In the past, NASA’s ODPO has emphasized avoiding the creation of orbital debris through mitigation techniques. However, in 2005, a study by Liou and Johnson using the LEGEND model showed that even if no future launches occurred, collisions between existing satellites would increase the 10-cm and larger debris population faster than atmospheric drag would remove objects. The “No New Launches” scenario highlights the eventual need for remediation of the existing debris population (also known as active debris removal, or ADR). The potential for collisions to damage the space environment was underscored by the Chinese ASAT test in 2007 and the accidental collision between Cosmos 2251 and Iridium 33 in 2009 (ODQN 14-4, ODQN 14-2, ODQN 13-2, ODQN 13-1, ODQN 12-1, and ODQN 11-2). These two events increased the SSN tracked orbital population by almost one third.
If the goal of remediation is to reduce the risk to the current fleet of operational spacecraft, remediation techniques need to focus on removal of small sized (but still damaging) debris. If the goal is to control the long-term growth of the debris population, ADR techniques need to concentrate on the removal of large, massive objects such as intact rocket bodies and non-functional satellites. These massive objects are the long-term source of fragmentation debris from on-orbit explosions and collisions. Studies have indicated that the removal of as few as five of the highest risk objects (defined as mass x probability of collision) per year can stabilize the long-term low Earth orbit debris environment (ODQN 15-3, ODQN 15-2).
Any successful ADR concept must be technologically feasible, economically affordable, and politically acceptable to the international community. In addition, debris removal activities should also be accomplished in a manner that does not unduly increase hazards to people and property on Earth from reentering debris.
The June 2010 National Space Policy for the United States of America directs NASA and the Department of Defense to “Pursue research and development of technologies and techniques… to mitigate and remove on-orbit debris…” However, it should be noted that, currently, no U.S. government entity has been assigned the task of removing existing on-orbit debris.
Many different methods for remediation of the orbital debris environment have been proposed over the years. These include the use of lasers, electro-dynamic or momentum exchange tethers, tugs, drag enhancement devices, and other, more exotic methods.