In 2010, the Scientific and Technical Subcommittee of the UNCOPUOS formed the Working Group on Long Term Sustainability (LTS) of Outer Space Activities, assigning it the task of formulating voluntary non-binding guidelines focusing on sustainable space utilization, space debris and space operations, space weather, and regulatory regimes. At its June 2016 meeting, the UNCOPUOS approved 12 of the proposed guidelines, while several remained on the UNCOPUOS agenda. Although the LTS Guidelines are voluntary, their adoption by the UNCOPUOS and consideration by the UNGA’s 4th Committee, are evidence of a growing awareness of their potential contribution to the evolution of space law applicable to all states. This paper explores whether the LTS Guidelines could evolve into customary legal norms as part of customary international law (CIL) and steps that could promote that evolution.