Statement of Dr. David Morrison. Ames Research Center. National Aeronautics and Space Administration. Before the Subcommittee on Space and Aeronautics Committee on Science House of Representatives
Mr. Chairman and Members of the Subcommittee:
It is an honor to return to this committee almost ten years after my first appearance in 1993. At that time I presented the conclusions of the NASA workshop that proposed a Spaceguard Survey to search for potentially threatening asteroids large enough to endanger civilization. Ten years ago there was very little recognition or support outside this committee for dealing with the asteroid impact hazard. I could not have predicted then that by 2002 we would already be past the halfway mark in discovering these large Earth approaching asteroids. Thanks to the Spaceguard Survey, we can now assert that we have reduced the risk from an unforeseen catastrophic impact by more than a factor of two. This is a notable achievement in an effort to protect humanity from the worst known class of natural disasters.
The nature of this risk was stated well by this Committee in 1991, when you wrote: "The chances of the Earth being struck by a large asteroid are extremely small, but since the consequences of such a collision are extremely large, the Committee believes it is only prudent to assess the nature of the threat and prepare to deal with it. We have the technology to detect such asteroids and to prevent their collision with the Earth."
The nature of the impact hazard
It is only during the past decade that we have come to appreciate that impacts by asteroids and comets (often called Near Earth Objects, or NEOs) pose a significant hazard to life and property. Comet impacts constitute only about 10% of the risk, so the focus of my remarks is on the more common impacts by Near Earth Asteroids, or NEAs. The most catastrophic of these are the extinction level events that can create a severe global environmental disaster. The impact of an asteroid about 10 miles in diameter (as large as the Washington beltway) 65 million years ago not only ended the existence of the dinosaurs, it wiped out more than 99% of all life on Earth. Fortunately for us, such mass extinction events are extremely rare. We can already state with assurance that there are no asteroids this large with orbits that could pose a threat to us. We are safe (for the present) from such impacts, but not from the smaller NEAs that actually dominate the current risk.
The greatest risk today is associated with NEAs large enough to perturb the Earth's climate on a global scale by injecting large quantities of dust into the stratosphere. These are not extinction level impacts, but they are still large enough to temporarily depress temperatures around the globe, leading to massive loss of food crops and possible breakdown of society. Various studies have suggested that the minimum mass impacting body to produce such global consequences is several tens of billions of tons, resulting in a ground burst explosion with energy in the vicinity of a million megatons of TNT -- many times greater than the sum off all the world's nuclear stockpiles. The corresponding threshold diameter for NEAs is between 1 and 2 km, or roughly one mile in diameter. Current investigation, including the Spaceguard Survey, focuses on these global threats. It is entirely appropriate that we deal first with the worst danger, even though the probability of an impact in this class is exceedingly small.
After NEAs that are large enough to risk a global catastrophe, we naturally turn our attention to smaller impacts that never-the-less would be capable of destruction on a vast scale, killing tens of millions of people. These are impacts by NEAs less than 1 km in diameter, but still large enough to devastate a large region. Such sub-kilometer NEAs are most dangerous, in fact, if they strike in the oceans. The resulting tidal wave or tsunami is an effective way to carry the energy of the collision to large distances from the point of impact. The tsunami from the ocean impact of a NEA 500 m in diameter could inundate many coastal cities in a single event. While not posing as great a risk as the global scale impact from NEAs more than 1 km in diameter, the danger from such ocean impacts may eventually be judged great enough to warrant action.
At even smaller sizes, NEA impact can still do a great deal of damage on a local scale. We have witnessed one example of such a small impact, which took place in Siberia in 1908. The energy of the explosion was about 15 megatons, and it destroyed more than 1000 square miles of forest. However, such impacts actually pose a much smaller risk than many other natural disasters, such as earthquakes and hurricanes.
It is fortunate for us that the greatest danger is posed by the largest NEAs, which are the easiest to discover. We are finding these at a rate that will allow us to retire that risk within a few more years (unless we find that one of these objects is on a collision course with Earth). As discovery techniques improve, we can shift our search toward smaller NEAs. How far to go depends on analysis of the costs and benefits of a particular defense scheme.
Based on our recent observations, astronomers have concluded that there are between 900 and 1300 NEAs larger than 1 km that could potentially pose a threat. We can estimate the risk we each run from these impacts, which is about 1 in a million per year. This is similar to the risk of one round-trip commercial air flight. The risk from smaller impacts is less, roughly one in ten million. These are all very low numbers. The asteroid impact hazard is an extreme example of a risk of very low probability but potentially catastrophic consequences.
Much effort has gone into estimating the statistical frequency of impacts and evaluating their consequences. However, from a policy perspective we do not need precise estimates of either the frequency of impacts or their consequences. We recognize that the actual risk is not statistical; if there is any sizable asteroid on a collision course with the Earth, it can be found and the impact predicted decades (or more) in advance. Our objective should be to find any large impactor far in advance, and thus provide decision-makers with options for dealing with the threat and defending our planet from a cosmic catastrophe. That is the purpose of the Spaceguard Survey.
The Spaceguard Goal
Half-a-dozen specially designed telescopes today are contributing to the Spaceguard Survey. As mandated by this Committee in 1995, the objective of the Spaceguard Survey is to find the NEAs larger than 1 km in diameter -- that is, to find any with the potential for global catastrophe if they collided with Earth. Specifically the Spaceguard goal is to find 90% of these NEAs by the end of 2008. The philosophy of Spaceguard is to monitor a large volume of space around the Earth using automated wide-field optical telescopes with advanced detectors and computational capability. Any asteroid that could hit the Earth will repeatedly pass close to our planet, providing plenty of opportunity for discovery. Once the NEA is discovered, its orbit is computed and its position is predicted for many decades in advance. Such long-term predictions are possible because the solar system is actually a very well behaved place; asteroids do not alter their orbits capriciously. If there is the possibility of collision in the future, we expect to have decades or even centuries of advanced warning.
Note that Spaceguard is not a last-minute warning system that attempts to find incoming objects on their final plunge toward impact. Such a system would be more complex and expensive than the current approach, and the few days or hours of warning it might provide would be insufficient to take defensive action in any case. The Spaceguard approach of cataloging all potentially dangerous NEAs is cost-effective and will yield the long lead times needed to effectively mitigate any future impacts.
Key Issues to Be Addressed After the Spaceguard Survey
The current Spaceguard program is focused on the NEAs that pose the greatest risk. Today the Spaceguard telescopes are finding many NEAs smaller than 1 km, but the level of completeness for such sub-kilometer asteroids is rather low. A logical next target might be NEAs in the range of 200-300 m diameter, since these pose the greatest tsunami danger. (Below this size, the total risk is much smaller.) Approximately 50,000 NEAs exist larger than 300 m in diameter, so the technical challenge is substantial. However, the exact target size, if any, could be above or below this range, and will need to be the subject of broad discussion within and outside the science community. Data from the existing Spaceguard Survey, as well as numerical simulations, will provide us with the information we need to make informed choices about future search goals. Once the target size is known, search strategies and requirements for smaller asteroids would need to be subject to trade studies and external review to ensure that we are getting the most effective survey possible for our investment.
We are the first generation of humans that both appreciates the long-term threat of cosmic impacts and has the technological capability to deal with it. However, this is one of many natural hazards that we face, and I believe that the costs as well as the effectiveness of the surveys need to be considered in the allocation of resources to deal with this hazard.
The search for NEAs is a little like taking out fire insurance for your home. You do not expect your home to burn. The great majority of us will never experience a fire. Yet we buy insurance to protect against even such an unlikely event, because our homes are too valuable to lose. In a similar way, we undertake the Spaceguard Survey, not because we expect an impact within our lifetimes, but because the consequences of an impact would be too horrendous to be acceptable.