asteroidCount = BinCounts[ Sort@Cases[(AstronomicalData[#, "SemimajorAxis"]/149597870691) & /@ Join[AstronomicalData["InnerMainBeltAsteroid"], AstronomicalData["MainBeltAsteroid"], AstronomicalData["OuterMainBeltAsteroid"]], x_?NumberQ], {2, 3.5, .005}];
ListPlot[asteroidCount, Joined -> True, Filling -> 0, Mesh -> All, Ticks -> {Table[{Rescale[x, {2, 3.5}, {0, 301}], x}, {x, 2, 3.5, .2}], Automatic}]