A　one-dimensional　collision　model　for　a　large-mass　structure　protected　by　high　porosity,　close-celled　aluminum　foam　subjected　to　low　velocity　impact　is　established　and　validated　with　drop　hammer　tests.　Based　on　the　proposed　model,　the　concept　of　critical　impact　velocity　is　introduced,　which　is　dependent　upon　the　mass　ratio　of　the　collision　system　as　well　as　foam　porosity.　Both　the　minimum　acceleration　and　critical　acceleration　of　the　protected　structure　are　obtained　under　different　impact　conditions.　It　is　demonstrated　that　closed-celled　aluminum　foams　are　suitable　for　protecting　large-mass　structures　subjected　to　low　velocity　impact.　When　the　initial　impact　velocity　is　lower　than　the　critical　one,　the　stress　imposed　by　the　foam　on　the　structure　will　not　exceed　the　plateau　stress　of　the　foam,　with　the　corresponding　acceleration　of　the　structure　significantly　reduced　if　the　foam　porosity　is　relatively　high.　On　the　other　hand,　when　the　initial　velocity　exceeds　the　critical　one,　due　to　foam　densification,　the　imposed　stress　increases　sharply　(about　5-15　times　of　foam　plateau　stress):　correspondingly,　the　acceleration　of　the　structure　increases　rapidly　(even　up　to　1000　g).　The　influence　of　impact　mass　ratio,　porosity　and　geometrical　dimensions　of　foam　protection　on　the　critical　velocity　and　acceleration　is　systematically　explored.