9 0 s , ▁it ▁was ▁thought ▁that ▁most ▁of ▁the ▁ 2 8 , 0 0 0 ▁launched ▁objects ▁had ▁already ▁decay ed ▁and ▁about ▁ 8 , 5 0 0 ▁remained ▁in ▁orbit . ▁By ▁ 2 0 0 5 ▁this ▁was ▁adjust ed ▁up ward ▁to ▁ 1 3 , 0 0 0 ▁objects , ▁and ▁a ▁ 2 0 0 6 ▁study ▁increased ▁the ▁number ▁to ▁ 1 9 , 0 0 0 ▁as ▁a ▁result ▁of ▁an ▁AS AT ▁test ▁and ▁a ▁satellite ▁collision . ▁In ▁ 2 0 1 1 , ▁NASA ▁said ▁that ▁ 2 2 , 0 0 0 ▁objects ▁were ▁being ▁track ed . ▁ ▁A ▁ 2 0 0 6 ▁NASA ▁model ▁suggested ▁that ▁if ▁no ▁new ▁launch es ▁took ▁place ▁the ▁environment ▁would ▁retain ▁the ▁then - known ▁population ▁until ▁about ▁ 2 0 5 5 , ▁when ▁it ▁would ▁increase ▁on ▁its ▁own . ▁Richard ▁Crow ther ▁of ▁Britain ' s ▁Def ence ▁E valu ation ▁and ▁Research ▁Agency ▁said ▁in ▁ 2 0 0 2 ▁that ▁he ▁believed ▁the ▁cas cade ▁would ▁begin ▁about ▁ 2 0 1 5 . ▁The ▁National ▁Academy ▁of ▁Sciences , ▁summar izing ▁the ▁professional ▁view , ▁noted ▁w ides p read ▁agreement ▁that ▁two ▁bands ▁of ▁LE O ▁space — 9 0 0 ▁to ▁ ▁and ▁— w ere ▁already ▁past ▁critical ▁density . ▁ ▁In ▁the ▁ 2 0 0 9 ▁European ▁Air ▁and ▁Space ▁Conference , ▁University ▁of ▁South am pton ▁research