Econintersect: There are four relatively large bodies ("proto planets" or "dwarf planets") in the Asteroid Belt between Mars and Jupiter: Vesta, Ceres Pallas and Hygiea. These bodies, and the thousands of smaller rocky bodies known as asteroids, are believed to be among the first formed from the primordial dust at the birth of the solar system, before any of the larger planets. Ceres is the largest of the four with a diameter slightly less than 1,000 km. The other three are all around 400 km. For comparison, Ceres is about 27% of the diameter of earth's moon and a little more than 3% of of the earth's diameter. The video below describes the history of the solar system, and especially the asteroid belt, which is being explored by NASA's Dawn mission now approaching Ceres.
- Exploring New Worlds with the Dawn Mission (Carol Raymond, von Kármán Lecture, YouTube)
- Asteroid Belt (Wikipedia)