were a bonanza of greater value than all the gold mines ever found.
Further, aiding my research was the data of Vesta the then newly orbiting Hubble Space Telescope was sending.  I found that Vesta, in particular, with its core, mantle and crust was like our Earth in miniature. I felt that an understanding of its evolution could help us know how our own planet formed and evolved. I used to often day dream: “But can we not send our instruments in a spaceship to the asteroid belt to find out?.........”
In late 1996 I was surprised when I was invited to the Johnson Space Center in Houston in Texas, to address scientists who were seriously examining the feasibility of doing what I had pictured in my mind’s eye. Most people felt that the chances of NASA agreeing to it were remote. I, who had by then developed a kind of a proprietary interest in Vesta- a feeling for which I should perhaps be pardoned - remained puzzled.
Years passed by and I became increasingly engrossed with Mars. Then suddenly one day in late December 2001, I came to know that the mission, which had been aptly named Dawn, has been approved by NASA! I was ecstatic. The road to Vesta had opened up.
Six years have elapsed since NASA ordered that the Dawn  spacecraft be built and now as I write this , this  craft is at the  launch  pad at  the   Cape Canaveral Airport Station in Florida , waiting to be blasted  off on July 7 afternoon (EDT) , on a Delta 7925H rocket.  It will be a few hours before dawn on Sunday July 8 in India when it leaves.
Unlike most spacecraft, which use chemical propellants, Dawn will be driven by an ion engine. It will take more time to go from Earth to Vesta and from Vesta to Ceres than if it had used chemical fuel. But it is worth having this longer trip. Dawn will use a significantly less expensive rocket than it would if it had to carry the much more massive propellants required for a conventional chemical propulsion system. In fact, Dawn without an ion engine would have been so costly that in all probability NASA would not have allowed it to fly at all!
After leaving Earth, the spacecraft will slingshot past Mars in March 2009. Next Dawn will ease itself into orbit around Vesta in October 2011 and then spend the next seven months examining this asteroid. Then in April 2012, the spacecraft will fire its ion drive again and resume its interplanetary cruise. As fortunately Ceres, Vesta and the Sun will in 2012 be so aligned (which arrangement happens every seventeen years), Dawn will be able to harness the forces that act on it in space and go to Ceres.
It arrives at this giant dwarf planet in February 2015. Ceres has a surface area almost as large as India’s. Consequently it will take five months for Dawn to finish the study of a world that is as huge as our sub continent. All this will have to be done from orbit because the considerably high gravity of Ceres will not allow the spaceship to make a controlled landing.
Dawn will cover three billion miles. It will also become the first spacecraft ever to enter into orbit around two different planetary bodies other than the Earth and Moon - a feat that will be made possible by the extraordinary manoeuvering capability of its remarkable ion propulsion system.
It will also set another mark. On 24th of August last year the International Astronomical Union (IAU) has revised the definition of planets, in accordance with which, both Ceres and Pluto find themselves in the dwarf planet category.   Dawn is scheduled to reach Ceres in Feb 2015, which is five months before July 2015 which is the month on which the  New Horizons spacecraft will reach  Pluto. This means that Dawn will become the first mission to reach a dwarf planet. It is amusing to find that the low-cost Dawn has ‘plutoed’ (to use the current word of the year) the New Horizons spacecraft, although the latter had left for Pluto a year and a half year earlier.
Further, after exploring Ceres, Dawn can be sent to explore a third target - like say the asteroid Pallas if  in 2015 NASA desires this.
What are the findings that scientists expect from  Dawn?
Cool and wet
Vesta and Ceres have many contrasting characteristics that are thought to have resulted from their being formed in two different regions of the infant solar system. The impression today is that Ceres had experienced a "cool and wet" (or icy) formation and might have sub-surface water. On the other hand Vesta has probably gone through a "hot and dry" (or rocky) phase and has a differentiated interior and primitive surface volcanism.
However, after the Dawn Mission the correct position is expected to emerge. Scientists today think that Ceres and Vesta followed very different evolutionary paths to reach their present states but after Dawn visits these bodies it could come up with deeper reasons behind the differing evolutions.
Dawn may also find Ceres having active hydrological processes leading to seasonal polar caps of water frost. Ceres also may be found to have a thin, permanent atmosphere distinguishing it from other minor planets.
Ceres and Vesta were first observed in 1801 and 1807 by Europeans Giuseppe Piazzi and Heinrich Olbers respectively. After a mere two centuries they are being specially visited by spacecraft laden with scientific instruments. What the human race will do with these two giant bodies of the asteroid belt during the centuries to come may be as far beyond imagination as the future of the American continent would have been to Columbus. Nevertheless, now that men have started planning bases and structures on the Moon it is possible to foresee that in time Ceres and Vesta will find themselves in environments their discoverers could never have imagined at all.
(The writer is an authority on the thermal evolution of asteroids and planets. His research on the asteroid Vesta helped NASA in deciding to send a spacecraft to the asteroid belt.  He’s also member of NASA’s Mars Exploration Rover
Mission.)

Did you know?
The first few asteroids were named after figures from Graeco-Roman mythology, but as such names started to run out, others were used —famous people, literary characters, the names of the discoverer's wives, children, and even television characters.
The first asteroid to be given a non-mythological name was 20 Massalia, named after the city of Marseilles. For some time only female (or feminized) names were used; Alexander von Humboldt was the first man to have an asteroid named after him, but his name was feminized to 54 Alexandra. This lasted until 334 Chicago was named. As the number of asteroids began to run into the hundreds, and eventually the thousands, discoverers began to give them increasingly frivolous names like 482 Petrina and 483 Seppina, named after the discoverer's pet dogs. This continues tho banned by the IAU.  (Sourced from Wikipedia)