Big Sky or Kessler Syndrome? Several decades ago, as the U.S. was just beginning to launch items into space, NASA officials relied on the big sky theory when faced with the question of accumulating debris. According to the theory, objects left in space would disperse and eventually re-enter the Earths atmosphere, where they would burn up before hitting the ground. Following this logic, there was no reason to be concerned about over-crowding the space around our planet.
The big sky theory was challenged in 1978 by a NASA scientist named Donald Kessler. Kessler published a paper titled Collision Frequency of Artificial Satellites: The Creation of a Debris Belt, which argued that the increasing number of man-made objects in space posed a huge threat. It wasnt just the slow growth of these objects, Kessler wrote, but the way in which inevitable collisions would create a domino-like effect. One big collision could generate thousands of pieces of debris, each of which might go on to strike other objects, leading to a chain reaction that would exponentially increase the number of items in space. This phenomenon, later dubbed the Kessler Syndrome, would produce a growing belt of debris.
The development of this belt of debris would have significant consequences, beginning with damage to existing satellites, as more and more are pelted with sharp objects traveling at high speeds. This could eventually disrupt satellites tasked with communication and weather observation functions, causing a noticeable impact for people on Earth. Even worse, any future space exploration missions or even service missions to repair existing objects in orbit would become far more dangerous.
Computer model of debris in low-Earth orbit
Right now, NASAs Orbital Debris Program Office is monitoring about 19,000 pieces of space debris larger than 10 centimeters. Because these are the biggest objects, they pose the greatest risk to Earth. This risk includes surviving re-entry into Earth and causing damage here, of striking a spaceship or the International Space Station, or of colliding with another piece of debris and causing the effect predicted by Kessler.
Unfortunately, there are also half a million items between one and 10 centimeters, and an almost incalculable number (projected at over 10 million) that are smaller than a centimeter. And it is not just the large pieces that create a riskthe International Space Station (ISS) is considered to be vulnerable to impact by objects only one centimeter across. The ISS was wisely designed with the ability to move out of the path of oncoming debris and has had to executive five such maneuvers in the last 30 months alone. Officials at NASA now believe that such debris poses the greatest threat to the ISS.
Evidence of the Kessler Syndrome at Play
Writing in 1978, Kessler predicted that the cascading effect of debris collisions would begin in 30 to 40 years. And now, right on time, we are starting to see evidence. The first event, in 2007, was not exactly what Kessler had expected; it was caused when China launched a rocket at one of its own defunct satellites, presumably as a show of military force. The rocket hit its target, and in the process created about 3000 pieces of debris that are now shooting through space and causing additional collisions. One large chunk came very close to hitting both the space shuttle Atlantis and the Hubble Space Telescope.
The second event was more in line with Kesslers original argument the idea that space would simply become more and more crowded until collisions became inevitable. On February 10, 2009, a U.S. communications satellite named Iridium 33 was struck by an out-of-service Russian satellite called Cosmos 2251. Both were traveling at 18,000 miles per hour, or five miles per second, and the impact created a cloud of debris consisting of thousands of individual pieces.
This collision and the Chinese rocket vastly increased the amount of debris in orbit around the Earth. Kessler said that these two events alone doubled the amount of fragments in Earth orbit and completely wiped out what we had done in the last 25 years to manage the threat generated by space pollution. Kesslers efforts included a set of rules and guidelines, which were subsequently adopted by many other nations, specifying which types of objects could be left in space.
Re-Examination and Clean-Up
After the satellite collision in 2009, NASA and the U.S. military began to take the issue of space pollution much more seriously. These organizations had previously been watching only 120 satellites for potential collisions, and Iridium 33 wasnt even on the radar. They quickly expanded their capacity and now monitor thousands of satellites and tens of thousands of pieces of debris.
In December 2009, Kessler and his colleagues organized the Conference on Orbital Debris Removal, which sought out a broad range of inventions and concepts for cleaning up the polluted space around our planet. Kessler was impressed with the results, saying Ive gone from being totally skeptical to thinking maybe something will work We can bring things down; its just going to cost a lot. NASA is pursuing a variety of avenues right now, and its 2011 budget proposal included funds for research grants in this area.
The proposed clean-up methods are numerous, and they are all still in the design and development phases. With that being said, here are a few of the most promising ideas:
- DARPA, the Defense Advanced Research Projects Agency, has outlined plans for an Electrodynamic Debris Eliminator featuring 200 nets, all of which are connected to a central unit. Debris captured in the nets could be sent back toward Earth to burn up in the atmosphere or even recycled on the spot. DARPA is planning test flights for 2013.
- The CubeSail, designed in Britain, is based around sail technology that relies on the force of sunlight to move. In the short term, the project team envisions a sail attachment for new satellites that could be used to move the object away from the debris belt either toward Earth or out into space. Further down the road, they hope to create special debris cleaners that use solar sails to navigate through space and gather objects.
- Tethers Unlimited, a space company in Seattle, proposed a vehicle named Rustler that would connect a miles-long attachment made of wire mesh to debris in space. Electrical current could be sent through the attachment and, relying on the principles of electromagnetic forces, the item would be pulled in by Earth's magnetic field and eventually burn up in the atmosphere.
A prototype CubeSail
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In the most general sense, the term space pollution includes both the natural micrometeoroid and man-made orbital debris components of the space environment; however, as "pollution" is generally considered to indicate a despoiling of the natural environment, space pollution here refers to only man-made orbital debris. Orbital debris poses a threat to both manned and unmanned spacecraft as well as the earth's inhabitants.
Environmental and Health Impacts
The effects of debris on other spacecraft range from surface abrasion due to repeated small-particle impact to a catastrophic fragmentation due to a collision with a large object. The relative velocities of orbital objects (10 kilometers per second [km/s] on average, but ranging from meters per second up to 15.5 km/s) allow even very small objects—such as a paint flake—to damage spacecraft components and surfaces. For example, a 3-millimeter (mm) aluminum particle traveling at 10 km/s is equivalent in energy to a bowling ball traveling at 60 miles per hour (or 27 m/s). In this case, all the energy
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would be distributed in an area of the same size as the particle, causing cratering or penetration, depending on the thickness and material properties of the surface being impacted. There has been one accidental collision between cataloged objects to date, but surfaces returned from space and examined in the laboratory confirm a regular bombardment by small particles. Space Shuttle vehicle components, including windows, are regularly replaced due to such damage acquired while in orbit. Debris also poses a hazard to the surface of the Earth. High-melting-point materials such as titanium, steel, ceramics, or large or densely constructed objects can survive atmospheric reentry to strike the earth's surface. Although there have been no recorded fatalities or severe injuries due to debris, reentering objects are regularly observed and occasionally found.
Debris is typically divided into three size ranges, based on the damage it may cause: less than 1 centimeter (cm), 1 to 10 cm, and larger than 10 cm. Objects less than 1 cm may be shielded against, but they still have the potential to damage most satellites. Debris in the 1 to 10 cm range is not shielded against, cannot easily be observed, and could destroy a satellite. Finally, collisions with objects larger than 10 cm can break up a satellite. Of these size ranges, only objects 10 cm and larger are regularly tracked and cataloged by surveillance networks in the United States and the former Soviet Union. The other populations are estimated statistically through the analysis of returned surfaces (sizes less than 1 mm) or special measurement campaigns with sensitive radars (sizes larger than 3 mm). Estimates for the populations are approximately 30 million debris between 1 mm and 1 cm, over 100,000 debris between 1 and 10 cm, and 8,800 objects larger than 10 cm.
The number, nature, and location of objects greater than 10 cm in size are provided in the fragmentation debris table and in the image of space debris around Earth. Low Earth orbit (LEO) is defined as orbital altitudes below 2,000 km above the earth's surface and is the subject of the image of space debris around Earth. Middle Earth orbit (MEO) is the province of the Global Positioning System (GPS) and Russian navigation satellite systems and is located at approximately 20,000-km altitude, whereas the geosynchronous Earth orbit (GEO) "belt" is inhabited primarily by communications and Earth—observation payloads around 35,800 km. The majority of objects in these orbital regions are in circular or near-circular orbits about the earth. In contrast, the elliptical orbit category includes rocket bodies left in their transfer (payload delivery) orbits to MEO and GEO as well as scientific, communications, and Earth-observation payloads. Of all objects listed in the
©NASA/Roger Ressmeyer/Corbis. Reproduced by permission.)
Remediation takes two courses: protection and mitigation. Protection seeks to shield spacecraft and utilize intelligent design practices to minimize the effects of debris impact. Mitigation attempts to prevent debris from being created. Active mitigation techniques include collision avoidance between tracked and maneuverable objects and the intentional reentry of objects over the oceans. Passive techniques include venting residual fuels or pressurized vessels aboard rockets and spacecraft, retaining operational debris, and placing spacecraft into disposal orbits at the end of a mission. Space salvage or retrieval, while an option, is currently too expensive to employ on a regular basis.
The United States and international space agencies recognize the threat of debris and are cooperating to limit its environmental and health hazards. The Interagency Space Debris Coordination Committee (IADC), sponsored originally by the National Aeronautics and Space Administration (NASA), has grown to include all major space-faring nations. The IADC charter includes the coordination and dissemination of remediation research, and strategies based on research results are being adopted by the worldwide space community.
Remediation strategies have resulted in a decline in the rate of debris growth in the 1990s although the overall population continues to grow. Continued work is necessary, however, to reduce the orbital debris hazard for future generations and continue the safe, economical utilization of space.
Committee on Space Debris, Aeronautics and Space Engineering Board, Commission on Engineering and Technical Systems, National Research Council. (1995). "Orbital Debris: A Technical Assessment." Washington, D.C.: National Academy Press. Also available from http://pompeii.nap.edu/books/0309051258/html/index.html
Johnson, Nicholas L. (1998). "Monitoring and Controlling Debris in Space." Scientific American 279(2):62–67.
Interagency Space Debris Coordination Committee (IADC) Web site. Available from http://www.iadc-online.org .