ography ▁▁▁ ▁Donald ▁K ess ler ▁( K ess ler ▁ 1 9 9 1 ), ▁" Col lis ional ▁C asc ading : ▁The ▁Lim its ▁of ▁Population ▁G row th ▁in ▁Low ▁Earth ▁Or bit ", ▁Adv ances ▁in ▁Space ▁Research , ▁Volume ▁ 1 1 ▁Number ▁ 1 2 ▁( Dec ember ▁ 1 9 9 1 ), ▁pp .   6 3 ▁– ▁ 6 6 . ▁ ▁Donald ▁K ess ler ▁( K ess ler ▁ 1 9 7 1 ), ▁" Est imate ▁of ▁Part icle ▁D ens ities ▁and ▁Coll ision ▁D anger ▁for ▁Space craft ▁Mov ing ▁Through ▁the ▁Ast ero id ▁B elt ", ▁Phys ical ▁Studies ▁of ▁Minor ▁Plan ets , ▁NASA ▁SP - 2 6 7 , ▁ 1 9 7 1 , ▁pp .   5 9 5 ▁– ▁ 6 0 5 . ▁Bib code ▁ 1 9 7 1 NA SS P . 2 6 7 .. 5 9 5 K . ▁ ▁Donald ▁K ess ler ▁( K ess ler ▁ 2 0 0 9 ), ▁web pages . char ter . net , ▁ 8 ▁March ▁ 2 0 0 9 . ▁ ▁Donald ▁K ess ler ▁( K ess ler ▁ 1 9 8 1 ), ▁" S ources ▁of ▁Or b ital ▁Deb ris ▁and ▁the ▁Project ed ▁Environment ▁for ▁Future ▁Space craft ", ▁Journal ▁of ▁Space craft , ▁Volume ▁ 1 6 ▁Number ▁ 4 ▁( J uly – August ▁ 1 9 8