1l2. The most direet vay to attenpt to negate the threat against strategicveapons would be to introduce a passive defence in the for; of lrardening. Thenissile silos in the soviet union and the united states have successively beenh€Jdened to leveIs estinated. to be above 100 atmospheres, which is presumabr-ycfose to the practical linoit for these installations. Certain connand and. control-posts may have hardness in the 200 atnosphere ( alnost I,OOO pSI) range. Hovever,a'y sueh instal-lation woul-d be vuLnerable to a direct hit ,nless it vas so deepthat the disturbance from a crater irnmediately above wourd not provide enough Ground n4otion or faul-t ing to damage the instalration. Depths o-f the order of2 km. are probably required, but no instal"fations or plans are knornn whichutiLize such super-hardening. 113. To nuuify or substantially invalidate the nuclear threat against civiliantargets by the provision of adequate shelters fox the popur-ation and" the hardeningof industrial structures is in practice inpossibJ-e, This protlem is more fur_lytreated in chapter IV. l-14. The problem of active defence against nuc],ear weapons primarily d.evofves uponthe detection of such weapons, their tocation to a.n accuracy which allows adef^ndlng reapon tc be directed against the attacking weapon, a',d the provision ofad.equate numbers of defending weapons to handle an attack. 115' The principal technique to counter these development s is to nake systemsmobile. This can be e:{pected to be a major aspect in development of bailisticmissile systems in the future. l-l-6. As has been indicated- earlier, the SALT r rreaty betveen the united. states andthe.USSR effeciively bans, indefinitely, the extensive deploynent of anantiba istic nissile system. fn addition, it specifical-:-y ir,rr" nobil_e ABM"including those deployed in space.A,/ 35 / 392 Engfish ANNCX Page \8 117. On the other hand, the technical fac-Lors invofved have in flarr.y cases combi.nedto make the use of nuclear veapons for defence less effective than the use of