▁so ak ▁up ▁an ▁AB M ▁system ▁was ▁of ▁little ▁consequence . ▁ ▁Through ▁the ▁ 1 9 7 0 s ▁a ▁number ▁of ▁alternative ▁concepts ▁to ▁defend ▁the ▁sil os ▁had ▁been ▁proposed . ▁Richard ▁Gar win ▁out lined ▁a ▁series ▁of ▁alternatives ▁to ▁large - scale ▁AB M ▁systems ▁like ▁Saf egu ard . ▁Among ▁these ▁ideas ▁were ▁a ▁" bed ▁of ▁n ails " ▁consisting ▁of ▁vertical ▁steel ▁sp ikes ▁surrounding ▁the ▁sil o ▁that ▁would ▁destroy ▁the ▁war head ▁before ▁it ▁hit ▁the ▁ground ▁and ▁triggered , ▁jam ming ▁systems ▁to ▁interrupt ▁rad ar ▁f uses , ▁the ▁dust ▁defense ▁where ▁small ▁nuclear ▁war head s ▁would ▁be ▁set ▁off ▁while ▁the ▁war head s ▁approached ▁and ▁throw ▁huge ▁amounts ▁of ▁dust ▁into ▁the ▁air ▁that ▁would ▁ab rade ▁the ▁war head s , ▁and ▁a ▁non - n uc lear ▁version ▁of ▁the ▁same ▁concept , ▁the ▁" cur tain ▁of ▁steel ▁pel lets ". ▁The ▁last ▁of ▁these ▁was ▁picked ▁up ▁by ▁Bernard ▁Feld ▁and ▁K osta ▁Ts ip is ▁in ▁a ▁major ▁article ▁in ▁Scient ific ▁American ▁in ▁ 1 9 7 9 . ▁They ▁proposed ▁replacing ▁the ▁shot gun - like ▁project iles ▁with ▁sw ar ms ▁of ▁small ▁un gu ided ▁rock ets ▁that ▁would ▁fire ▁at ▁a ▁range ▁of ▁about ▁. ▁As ▁they ▁were ▁un gu ided , ▁they ▁suggested ▁that ▁they ▁would ▁not ▁be ▁considered ▁inter ceptor ▁miss iles ▁under ▁the ▁AB M ▁treat y , ▁but ▁also ▁suggested ▁that ▁a ▁ren