ASTEROIDS COMETS & METEORS:
Asteroid impact The solar system is filled with a barrage of comets and asteroids. Some of these cross the paths of the other planets as well as the earth s. The earth travels around in a swarm of projectiles. Comets and asteroids can and do strike the surface of the earth. Evidence from spacecraft exploring the outer planets and their satellites reveal the presence of craters scaring their surfaces.
These help to support the idea that all the planets are under constant attack by these flying projectiles. It was nearly sixty years ago that the first earth-crossing asteroid was discovered. Improvements in telescopes and technology have increased our knowledge about cosmic projectiles. Since then dozens of near-Earth asteroids and short period comets are discovered each year. The exact nature of the role of asteroids is still under debate.
Few scientists can deny the increasingly important role that asteroids and comets have played in the formation of the earth and life itself on this planet. The scientific community was not paying attention to this threat until 1980. That s when Luis Alvarez and others proposed that such an impact, and the global pall of dust, resulted in the mass extinction of life forms on earth, ending the age of dinosaurs (Alvarez and others, 1980). Widespread public interest was aroused as additional papers and discussions via scientific literature followed. In 1981, NASA organized a workshop Collision of Asteroids and Comets with the Earth: Physical and human Consequences at Snowmass, Colorado (July 13-16, 1981). In response to the close passage of asteroid 1989 fc, the American Institute of aeronautics and astronautics (AIAA, 1990) recommended studies to increase the detection rate of near-Earth asteroids, and how to prevent such objects striking the Earth.
The AIAA brought these recommendations to the attention of the House Committee of Science, Space, and Technology, leading to the Congressional mandate for this workshop included in the NASA 1990 Authorization Bill. In conjunction with these political developments, a small group of dedicated observers significantly increased the discovery rate of Near-Earth asteroids and comets, and several of these discoveries were highlighted in the international press. Other recent activity has included the 1991 International Conference on Near-Earth Asteroids. (San Juan 1991 Capistrano, California, June 30 july 3), a meeting on the Asteroid Hazard held in St.
Petersburg, Russia (October 9-10, 1991), and a resolution endorsing international searches for NEO s adopted by the International Astronomical Union (August 1991). Despite a wide spread perception that asteroid impact is a newly recognized hazard, the basic nature of the hazard was roughly understood half a century ago. In 1941, Fletcher Watson published an estimate of the first three Earth-approaching asteroids (Apollo, Adonis, And Hermes). A few years later, Ralph Baldwin (1949), in his seminal book The Face of the Moon, wrote Since the Moon has always been the companion of the Earth, the history of the former is only a paraphrase of the history of the latter [Its mirror on Earth] contains a disturbing factor. There is no assurance that these meteoritic impacts have all been restricted to the past. Indeed we have positive evidence that [sizable] meteorites and asteroids still abound in space and occasionally come close to the Earth.
The explosion that formed the [lunar] crater Tycho would, anywhere on Earth, is a horrifying thing, almost inconceivable in its monstrosity. Watson and Baldwin (both of whom are still alive) were prescient, but in their time few other scientists gave much thought to impacts on the Earth. Recently, however, there has been a gestalt shift that recognizes extraterrestrial impact as a major geological process and, probably, an important influence on the evolution of life on our planet. Also new is our capability to detect such objects and to develop a space technology that could deflect a potential projectile before it struck the Earth. The NASA International Near-Earth Object Detection Workshop was organized in the spring of 1991 and held three formal meetings; on June 30 July 3 at the San Juan Capistrano Research Center, and on November 5 in Palo Alto, California.
The group has membership of 24 individuals from four different continents. As described in the following chapters of this report, the workshop group has analyzed the nature of the hazard and defined a practical program for the detection of potentially catastrophic impacts. The greatest risk is from the impact of the largest objects those with diameters greater than 1 km. Such impacts, which occur on average from once to several times per million years, are qualitatively as well as quantitatively different from any other natural disasters in that their consequences are global, affecting the entire planet. How, then, should we approach the problem of discovering and tracking these objects About 90% of the potential Earth-impacting projectiles are near-Earth asteroids or short-period comets, called collectively NEOs (Near Earth Objects). The other 10% are intermediate or long-period comets (those with period longer than 20 years), which are treated separately since they spend so little time in near-Earth space.
The NEOs have orbits that closely approach or intersect that of the Earth. Their normal orbit motion brings them relatively near the Earth at intervals of a few years, permitting their discovery. The objective of an NEO survey is to find these objects during their periodic approaches to the Earth; to calculate their long-term orbital trajectories, and to identify any that may impact the Earth over the next several centuries. If any appear to be on Earth-impact trajectories, there will generally be a period of at least several decades during which to take corrective action. It should be emphasized that we are discussing neither a short-range search nor a quick-response defense system. The chance that an NEO will be discovered less than a few years before impact is vanishingly small.
These help to support the idea that all the planets are under constant attack by these flying projectiles. It was nearly sixty years ago that the first earth-crossing asteroid was discovered. Improvements in telescopes and technology have increased our knowledge about cosmic projectiles. Since then dozens of near-Earth asteroids and short period comets are discovered each year. The exact nature of the role of asteroids is still under debate.
Few scientists can deny the increasingly important role that asteroids and comets have played in the formation of the earth and life itself on this planet. The scientific community was not paying attention to this threat until 1980. That s when Luis Alvarez and others proposed that such an impact, and the global pall of dust, resulted in the mass extinction of life forms on earth, ending the age of dinosaurs (Alvarez and others, 1980). Widespread public interest was aroused as additional papers and discussions via scientific literature followed. In 1981, NASA organized a workshop Collision of Asteroids and Comets with the Earth: Physical and human Consequences at Snowmass, Colorado (July 13-16, 1981). In response to the close passage of asteroid 1989 fc, the American Institute of aeronautics and astronautics (AIAA, 1990) recommended studies to increase the detection rate of near-Earth asteroids, and how to prevent such objects striking the Earth.
The AIAA brought these recommendations to the attention of the House Committee of Science, Space, and Technology, leading to the Congressional mandate for this workshop included in the NASA 1990 Authorization Bill. In conjunction with these political developments, a small group of dedicated observers significantly increased the discovery rate of Near-Earth asteroids and comets, and several of these discoveries were highlighted in the international press. Other recent activity has included the 1991 International Conference on Near-Earth Asteroids. (San Juan 1991 Capistrano, California, June 30 july 3), a meeting on the Asteroid Hazard held in St.
Petersburg, Russia (October 9-10, 1991), and a resolution endorsing international searches for NEO s adopted by the International Astronomical Union (August 1991). Despite a wide spread perception that asteroid impact is a newly recognized hazard, the basic nature of the hazard was roughly understood half a century ago. In 1941, Fletcher Watson published an estimate of the first three Earth-approaching asteroids (Apollo, Adonis, And Hermes). A few years later, Ralph Baldwin (1949), in his seminal book The Face of the Moon, wrote Since the Moon has always been the companion of the Earth, the history of the former is only a paraphrase of the history of the latter [Its mirror on Earth] contains a disturbing factor. There is no assurance that these meteoritic impacts have all been restricted to the past. Indeed we have positive evidence that [sizable] meteorites and asteroids still abound in space and occasionally come close to the Earth.
The explosion that formed the [lunar] crater Tycho would, anywhere on Earth, is a horrifying thing, almost inconceivable in its monstrosity. Watson and Baldwin (both of whom are still alive) were prescient, but in their time few other scientists gave much thought to impacts on the Earth. Recently, however, there has been a gestalt shift that recognizes extraterrestrial impact as a major geological process and, probably, an important influence on the evolution of life on our planet. Also new is our capability to detect such objects and to develop a space technology that could deflect a potential projectile before it struck the Earth. The NASA International Near-Earth Object Detection Workshop was organized in the spring of 1991 and held three formal meetings; on June 30 July 3 at the San Juan Capistrano Research Center, and on November 5 in Palo Alto, California.
The group has membership of 24 individuals from four different continents. As described in the following chapters of this report, the workshop group has analyzed the nature of the hazard and defined a practical program for the detection of potentially catastrophic impacts. The greatest risk is from the impact of the largest objects those with diameters greater than 1 km. Such impacts, which occur on average from once to several times per million years, are qualitatively as well as quantitatively different from any other natural disasters in that their consequences are global, affecting the entire planet. How, then, should we approach the problem of discovering and tracking these objects About 90% of the potential Earth-impacting projectiles are near-Earth asteroids or short-period comets, called collectively NEOs (Near Earth Objects). The other 10% are intermediate or long-period comets (those with period longer than 20 years), which are treated separately since they spend so little time in near-Earth space.
The NEOs have orbits that closely approach or intersect that of the Earth. Their normal orbit motion brings them relatively near the Earth at intervals of a few years, permitting their discovery. The objective of an NEO survey is to find these objects during their periodic approaches to the Earth; to calculate their long-term orbital trajectories, and to identify any that may impact the Earth over the next several centuries. If any appear to be on Earth-impact trajectories, there will generally be a period of at least several decades during which to take corrective action. It should be emphasized that we are discussing neither a short-range search nor a quick-response defense system. The chance that an NEO will be discovered less than a few years before impact is vanishingly small.
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