The present-day classification was initiated by Clark R. Chapman , David Morrison , and Ben Zellner in 1975 ( 1 ) with three categories : C for dark carbonaceous objects , S for stony ( silicaceous ) objects , and U for those which did not fit into either C or S. This classification has since been expanded and clarified . The present-day classification was initiated by Clark R. Chapman , David Morrison , and Ben Zellner in 1975 with three categories : C for dark carbonaceous objects , S for stony ( silicaceous ) objects , and U for those that did not fit into either C or S. This classification has since been expanded and clarified .