Minding the (Space) Environment
Ray A. Williamson, PhD, is editor of Imaging Notes and Executive Director of the Secure World Foundation, an organization devoted to the promotion of cooperative approaches to space security (www.SecureWorldFoundation.org).
Most remote sensing satellites are devoted in some way to monitoring and managing Earth’s environment. Indeed, the broad, synoptic view of moderate resolution, multispectral satellite cameras is ideally suited to capturing the data needed to alert the global community to large scale deforestation, river and ocean pollution, and other environmental threats to life and property. Atmospheric sensors provide us with information on threats to the protective cover that makes life on Earth possible: ozone loss in the upper atmosphere, buildup of CO2 and other greenhouse gases in the lower layers, and severe weather. We all depend on, and can be proud of, the important work these spaceborne sensors do for us in following changes in Earth’s environment.
Other Earth observation satellite systems provide valuable reconnaissance and commercial information to users. All of these satellite systems are increasingly at risk.
Risk of Collision
Outer space seems to be vast and mostly empty, with collisions between working satellites only a remote possibility. Yet it now appears that traffic in certain orbits has reached the point where constant vigilance is necessary to avoid collisions. The sun-synchronous polar orbits used for most Earth-observing satellites are particularly at risk.
Case in point: On July 4, 2007, NASA found it prudent to move the NASA-Canadian CloudSat satellite in order to avoid a possible collision with the Iranian Sinha-1 remote sensing satellite. CloudSat, launched in April 2006, is an experimental satellite devoted to providing, among other things, new data about the relationship of clouds to storms by using advanced radar. The maneuver reduced the risk of collision.
A few days later, NASA moved CloudSat back into its earlier orbit in order to synchronize its orbit with the Cloud-Aerosol LiDAR and Infrared Pathfinder (CALIPSO) satellite, a joint project between NASA and the French Centre National d’Etudes Spatiales (CNES). Working together, the two satellites provide "new, never-before-seen 3D perspectives of how clouds and aerosols form, evolve, and affect weather and climate" (http://www-calipso.larc.nasa.gov). Both fly in formation with three other environmental spacecraft in the so-called A-Train.
Risk of Space Debris
Orbital debris is an even greater threat to working satellites than is the threat of collision. Debris experts estimate that more than 16,000 items of debris 10 cm in length or greater now speed around Earth in various orbits. The number of untrackable smaller pieces is orders of magnitude higher. Yet, even small bits of debris can be highly destructive, because the impact velocities between the debris and a functioning satellite approach an average of 10 km per second.
Debris creation is an unavoidable by-product of launching and operating spacecraft in Earth orbit. Hence, debris experts expect the orbital debris population to increase steadily as more countries enter the world of space activities. Fortunately, even non-experts have come to realize that it is important to limit the space debris generated to the absolute minimum. That is why, after years of study and discussion, the United Nations Committee on the Peaceful Use of Outer Space (COPUOS) passed a non-binding resolution in June 2007 formally calling for the reduction of new debris generation and the study of means to remove debris from orbit. That resolution went to the U.N. General Assembly and was approved in October 2007.
Figure 1 The CALIPSO mission operations team uses commercial visualization software to carefully analyze the risk of collision with space debris. Image is courtesy of NASA’s Langley Research Center, NASA’s Goddard Space Flight Center Conjunction Assessment Team, and AGI.
That action by the U.N. was an important, perhaps crucial, small step in providing long-term governance of our orbital environment. I am sure that part of the impetus behind its passage was the Chinese anti-satellite test on January 11, 2007, in which China destroyed an aging Chinese weather satellite, Feng Yeng-1. That test added some 2000 pieces of trackable debris to the deadly mix in sun-synchronous polar orbit, sharply increasing the long-term danger to working satellites. Orbital debris at those altitudes takes tens of years to fall back to Earth. In the meantime, satellites that we depend on for critical environmental, security, and business-related information are at increased risk.
Larger steps toward space governance are needed. As additional countries and private companies launch spacecraft into orbit, popular orbits like the polar, sun-synchronous and geosynchronous orbits will become much more crowded and will need an international space traffic management system to keep them relatively collision free.
As the case of the CloudSat and Sinha-1 satellites reveals, a quasi space management regime exists now, mostly controlled by the United States. The U.S. Air Force maintains ground-based optical and radar observatories that keep track of the 18,000 or so working satellites and larger debris—so-called space situational awareness, or SSA. Through NASA, it publishes an open catalogue of orbital elements that commercial and non-U.S. satellite operators can use to guide their spacecraft and avoid collisions. However, this open catalogue holds much less information on orbits of working satellites and debris than the full catalogue (classified satellite information is withheld, as are the orbital elements of debris of uncertain provenance).
Commercial entities and other national agencies can request and receive guidance from the U.S. Air Force in planning needed spacecraft maneuvers. However, satellite operators complain that the Air Force is often slow to respond to requests. That is understandable, given that maintaining SSA against ever increasing amounts of debris and satellites is extremely expensive and the Air Force budgets for maintaining such a capability have not kept up with the need.
Further, other countries do not want to depend on the United States for such critical information. As a result, many countries are now developing their own SSA capabilities. Because the U.S. military advantage in maintaining a closed catalogue is therefore declining, it would be in the U.S. interest to lead the way in a cooperative program for SSA, first with close allies, and then broadening to other space-capable nations as experience is gained. This could be an important first step in developing an international space traffic management system for space, a system that would provide much greater safety and security for the many Earth observation satellites in the increasingly crowded sun-synchronous polar orbits.