Have a look at the video animation below, capturing the exponential increase of asteroid discoveries over time. The animation starts in 1980 when only a handful of objects between the orbits of Mars and Jupiter were known. As new asteroids are discovered they are added to the map and highlighted white so that they can be easily distinguished from already known ones.
* Earth Crossers are Red, Earth Approachers (Perihelion less than 1.3AU) are Yellow, All Others are Green
Notice how the pattern of discovery follows the Earth around its orbit. Most discoveries are made in the region directly opposite the Sun, when asteroids are close to opposition. At the time of opposition Main Belt Asteroids are not only closest to Earth in their orbit around the Sun, but also most of their surface is illuminated (when seen from the Earth) and therefore they are reaching their maximum brightness.
Furthermore, you’ll also notice some clusters of discoveries on the line between Earth and Jupiter, these are the result of surveys looking for Jovian moons. Similar clusters of discoveries can be tied to the other outer planets, but those are not visible in this video.
As the video moves into the mid 1990’s we see much higher discovery rates as automated sky scanning systems come online. Most of the surveys are imaging the sky directly opposite the sun and you’ll see a region of high discovery rates aligned in this manner.
At the beginning of 2010 a new discovery pattern becomes evident, with discovery zones in a line perpendicular to the Sun-Earth vector. These new observations are the result of the WISE (Widefield Infrared Survey Explorer) which is a space mission that’s tasked with imaging the entire sky in infrared wavelengths.
Currently we have observed over half a million minor planets, and the discovery rates show no sign that we’re running out of undiscovered objects.