In the early twenty-first century, there are approximately 5000 commercial and private airplanes in the air at any one moment. The task of the U.S. Air Traffic Control System is to ensure the safe operation of these commercial and private aircraft. Air traffic controllers coordinate the movements of these planes, keep them at safe distances from each other, direct them during take-off and landing from airports, reroute them around bad weather, and ensure that air traffic flows smoothly. Other nations around the world maintain and operate similar air traffic control systems.
As space travel becomes a more common activity, it may become essential to institute a similar traffic control system for spacecraft. However, a more urgent problem is presented by the number of individual objects that are in orbit around Earth. The Space Surveillance Network (SSN) is currently tracking around 7,000 artificial objects circling Earth. The risk of collision with an object in space increases rapidly as the number of objects increases. A bit of space debris as small as a paint chip can do severe damage if it collides with a satellite because the relative velocity between the two objects can be as high as 25,000 kilometers per hour. The National Aeronautics and Space Administration (NASA) has calculated that the probability of a collision between a space station-sized satellite and a piece of orbital debris is 46 percent over the lifetime of the spacecraft unless avoidance techniques are used.
A space traffic control system would therefore have two separate missions. The current role of tracking and cataloging functioning and nonfunctioning orbital objects currently performed by SSN would need to be transferred to the new space traffic control system. This mission would also include a system for predicting and warning satellite operators of potential collisions between different spacecraft or between spacecraft and debris.
The future role of the space traffic control system would also include the monitoring of all space traffic and determination of the best orbits for the insertion of new satellites into Earth orbit. Moreover, it would guide and monitor the greatly increased numbers of both cargo-and passenger-carrying space vehicles anticipated in future decades. Such a system would require full utilization of both current and advanced technology. New satellites and spacecraft would carry transponders similar to the devices carried by modern aircraft. These would transmit identifying information back to the space traffic control radar system. Older satellites without transponders and nonfunctioning space debris would be tracked by more sophisticated equipment.
In order for the new system to function efficiently, it must have complete access to all of the information currently maintained in the SSN and the North American Aerospace Defense Command databases. Of greater importance, however, will be making the program international. Currently, each nation provides its own air traffic control system and aircraft flying over international boundaries are "handed off" from one system to another as the aircraft crosses the boundary. While the United States has taken the lead in monitoring and tracking space debris and functioning satellites, other countries must fully participate in any space traffic control system. It must be a truly international effort, supported by firm treaties between the nations with launch capabilities. At present, the United States, the Russian Federation, the European Space Agency, China, and Japan are all capable of launching payloads into any Earth orbits. These countries must all cooperate in the design and implementation of a space traffic control system.
see also Navigation (volume 3); Space Debris (volume 2); Tracking of Spacecraft (volume 3).
"Addressing Challenges of the New Millennium."6 thInternational Space Cooperation Workshop Report. AIAA:7-14, March 2001.
Johnson, Nicholas L. "The Earth Satellite Population: Official Growth and Constituents." In Preservation of Near-Earth Space for Future Generations, ed. John A. Simpson. New York: Cambridge University Press, 1994.
Kessler, Donald J. "The Current and Future Environment: An Overall Assessment," in Preservation of Near-Earth Space for Future Generations, ed. John A. Simpson. New York: Cambridge University Press, 1994.
Nieder, Raymond L. "Implication of Orbital Debris for Space Station Design," AIAA90-1331, 1990.
U.S. Congress, Office of Technology Assessment. Orbiting Debris: A Space Environmental Problem (Background Paper), OTA-BP-ISC-72. Washington, DC: U.S. Government Printing Office, 1990.