19. The rocket Eay carry one or sereral valhead.s, r^rhich nay be independentl-ytar6ete,l- the nultiple. initependently targetabLe re_entry .,Lniol. (Ufnvl ".r"1., vas tlevelopeal by the united states in the late 1g6os ana- is a"oioy"a-J"" ty tr,usoviet union. rn a Mr*vear systen, the separate re- entry vehici.es- are usuallycarried on a 'bustt r,rhich rel eases the Rvs one by one after eaking pre-seleetedchanges io speed. anal orientation so as to direci the RVs to their -="prrrtu targets.These RVs can reportedly land insicle an area of perhaps 150 ]""; -;y- t'OO-;r. Thus " they are not as compreteJ.y independent in a*ivar trne or location as they wourdbe vere thev on different rcBr'rs, and they provide less tarletin!- ir""itiritv. 20" triith increasing nissire accuracy and many Rvs per missile, MrRV has raiseilthe spectre that a frection of one siders rcBi{ forcis rnay in a ,'first strike',destroy the opponentrs rcBl.ls stil"l housed in their hardeneil silos. ?his woult! bepossible v-ith sufficient accuracy and reriabir.ity of the attacking Rvs, and. ifthe rcB4s to be attacrred. '.rere not raunched. before they nere destroyed. rtrissitua'bion is therefore considerecr. to be potentially unstabr-e, since in tine ofcrisis each sicle uay consid.er reunehing its r'issitls rather ihan risk theirdestruction, 2r' rf a target is vulnerabre to a particular pre.sure rever of the air bIast,its destruction mqy be achieved within a certain rexinum area around. the pointof d.etonation, The size of this area increeses with the weapon yield (e.g., by afactor 4 for an B-fold increase in yield or a factor loo for a r,ooo-for.d. increasein yield). By contrast, the area of destruction due to br-ast increases inproporti.on to the nuuber of veapons. This neans in practice that the degtructionis increasetl by increaslng the nurrber of varheads and lorering their ind.ividualRV yield; i.e., one Large varhead is not so effective as severel srnal-Ier ones ofthe same_total yield spread out over the taxget area. This is also iuustrsted by