From: Ames Research Center
Posted: Monday, August 15, 2005
Following are three items: (1) a note on the discovery progress being made in the Spaceguard Survey, (2) a New York Times editorial on Apophis, and (3) a collection of personal opinions on policy issues from a recent paper of mine in Science and Global Security.
SPACEGUARD SURVEY PROGRESS
Next month, the number of known near-Earth asteroids (NEAs) larger than 1 km diameter (or, more precisely, brighter than absolute magnitude 18) should pass the 800 mark. As of August 8 the NASA NEO Program Office website
If the population of NEAs larger than 1 km is 1100 (the consensus figure from several studies), 800 represents 73 percent completeness. It is interesting to note that 800 is already more than the total number of NEAs larger than 1 km predicted in some analyses carried out just a few years ago. For the metric of the Spaceguard Survey, which aims to find 90 percent of NEAs larger than 1 km, we are now more than 80% of the way to that goal.
The recent pattern of discovery also shows an expected reduction in the discovery rate for the larger NEAs. As the survey becomes more complete, there are fewer large NEAs to be found and a larger fraction of those detected are rediscoveries of asteroids already catalogued. The total number of NEAs larger than 1 km found each year has trended downward since 2000, with annual totals of 131, 91, 101, 69, and 57.
Although the number of large NEAs discovered each year is smaller, the total NEA discovery rate has continued to show a gradual increase. From 2000 through 2004 the annual totals are 362, 439, 485, 439, and 532. This shows that the discovery systems of the Spaceguard Survey have improved over time. However, the fraction of the new NEAs that are larger than 1 km has declined from about 25 percent of discoveries to near 10 percent. The reason we are finding fewer large NEAs is not a failure of the search systems but rather a real depletion in the population of undiscovered large NEAs. It is, thus, a measure of the success of the program.
New York Times Editorial
APOPHIS AND US
While the shuttle is in space and our attention is tilted upward, it's worth thinking about what else is up there, especially an asteroid named 99942 Apophis. When this object - a little more than a thousand feet across - was first discovered last year, astronomers estimated that it had a distant chance of striking Earth in 2029. After closer observation, it seems likely that this asteroid will still pass very near our planet, but without striking it. There is still a possibility, however, that as it swings by it may hit a gravitational "keyhole," shifting its orbit far enough to make it strike Earth in 2036.
Beyond the question of whether Apophis deserves to be added to our regular list of doomsday worries, there's the practical matter of whether we can do anything about it. Hollywood heroics - trying to blast it out of the sky - might do more harm than good. NASA scientists think they have the technology to safely give it a nudge that might shift the orbit a few thousand feet. The trick is figuring out exactly when the asteroid will be in the exact place where such a modest bump would be effective.
One possibility is to put a radio beacon on the asteroid, as if it were a member of a wolf pack in Yellowstone. That would let astronomers refine their predictions of the asteroid's potential orbit when it next approaches Earth, in 2012-2013. Given NASA's recent success in firing a probe at the comet Temple I, planting a beacon on an asteroid seems doable.
You don't need to be a science-fiction writer to see a curious convergence here - the approach of a possibly Earthbound asteroid and the emergence of the scientific and technological capacity to cope with it. That, of course, is merely coincidence. Time will tell whether the risk from 99942 Apophis increases or whether it diminishes naturally, its orbit deforming away from harm, in ways that are well within the realm of probability.
Concluding section (text only, footnotes and references omitted) from a paper published in Science & Global Security, Vol 13 No 1-2, pages 87-104 (2005)
DEFENDING THE EARTH AGAINST ASTEROIDS: THE CASE FOR A GLOBAL RESPONSE
CONCLUSIONS: PUBLIC POLICY ISSUES
The preceding sections of this paper hint at a number of policy issues that are summarized in this concluding section. For this discussion, I assume that the current Spaceguard Survey will continue beyond its 2008 target of 90 percent discovery of NEAs larger than 1 km, and that other telescopes will probably re-enforce this effort, thus retiring most of the risk of global-scale impacts from undiscovered asteroids. The following questions are all addressed to what steps we should undertake beyond Spaceguard.
1. Is it important to extend asteroid surveys to sub-km impactors, perhaps down to the limit of penetration of the Earth atmosphere? Such an undertaking is consistent with an imperative for governments to make an effort to identify and protect their populations from preventable disasters. It may or may not be cost effective, depending on accounting assumptions. This effort would be considerably less cost-effective than the current Spaceguard Survey, since we would need to spend at least an order of magnitude more funds to protect against a risk that is at least an order of magnitude smaller than that of NEAs larger than 1 km.
2. Should we begin to develop technologies for deflecting asteroids? To date, essentially no funds have been spent for this purpose. Many would argue that it is prudent to begin such research before an actual threat is identified. Others argue that since these technologies are unlikely to be needed within the next few decades, it is a waste of resources to do any work at present. The most compelling case is probably to accelerate our study of NEAs, including visits by spacecraft. The knowledge gained by such scientific exploration is also needed to make plans for future deflection efforts, if they are required.
3. Should we test asteroid deflection technologies? Edward Teller was an advocate during the final decade of his life for conducting such experiments. He argued not only that such experiments were needed to test deflection schemes, but also that the experience gained in planning such an international test project would be invaluable if and when we faced the real thing -- especially if the options for defense included nuclear explosives. The recent proposal by the B612 Foundation for a first test of a space tug represents such an experimental approach.
4. Who should be in charge of these efforts, from possible extensions of the Spaceguard Survey to potential testing of defensive systems? Is NASA the correct agency? What should be the role of the Department of Defense? For that matter, are these topics the responsibility of the U. S. government? To date, there is no official position or plan that allocates overall responsibility within the government.
5. Should civil defense and disaster relief agencies be planning to deal with the aftermath of an impact explosion that occurs without warning? Today, no warning would be expected for sub-km impacts. Who should assume responsibility in planning for mitigation if such a disaster should occur?
6. How important is international participation? While the impact hazard has been discussed internationally by the United Nations, the Council of Europe, the Organisation for Economic Co-operation and Development, the International Astronomical Union, and the International Council of Scientific Unions, no concrete action has been taken. The most comprehensive study of the problem outside the US was carried out in the UK. However, of the 14 recommendations in the UK NEO Task Group Report, only one has been fully implemented -- the establishment of a British National Center for public education on the impact hazard. Perhaps it is the proper role of the only superpower to assume unilateral responsibility for the protection of our planet from cosmic impacts.
7. Which impacts (if any) do not require mitigation, and who will make the decision? Suppose the astronomers discover a 100 m NEA that will impact in the ocean -- even if the science community concludes that there is no danger from tsunami, will that satisfy the public? Or suppose that a land impact is predicted; if the target area is deserted it may be easy to decide to let it hit, but suppose there are cities or other major infrastructure such as dams in the target area. Who will decide whether a multi-tens-of-billions of dollars effort should be undertaken to deflect the asteroid? Who will pay for it?
8. If a sub-km impactor is identified and a decision is made to change the orbit, there are a number of scenarios that could be complex and divisive. Suppose the initial target is identified as being in Country A. To change the asteroid orbit we must supply continuous thrust that gradually moves the impact point off the planet. But in this process the impact point crosses Nations B, C, and D, which were originally not at risk. Who will the nations trust to carry out the deflection maneuver? And what if the maneuver is only partially successful and the asteroid ends up striking Nation C rather than missing the Earth? Who is responsible?
9. In any of these examples, will the population of the United States or any other country trust either scientific judgments or the decisions of public officials? If an asteroid is discovered with an initial well-publicized non-zero chance of collision, and subsequent observations ultimately convince the scientific community that it will miss by a very small margin, will the public believe them? Or suppose an asteroid is found that is indeed on a collision course but the scientists estimate that it is only 30 m in diameter and thus will disintegrate harmlessly at high altitude. Will the people who live at ground zero trust this conclusion? What level of proof (or acceptance of responsibility) will be required? (Many would find something suspect about the phrase "I'm from the (U.S.) government and am here to protect you from asteroids").
10. Is the public likely to support continued and perhaps accelerated government spending to protect the Earth from asteroids? It is difficult to sustain interest and support in the absence of known threats, and there has never been an asteroid impact in a populated area in all of recorded history. In recent years, there have been a number of media-inspired scare stories, mostly based on very preliminary orbits, with the "threat" disappearing within a day or two. Such stories may sustain public interest, but they can also backfire if the public or the media conclude either that the astronomers don't know what they are doing or that they are "crying wolf" to attract public attention. Communicating the nature of this hazard, with no historical examples but possible fatalities of a billion or more people, is challenging. Yet if we are to create and sustain international programs for planetary defense, public understanding and support is required.
We cannot today answer the above questions. All would profit by a wider dialogue and the participation of individuals and groups who may never have been exposed to this unique natural hazard.
NEO News (now in its tenth year of distribution) is an informal compilation of news and opinion dealing with Near Earth Objects (NEOs) and their impacts. These opinions are the responsibility of the individual authors and do not represent the positions of NASA, the International Astronomical Union, or any other organization. To subscribe (or unsubscribe) contact firstname.lastname@example.org. For additional information, please see the website http://impact.arc.nasa.gov. If anyone wishes to copy or redistribute original material from these notes, fully or in part, please include this disclaimer.
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