The 2011 National Security Space Strategy declared, "Space is vital to U.S. national security and our ability to understand emerging threats, project power globally, conduct operations, support diplomatic efforts, and enable global economic viability." Maintaining the benefits afforded by space is central to a wide range of U.S. national interests. These include significant government (military, intelligence, and civil ) and commercial interests. In recent years, recognition has been growing among space-faring nations and the international scientific community that the mass of man-made debris in Low-Earth Orbit (LEO) has reached a critical density that will lead to a slow but unstoppable growth of space debris that could have profound implications for those interests. Significant debris-generating events in the past decade have heightened the issue, and a number of national and international studies confirm that the amount of current LEO debris is both unstable and increasing. This instability in the space environment presents a threat to U.S. national interests in space. U.S. Deputy Secretary of Defense William J. Lynn, III, summarized the national security implications of interference with our satellites and space capabilities: "Space systems enable our modern way of war. They allow our warfighters to strike with precision, to navigate with accuracy, to communicate with certainty, and to see the battlefield with clarity. Without them, many of our most important military advantages evaporate." Although space debris is not the sole threat to U.S. space assets, this report focuses on the threat posed by orbital debris, the steps that have been taken thus far to mitigate it, and what might be done to ensure the long-term sustainability of the space environment. Although Congress may not need to make key decisions immediately, Congress may want to begin to focus on measures the United States may take to mitigate the threat associated with orbital debris. Decades of human space flight—primarily U.S. and Russian space activities—have littered the Earth's orbit with debris. NASA defines orbital debris as "all man-made objects in orbit about the Earth which no longer serve a useful purpose." Examples include derelict spacecraft, abandoned space launch vehicle stages, mission-related debris, and fragments created as a result of explosions or collisions. The U.S. Space Surveillance Network is the leading space object tracking system in the world and catalogues objects as small as about 10 cm (softball size) in LEO and as small as 1 meter in Geosynchronous Orbit. Today, the Space Surveillance Network tracks more than 23,000 objects 10 cm in diameter or larger in orbit around the Earth. Of those, only about 1,100 (5%) are active satellites. The rest is orbital debris. In addition to the debris tracked by the Space Surveillance Network, there are hundreds of thousands of pieces of debris smaller than 10 cm, which are considered too small to track or catalogue, but are still capable of damaging satellites and the International Space Station. Prior to 2007, the principal source of space debris was the explosion of old launch vehicle upper stages that had been left in orbit with unspent energy sources. Explosions of this type were prevalent in the 1970s and 1980s but have since slowed due to increased mitigation techniques practiced worldwide. Since 2007, two significant debris-generating events have greatly increased the amount of debris in orbit. On January 11, 2007, the Chinese government launched an interceptor missile in an anti-satellite weapon test that destroyed their decommissioned Fengyun-1C weather satellite. This intentionally destructive event created the most severe orbital debris cloud in space flight history, generating more than 3,000 pieces of debris larger than 10 cm and an additional 150,000 pieces larger than 1 cm. The majority of debris particles were thrown into long-duration orbits, and remnants of this event will likely remain in orbit for at least a century. One expert reported it likely that debris from the Chinese ASAT test damaged a small Russian satellite six years later in 2013. But the Pentagon later reported that this Russian satellite most likely broke apart for different reasons and not from debris from the Chinese ASAT test. Another space expert suggested, however, it was more likely the satellite did not break up on its own and may have been hit by debris too small to be tracked and identified by the U.S. Space Surveillance Network. Two years after this incident, the first accidental hypervelocity collision occurred between two intact spacecraft. On February 10, 2009, an operational U.S. Iridium communications satellite collided at a near right angle with a defunct Russian Cosmos satellite. The collision resulted in roughly 2,100 new pieces of debris larger than 10 cm. Collision probability reports calculated by the Center for Space Standards & Innovation had predicted a "close approach" between the satellites on the day of the event. This did not cause particular alarm, however, because the approach of these two satellites was not the top predicted close approach that day, nor even the top predicted close approach of any of Iridium's satellites for the coming week. Nevertheless, at the time the approach was predicted to occur, Iridium abruptly lost contact with its spacecraft and the U.S. Space Surveillance Network began detecting debris, confirming that an actual collision had occurred. These two events increased the amount of objects in Earth orbit cataloged by the Space Surveillance Network by one-third. These collisions alone have been described as essentially negating the results of more than 20 years of international compliance with debris mitigation guidelines. As shown in Figure 1 , following these two events, the amount of cataloged fragmentation debris in orbit more than doubled after having remained nearly constant for more than 20 years. Since the 1960s, the United States and Russia have conducted a significant number of ballistic missile defense (BMD) tests in space resulting in varying amounts of debris. It appears, however, that the debris created by launching targets and interceptors into space, together with any resultant orbital debris from those tests and intercepts, is unlikely to be consequential in the context of this report. This is especially true of current missile defense testing. Although the 1963 Limited Test Ban Treaty banned nuclear testing in space, the United States conducted a number of BMD tests in the late 1960s and early 1970s to validate the effectiveness of a nuclear-tipped BMD system deployed briefly in 1975-1976. There are no available data on how much these tests may have contributed, if at all, to the amount of existing space debris. Since the early 1980s, the United States has conducted a number of hit-to-kill BMD flight and intercept tests outside the Earth's atmosphere in space. In February 2008, a BMD-capable Aegis cruiser used a modified version of the Aegis BMD system to successfully shoot down an inoperable U.S. surveillance satellite that was in a deteriorating orbit. According to the Missile Defense Agency (MDA) "nearly 100 percent of the debris safely burned-up during reentry within 48 hours and the remainder safely re-entered within the next few days." Currently, MDA plans and designs flight tests to mitigate and minimize any potential hazards. According to MDA, debris modeling predicts that almost half of the mass would be vaporized during the intercept collision and about a quarter of the mass would be dispersed into fine particles with masses of less than a few tens of grams that then burn up on reentry. The remaining mass is predicted to be in pieces large enough to survive reentry and reach the ground. It is not known how long any of this debris remains in orbit before it reenters the atmosphere. It should be pointed out, however, that of the 10 most significant events that generated space debris, none are attributed to any U.S. BMD tests. Orbital debris can instantly destroy or disable space-based resources. A collision with a 10 cm object would catastrophically damage a typical satellite, a 1 cm object would likely disable a spacecraft or penetrate the shields of the International Space Station, and a 1 mm object could destroy satellite sub-systems. Once a risk of collision has been identified, the only effective way to mitigate the risk is to move the spacecraft out of the way. Moving a satellite, however, entails costs. Avoidance maneuvers cost fuel, shorten a spacecraft's lifetime, and may disrupt data and service continuity. A 2013 European Commission Memorandum reported that some European space agencies operating satellites carry out one satellite collision avoidance maneuver each month on average. It is important to note, however, that most objects in space are not under control, and their orbital paths cannot be altered. This means that the majority of potential collisions cannot be avoided using evasive maneuvers. In 2011, U.S. Deputy Secretary of Defense William J. Lynn, III, summarized the threat from orbital debris: "Whether or not we can see it, the debris is there. The danger is that each collision exponentially raises the potential for another, such that a debris cascade could someday render entire orbits unusable." But this danger has been known since the late 1970s. Current research suggests that the current LEO debris environment is unstable: The amount of orbital debris will continue to grow, in spite of debris mitigation measures, because collisions will generate new debris faster than it is removed by natural forces. A number of studies, both national and international, have concluded that orbital debris has already reached a "tipping point." A 2011 study by the National Research Council of the National Academies reported that NASA's models showed the orbital debris population would continue to increase as a result of random collisions involving non-operational intact debris. The study concluded that the amount of debris currently in orbit will continually collide with itself, leading to the growth rate of debris and increases in spacecraft failures. The study noted that the debris increase thus far has occurred most rapidly in LEO, but Geosynchronous Orbit may potentially suffer the same fate over a longer time period.