Status Report

NEO News (01/15/04) Risk Reduction

By SpaceRef Editor
January 17, 2004
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This edition of NEO News presents two perspectives on reducing the impact hazard. The first is a dialogue about whether the Spaceguard Survey has actually reduced risk and made us safer. The second is excerpted from an article in which Jim Oberg proposes that the identification and reduction of environmental risks such as asteroid impacts would be a proper long-term goal for NASA.

David Morrison


by David Morrison

Proponents of the Spaceguard Survey have, since the idea was first advocated more than a decade ago, stressed the practical value of this survey. Our purpose is not to improve our scientific understanding of NEAs and the impact frequency, although these are legitimate byproducts of the survey. Rather, we are trying to find and examine NEAs, one at a time, to determine if any will collide with the Earth during the next century. We do this so that we will have sufficient warning of a future impact to avert it, thereby protecting the planet and its inhabitants from this particular cosmic hazard. The implication is, therefore, that as we carry out this survey we are contributing to the safety of the world and reducing the risk of an impact catastrophe.

Is it correct, however, that the Spaceguard Survey is reducing risk and making us safer? Are we really any better off than we would have been without the survey? Or, to carry the argument to its logical conclusion, if we ultimately find every NEA larger than 1 km and show that none will hit the Earth in the next century, have we effectively retired or eliminated this risk? And if so, does it also follow that when we have discovered half the population and shown that their orbits are safe (as is the case today), we can claim that we have removed half the risk?

Several persons have questioned this logic, leading to an interesting dialogue about the nature of the impact hazard. David Tholen of the University of Hawaii wrote (in CCNet for January 14, 2004) that the impact risk does not change, only our knowledge of it. “The risk to Earth is the same now as it was five years ago, and is the same as it will be five years from now. What will change is our knowledge of the risk. If, for example, we were to find that there are no objects larger than 1 km on an impact trajectory with Earth, then the risk is zero and has been zero all along; we simply didn’t know that. As far as I am concerned, the only way to change the risk is to change the orbits of the NEOs. We can reduce the risk by moving an object on a collision course such that it will not collide. Or we can increase the risk by moving an object such that it will collide. In the absence of such changes, the risk remains the same. To use another example, back in 1990 we thought there were 2000 objects larger than 1 km in diameter in near-Earth orbits. Now we think the real number is closer to 1000. The risk posed by the actual population didn’t change. Our knowledge of the population did change.”

A similar question was raised by Rusty Schweickart of the B612 Foundation, who wrote “Reducing risk, it seems to me, could only result from changing the environment for the better… not simply knowing more about it. The risk can’t have changed since we haven’t done anything to reduce the incidence of asteroids hitting the Earth. What we have done is determined that the risk we actually have is lower than what probability would have led us to believe before we discovered the NEAs that we now know about. Our actual risk didn’t change… only the accuracy of our knowledge of it.”
One possible way to clarify this issue is to note that what is being reduced is the risk of impact by an unknown NEA. For a known NEA with a well-determined orbit, the concept of risk (as a probability) is meaningless. The Spaceguard Survey steadily reduces the unknown population by moving NEAs from the unknown category, where we can associate a risk with them, to the known category, where they either pose zero risk or, at the opposite extreme, are predicted to collide with the Earth during the next century.

Alan Harris of the Space Science Institute notes, “With asteroids, the situation is completely deterministic. Any single object either is, or is not, on an impact course in the next century. If we find one and certify that it isn’t going to hit, then our estimate of impact probability for the next century is reduced by one over the number of remaining undiscovered objects. The per-object probability of impact is a constant.  As we find more objects and certify that those particular objects are not going to hit, then our risk — the per object probability times the number of remaining undiscovered objects — goes down. Discovering NEAs does not reduce the per-object probability of impact, but it does reduce the number of undiscovered objects you have to multiply that probability by to get the remaining risk.”

Journalist Oliver Morton puts it this way:  “Spaceguard has reduced the risk of impact by a previously unknown body. At the same time, it has not increased any risks. Imagine the universe of risks as a set of non-overlapping categories such as “impact risk due to a previously unknown object.” Reducing one category while not enlarging another reduces the universe of risk and makes us safer. However, there’s a corollary to this. Spaceguard clearly *could* increase the risks in another category — “impact risk due to a known object”, and such a discovery would make us less safe. But it’s certainly something that it’s better to know than not to know; becoming less safe would not be a failure. My conclusion is that yes, we are safer today because of Spaceguard’s results so far (and in all likelihood we’ll be safer still at the end). But that that’s not the point. Spaceguard is not meant to make us safe. It’s meant to give us an accurate assessment of how safe we are.”

Clark Chapman of SWRI in Boulder makes the following point: “If nothing else had changed, and if we actually found 90 percent of large objects — which dominate the mortality — then the chance of dying would indeed become a factor of 10 smaller, provided that none of those 90 percent are on an impact course in the next century. I used to show a slide that said that it was more likely that a mile-wide asteroid would hit Earth “next year” than that the next poker hand you would get would be a Royal Flush.  More recently, I have had to amend that slide because Spaceguard has resulted in the chances being somewhat smaller of such an impact than being dealt the Royal Flush.”

If we accept this logic, then it is reasonable to ask how the asteroid impact risk today compares with that estimated in 1992, when the Spaceguard Survey Report was completed (see also the paper by Chapman and Morison in Nature, 1994, on assessing the impact hazard). There are two components to the changes in our assessment of this risk, one from a revised estimate of the NEA population and associated impact frequency, and one from the actual discoveries of the Spaceguard Survey.

Even if we do not count the identification during the last decade of specific NEAs as non-threatening, we would evaluate the risk as lower today as a consequence of a better understanding of the NEA population and its orbital dynamics. Perhaps the simplest way to think of this is in terms of number of NEAs larger than 1 km, for which our best estimate has dropped a factor of two. The average interval between impacts increases by a factor of two, and the a priori lifetime risk of death from an impact (in the terms of the Chapman and Morrison paper) drops from about 1 in 20,000 to 1 in 40,000. It is probably also prudent to gain another factor of two by taking a diameter of 2 km rather than 1.5 km as the threshold for a global catastrophe. The net result of these two shifts is to change the average interval between civilization-threatening impacts from 0.5 million years to about 2 million years. Of course, the uncertainties in these numbers are actually much greater, since in fact we know rather little about just what would trigger a collapse of civilization or what the probable fatalities would be in the aftermath of such an event. The actual values are therefore dependent on other poorly-known factors.

The recent report of the NASA NEO Science Definition Team (August 22, 2003) presents detailed models of the impact risk across a very broad range of impactor sizes. They determined what size impactors pose the greatest risk in various categories, and hence what mitigation remedies would be required to retire 90 percent (or any other fraction) of the remaining impact hazard. Figures 3.10 and 3.11 of the NASA SDT report show the overall hazard and the residual hazard, respectively, as a function of impactor size. The residual hazard is that hazard that will remain after five more years of the LINEAR survey (continuing at its present level and not counting any of the other survey telescopes). For nominal cases, this residual hazard is equivalent to annual fatalities of approximately 300, with the risk divided between the remaining undiscovered NEAs larger than 1 km and the large population of sub-km NEAs. This anticipated 2008 residual risk is about an order of magnitude lower than that suggested in the 1992 Spaceguard Report, partly as a result of the above discussed revisions in impact frequencies and partly as a result of the assumed 75 percent completion of the Spaceguard Survey in 2008. I emphasize, however, that these conclusions depend on the concept that risk really is retired by the discovery of NEAs and consequent reduction in the unknown population.

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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 For additional information, please see the website If anyone wishes to copy or redistribute original material from these notes, fully or in part, please include this disclaimer.

SpaceRef staff editor.