Asteroids, or Planetoids, a number of small planets existing between the orbits of Mars and Jupiter. The existence of a planet in this zone had been surmised by Kepler as far back as 1596, but it was not till the beginning of this century that the first of the planetoids was discovered. More than two hundred have since been found, the four largest being Vesta, Ceres, Pallas, and Juno. They are at an average distance of about 250,000,000 miles from the sun. A very good telescope is necessary in order to see them, as the largest of them is only about 230 miles in diameter.