“…because as we know, there are known knowns; there are things that we know that we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns, the ones we don’t know we don’t know.” —Donald Rumsfeld, United States Secretary of Defense, 2002
There is widespread belief that the population of man-made space debris in low Earth orbit (LEO) will continue to grow even with the widespread adoption of mitigation measures, such as those recommended by the Inter-Agency Space Debris Coordination Committee (IADC). This growth is expected to result from accidental collisions involving large intact objects. Recent modelling studies have suggested that removing such objects, through a process called Active Debris Removal (ADR), may reduce the number of collisions and prevent the growth of debris in LEO. However, some assumptions about future launch traffic, compliance with mitigation guidelines and solar activity, amongst others, were made within these studies to constrain the many degrees of freedom. When these constraints are relaxed, predictions of the future space debris population become much more uncertain; the benefits of ADR may be lost entirely, or the process may not be needed at all. Within this context, there are also unknowns that are likely to exacerbate the uncertainty in predictions, including long-term changes in solar irradiance and thermospheric mass density. The impact of these, in particular, is not acknowledged but taken together with other sources of uncertainty, they may have a critical influence on our understanding of the future space debris environment.