ets . ▁Using ▁spect ro sc opy , ▁comp ounds ▁such ▁as ▁alk ali ▁met als , ▁water ▁v apor , ▁carbon ▁mon ox ide , ▁carbon ▁dio x ide , ▁and ▁m eth ane ▁have ▁all ▁been ▁discovered . ▁ ▁Ast ero ids ▁Ast ero ids ▁can ▁be ▁class ified ▁into ▁three ▁major ▁types ▁according ▁to ▁their ▁spect ra . ▁ ▁The ▁original ▁categories ▁were ▁created ▁by ▁Clark ▁R . ▁Chap man , ▁David ▁Mor r ison , ▁and ▁Ben ▁Z ell ner ▁in ▁ 1 9 7 5 , ▁and ▁further ▁expanded ▁by ▁David ▁J . ▁Th olen ▁in ▁ 1 9 8 4 . ▁In ▁what ▁is ▁now ▁known ▁as ▁the ▁Th olen ▁classification , ▁the ▁C - types ▁are ▁made ▁of ▁carbon ace ous ▁material , ▁S - types ▁consist ▁mainly ▁of ▁sil ic ates , ▁and ▁X - types ▁are ▁' met all ic '. ▁ ▁There ▁are ▁other ▁class ifications ▁for ▁unusual ▁astero ids . ▁ ▁C - ▁and ▁S - type ▁astero ids ▁are ▁the ▁most ▁common ▁astero ids . ▁In ▁ 2 0 0 2 ▁the ▁Th olen ▁classification ▁was ▁further ▁" e vol ved " ▁into ▁the ▁S MA SS ▁classification , ▁expand ing ▁the ▁number ▁of ▁categories ▁from ▁ 1 4 ▁to ▁ 2 6 ▁to ▁account ▁for ▁more ▁precise ▁spect ro scop ic ▁analysis ▁of ▁the ▁astero ids . ▁ ▁Com ets ▁ ▁The ▁spect ra ▁of ▁com ets ▁consist ▁of ▁a ▁reflected ▁solar ▁spectrum ▁from ▁the ▁dust y ▁clouds ▁surrounding ▁the