▁creating ▁over ▁ 2 0 0 0 ▁large ▁de br is ▁fragments . ▁These ▁de br is ▁cross ▁many ▁other ▁or bits ▁and ▁increase ▁de br is ▁collision ▁risk . ▁ ▁It ▁is ▁the or ized ▁that ▁a ▁sufficiently ▁large ▁collision ▁of ▁space craft ▁could ▁potentially ▁lead ▁to ▁a ▁cas cade ▁effect , ▁or ▁even ▁make ▁some ▁particular ▁low ▁Earth ▁or bits ▁effectively ▁unus able ▁for ▁long ▁term ▁use ▁by ▁orbit ing ▁sat ell ites , ▁a ▁phenomen on ▁known ▁as ▁the ▁K ess ler ▁synd rome . ▁The ▁theoretical ▁effect ▁is ▁project ed ▁to ▁be ▁a ▁theoretical ▁run away ▁chain ▁reaction ▁of ▁coll isions ▁that ▁could ▁occur , ▁expon entially ▁increasing ▁the ▁number ▁and ▁density ▁of ▁space ▁de br is ▁in ▁low - E arth ▁orbit , ▁and ▁has ▁been ▁hypoth es ized ▁to ▁ens ue ▁beyond ▁some ▁critical ▁density . ▁ ▁C rew ed ▁space ▁miss ions ▁are ▁mostly ▁at ▁ ▁alt itude ▁and ▁below , ▁where ▁air ▁drag ▁helps ▁clear ▁zones ▁of ▁fragments . ▁The ▁upper ▁atmosphere ▁is ▁not ▁a ▁fixed ▁density ▁at ▁any ▁particular ▁orb ital ▁alt itude ; ▁it ▁var ies ▁as ▁a ▁result ▁of ▁atmos pher ic ▁t ides ▁and ▁exp ands ▁or ▁contract s ▁over ▁longer ▁time ▁periods ▁as ▁a ▁result ▁of ▁space ▁weather . ▁These ▁longer - term ▁effects ▁can ▁increase ▁drag ▁at ▁lower ▁alt itudes ; ▁the ▁ 1 9 9 0 s ▁expansion ▁was ▁a ▁factor ▁in ▁reduced ▁de br is ▁density . ▁Another ▁factor ▁was ▁fewer ▁launch es ▁by ▁Russia ; ▁the