Abstract: UNIDROIT is drafting the Protocol to the Convention on International Interests in Mobile Equipment on Matters Specific to Space Assets under the background of commercialization of outer space. The drafters have not made consensus on the definition of the space assets. The relationship between space assets and space objects under the Registration Convention merits discussion. In order to define space assets， the drafters need to consider the structural and value independence of the assets， run around sensitive political issues， avoid ambiguity， comply with the purpose of the Protocol and take into account of the development of space technology. The definition of space assets should also assort with the UN outer space treaty system.