This article studies necessarily conditions to establish reasonable safety zones in outer space. Studies, proposals, an international agreement and precedents in other fields of international law are considered for that purpose. The conclusions include: safety zones should be established for the safety and sustainability of space and restrictions pertaining to safety zones should be equally imposed to all stakeholders; safety zones could be lawfully established when they constitute an essential part of a certain space activity which is in accordance with international law. Elements for a reasonable safety zone also seem to include the implementation of TCBM and the operation in a restricted manner by a State having established one. Appropriateness test should be met in addition to the legality test.