Statement of David Morrison
Chief, Space Science Division
NASA Ames Research Center

Before the Subcommittee on Space of the Committee on Science, Space, and Technology

U.S. House of Representatives, One Hundred Third Congress

March, 24, 1993

Cosmic impacts pose a significant hazard of loss of human life and property. On a statistical basis, these risks are at least as large as many other natural hazards that are taken very seriously by modern society. Large but very infrequent impacts, primarily by asteroids, can produce short-term climate effects that endanger world food supplies and might lead to the death of a substantial fraction of the Earth's population.

This Committee recognized the impact hazard in 1991 when it asked NASA to convene a workshop to investigate ways that the discovery of potentially hazardous asteroids could be accelerated. I chaired the International Near Earth Object Detection Workshop, an international working group of 24 scientists and engineers that carried out this study. Our report, entitled The Spaceguard Survey, was completed in January 1992 and delivered to this Committee a year ago. My testimony this afternoon summarizes the conclusions of that report and notes the progress that has been made on this problem since the report was submitted.

The first task of the Spaceguard working group was to assess the risk due to cosmic impacts of different sizes. If we wish to reduce the risk from such impacts, it is essential to know more about the nature of the threat. At one extreme are the very large but extremely rare impacts such as the 10-15 km diameter asteroid that wiped out the dinosaurs 65 million years ago. A similar impact today would endanger all life on our planet. At the other extreme are meteorites -- rocks falling from the sky -- which could injure or kill an individual. However, analysis shows that neither the large, extinction-threatening impacts nor the small space rocks constitute a major risk. Our primary focus is on impacts of intermediate size, corresponding to projectiles with diameters from a few tens of meters up to several kilometers.

The lower threshold for danger is defined by the atmosphere of the Earth, which effectively shields us from impacts by projectiles less than a few tens of meters across -- projectiles with explosive energies less than about 10 megatons of TNT. Above this threshold, an incoming asteroid or comet can penetrate to the lower atmosphere or surface and explode with the force of the largest nuclear explosives. An example is provided by the 1908 Tunguska airburst explosion in Siberia, estimated to have released 10-20 megatons of energy. Such impacts could devastate a large city. Fortunately, however, most such impacts -- like the Tunguska blast -- will not strike in densely populated parts of the world. From current statistics of asteroids and comets, we estimate that such an impact occurs over land about once per millennium, and that we might expect a U.S. city to be destroyed by such an impact about once every several hundred thousand years. (This is equivalent to the statement that the risk of some U.S. city being hit in any year is 1 in several hundred thousand.) Of potentially greater concern are impacts in the oceans, which would raise tsunamis that could endanger coastal populations -- a subject that requires further research.

The greatest risk is associated with still larger but rarer impacts, which are capable of causing global environmental catastrophe. Above some threshold -- estimated as lying between projectile diameters of 1 to 3 km -- impacts can produce global effects in addition to their immediate blast damage. Injection of fine dust into the stratosphere as well as other poorly-understood environmental consequences would lead to short-term surface cooling and possibly changes in atmospheric chemistry. In such a scenario, widespread crop failures would lead to global starvation and epidemics. Impacts of this magnitude contribute the greatest risk, since their effects are global rather than local. If such an impact took place anywhere on Earth during our lifetimes, we would each be in danger, independent of where the projectile struck.

Statistical estimates of human risk associated with global ecological catastrophe suggest that each human on this planet runs a risk of perhaps 1 in 20,000 of dying from this cause as opposed to all of the other natural or accidental causes of death. This risk is as great as that associated with death from many other natural disasters such as hurricanes, tornadoes, earthquakes, or floods. Prudence suggests that we should be concerned about such impacts and seek ways of avoiding them or mitigating their consequences.

Thus far I have been discussing statistical risks. In fact, we do not know today if there is a real comet or asteroid out there that might hit the Earth within our lifetimes. Even for the larger near-Earth asteroids -- those larger than 1 km in diameter -- we have discovered only about 100 of the estimated population of about 2000. The others are there, but we have not yet located them or charted their orbits, so we do not know if any of them present a near-term danger.

The Spaceguard working group spent considerable time investigating ways that we could discover and track these asteroids, drawing upon the experience and expertise of the international astronomical community. Our objective was to devise a cost-effective way to find any near-Earth asteroids that might be capable of precipitating a global ecological disaster -- those with diameters larger than 1 km. Since these objects can be detected by ground-based telescopes, there is no requirement for much more expensive space-based systems or esoteric technologies. What is required is a comprehensive international survey program that makes use of the most recent advances in detectors and automatic data processing to find faint moving objects and track them against the background of millions of stars. We must move from the present approach, in which a handful of dedicated individuals carry out limited searches using primarily photographic techniques, to a properly-funded automated survey.

The Spaceguard Survey Report specifically describes how a network of six specially constructed telescopes, each with an aperture of 2-3 m and equipped with modern large-format CCD detectors, could carry out the required survey in the minimum practical time, which is about two decades. At the end of the survey we would know whether any of these asteroids poses a danger to us. If we find one or more threatening objects, we would have decades of warning in which to plan ways to deflect or destroy it.

In addition to the discovery of new objects, The Spaceguard Report notes that we need to improve our capability for tracking and orbit determination. Any objects that appear to be on possible collision course with the Earth will require precise tracking by radar and optical means. Comets are a particular source of uncertainty because their orbits are difficult to define.

We emphasize that the Spaceguard Survey does not represent a short-term or local defense strategy. Our objective is a complete catalogue of potentially threatening asteroids. The most cost-effective approach to this inventory consists of a coordinated international network of specialized telescopes for discovery, confirmation, and follow-up observations.

The Spaceguard working group also notes the need to better define the impact risk. Our current understanding of the problem is dependent on the asteroids and comets that have been discovered by a few dedicated observers. Modest expenditures for additional personnel and modern computing facilities can accelerate their discovery success and provide better statistics on asteroid sizes and orbits. For small objects (tens of meters diameter) that are too faint for astronomical detection, our knowledge of impact statistics would be greatly improved by access to data on upper atmospheric entry that could be collected by down-looking surveillance assets in Earth orbit. One important byproduct of the Spaceguard Survey will be the discovery of hundreds of new asteroids with orbits that are relatively accessible to robotic or crewed spacecraft. If the exploration of asteroids or the utilization of asteroidal resources should become an important focus in our future space program, the discovery of appropriate asteroid targets could more than pay for the cost of the Spaceguard Survey in reduced costs of reaching them.

In the year since the publication of The Spaceguard Survey Report, a number of events have taken place. Public awareness of the impact problem has increased, especially following the cover story in Newsweek this past November. In addition, the NASA Solar System Exploration Division has provided new funds in FY 93 for the development and testing of instrumentation concepts for automated detection of asteroids. Several international meetings have been held, including a conference in Tucson in January of this year and one planned for Italy in April. A working group has been formed under the auspices of the International Astronomical Union. In Australia, the government has recently allocated half a million dollars to support asteroid detection at the Anglo-Australian Observatory.

Many technical details are in The Spaceguard Survey Report, which you have. For example, the special problems posed by long-period comets, which I have not mentioned here, are detailed in that report. Let me conclude this testimony by emphasizing a few key points:

The impact hazard is real, and it is of a magnitude at least as great as many other natural hazards. A large impact would generate the ultimate environmental disaster. As a result of the extensive recent coverage given the issue in Time, Newsweek, and other media we can expect growing public interest in cosmic impacts.
Over long time spans, impact catastrophes are inevitable. What happened to the dinosaurs can happen to us. Within our lifetime, however, we do not know if an impact will take place. The primary objective of any program to deal with this hazard is to determine whether or not a near-term impact is likely.
Over the next two decades, it is within our capability to carry out a comprehensive search (the Spaceguard Survey) with a high probability of detecting an object years to decades before it actually strikes our planet.
In addition to their impact potential, asteroids and comets are of great intrinsic interest to science, and the asteroids may in the future become the focus of robotic and crewed space missions. The Spaceguard Survey is likely to find many more objects that are easily accessible than that are threatening. But either as friends or foes, we need to know more about these cosmic neighbors.
Unlike most other natural disasters, cosmic impacts can be avoided. We have the means to protect our planet if we choose to do so.