NEO News (07/01/08) Population and impact frequency

From Dave Morrison.

NEO News (07/01/08) Population and impact frequency

This edition of NEO News contains one of our periodic updates on the progress of the Spaceguard survey, a topic that is intertwined with estimates of the population of NEAs as a function of size. As in the past, the most detailed such estimates come from Alan Harris. Reproduced below is his recent article on this subject, published in Nature last week. I have not included any of the graphics (two figures) or the references; for these, please reference the original article in Nature.

All estimates of NEA populations and impact rates are plagued by uncertainties concerning the albedos of NEAs and hence the conversions from the observed quantities (magnitudes) to size and energy. The are also uncertainties in the magnitude scales used by different observing groups. Models include different assumptions, and it is thus difficult to compare detailed numerical results. The results from Harris are internally self-consistent, but other self-consistent models yield slightly different conclusions, even using the same observational data. Harris reports (for early June) that 742 near-Earth asteroids of diameter greater than 1 km have been discovered. He estimated a total of 940, concluding that the Spaceguard Survey has identified about 79% of NEAs larger than 1 km. Lindley Johnson of NASA Headquarters, using the same data, uses the discovery of 744 (as of late June) but adopts a different total population of NEAs >1 km to yield a completeness of 86%. Either way, the Spaceguard Survey has been a great success.

The most important innovation in the Harris models discussed below is in the use of new astronomical data from the surveys to conclude that the size frequency distribution of NEAs is not a straight-line power law (his Figure 1 in the Nature paper). The dip in population of sub-km NEAs results in lower risks, as he discusses below.

In spite of the differences between models, it seems to me that with all the new data from Spaceguard, the population and impact frequency numbers are now pretty solid. But how we interpret them is an individual matter. Some feel that the risk from sub-km NEAs is unacceptable, others that it is negligible. It is in this area of interpretation that we see the strongest divergence of opinion about the magnitude of the remaining NEA impact risk.

David Morrison

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Nature: Vol 453 pg 1178 June 2008

COMMENTARY: WHAT SPACEGUARD DID
by Alan Harris

The sky isn't falling, but there are still good reasons for keeping an eye on it. In 1991, a NASA-sponsored international working group convened to develop a thorough survey of near-Earth objects (NEOs) - predominately asteroids with an orbit that brings them within 1.3 astronomical units of the Sun. The objective of the survey would not be a mere sampling of the large asteroids that might constitute a risk to Earth, but rather a census. The report defined the 'Spaceguard Survey'. Spaceguard's goal was to identify most NEOs larger than 1 kilometre in diameter within a decade. The impact of an asteroid larger than 1 kilometre in diameter has the potential to cause a global climatic perturbation, similar to a 'nuclear winter', and could lead to billions of deaths worldwide. Such events, although less frequent than smaller 'impacts' such as the Tunguska event in Russia (see page 1157), nevertheless present a greater risk of death, even to individuals. Moreover, they carry the additional risk of ending civilization. So, it is clear to most why a survey might be important.

The idea was slow to catch on within NASA, but by May 1998, Carl Pilcher, the Science Director of Solar System Exploration in the NASA Office of Space Science, testified before the Subcommittee on Space and Aeronautics of US Congress that "NASA is committed to achieving the goal of detecting and cataloguing 90% of NEOs larger than 1 kilometre in diameter within ten years". This for many was the formal start of Spaceguard, so it is appropriate, a decade later, to ask whether its goals have been met. The pedantic answer is no, but in terms of risk reduction - or more precisely, knowing whether an impact will, or will not, occur in our lifetimes - Spaceguard identified a fraction of NEOs responsible for more than 90% of the potential impact risk, and found that impacts from that fraction pose a negligible risk in the next 50-100 years. The remaining short-term risk is almost entirely from any remaining undiscovered NEOs. In that sense, the Spaceguard Survey has been a remarkable success.

Two years ago, I was commissioned by NASA, through the NEO Program Office at the Jet Propulsion Laboratory in Pasadena, California, to assess the progress of Spaceguard. I filed my final report with NASA in March 2007 and have presented a brief summary of the main results. It is easy enough to keep count of the number of discovered objects larger than a given size, but to know when 90% have been found, one must estimate the total population. This is a bit of a bootstrap process, using the survey itself to estimate everything out there. In the simplest terms, if we scan the sky tonight, the number of detections of already-known objects compared to the total number of objects detected during a test interval gives us a measure of completeness. In detail, it is not so simple because not all NEOs are equally detectable.

How is Spaceguard doing? As of 10 June 2008, 742 near-Earth asteroids of diameter greater than 1 kilometre had been discovered. In my report I estimated a total of 940, and so the Spaceguard Survey has identified about 79%; not quite 90%, but not bad considering the uncertainties and the efforts required to reach 90%. Meanwhile the estimated risk of impact is dwindling. In the very largest size range, asteroids about 10 kilometres in diameter, the three already discovered are almost certainly all that exist. These would produce an impact similar to that which killed the dinosaurs 65 million years ago, with an estimated impact interval of around 108 years - roughly the last time dinosaurs walked on Earth. Oddly, an object that might cause a Tunguska-like event - roughly 50 metres in diameter - should collide with Earth only about every 1,500 years, and the last event we saw was only 100 years ago.

Recently, Mark Boslough at Sandia National Laboratories, in Albuquerque, New Mexico, suggested that the energy of the Tunguska event may have been as low as 3 megatonnes. That adjustment reduces the expected time between similar events to perhaps about once in 500 years, still leaving the chances of an event within a century as unlikely. 'Statistics of one' cannot be held too rigorously to formal probability estimates, but our view of the skies has produced a strong predictor for the frequency of impacts. It is so strong, in fact, that it could and should rule out some suggestions of past impacts such as the multiple kilometre-sized objects claimed by some to have pelted Earth during the Holocene period. Such an event is inconsistent with what we see in the skies, by about two orders of magnitude.

Another NASA study in 2003, estimated the expected damage from impacts of various sizes. Using those values of expected damage, and the impact frequency from the newly derived population, I estimated the 'risk spectrum' of impacts over the entire size range of those that can penetrate the atmosphere. Figure 2 shows that 'spectrum', first for the entire population, that is, the 'intrinsic risk' before any NEOs had been discovered, and secondly the 'residual risk' from the fraction of the NEO population that remains undiscovered. Since the objects that have been discovered have been found to have no, or a vanishingly small, probability of hitting Earth in the next 50 or more years, we can think of that fraction of the intrinsic risk as 'retired' for the short term over which we can predict impact trajectories, about a human lifetime.

Figure 2 shows that the risk from large impacts -- the kind that would cause global climatic disaster and potentially bring down our civilization - has been dramatically reduced, by more than an order of magnitude. In the smaller size range, from several-hundred-metre-diameter objects that could cause massive tsunamis if they crashed into an ocean, down to sub-hundred-metre objects the size of that in the Tunguska event - which could cause ground damage from airbursts - current surveys have done little to retire the risk. But the intrinsic risk from these events is very small, and in fact resembles that of other natural disasters such as tsunamis, earthquakes and volcanic eruptions in that they do not pose a global threat to life as we know it.

In the 2003 NASA report, the recommendation was made for a new survey to reduce the assessed residual impact risk from objects less than 1 kilometre in diameter by a further order of magnitude. It was estimated at that time that to achieve this goal would require discovering 90% of NEOs larger than 140 metres in diameter. This has become the new mantra of survey plans, but perhaps this should be reconsidered. Because of the steep dip in the population curve in the size range between about 50 metres and about 500 metres, the intrinsic impact frequency, and hence the impact risk, is about three times lower than was estimated in the 2003 report. So, in a way, two-thirds of the risk assumed to exist in those reports is gone already, without even looking at the sky. In the earlier reports, the 'residual risk' to be addressed by a next-generation survey was assumed to be approximately 300 fatalities per year, but using my new population estimate that figure drops to around 80 per year. In comparison to other risks in life, this is negligible.

What is the risk that your death will come from the sky? Before the Spaceguard Survey, it was thought to be comparable to the risk of dying in a commercial aeroplane accident. Currently, however, the residual risk from the remaining undiscovered NEOs is more comparable to the risk of death from a fireworks accident. At some point one has to ask how far down we need to drive the residual risk, especially because the cost of doing so increases steeply as the size of impactors decreases.

Be that as it may, plans are continuing for next-generation surveys and they may serve another purpose. The Large Synoptic Survey Telescope (LSST), a ground-based, wide-field instrument with an 8.4 metre aperture, is planned to enter service by about 2012. Recognizing the diminishing value of driving our assessement of the impact risk so low, the LSST project has adopted the NEO survey as only one of many scientific goals for the telescope, and in particular has emphasized the scientific value of a Solar System survey.

Indeed, the Spaceguard Survey itself has yielded scientific results aside from the value of impact risk reduction. Recently William Bottke and his colleagues at the Southwest Research Institute in Boulder, Colorado, used orbital statistics of asteroids discovered by the surveys to propose that the event that killed off the dinosaurs came from an 'impact shower' resulting from the collisional breakup that produced the Baptistina asteroid family. The size frequency distribution of impactors is itself interesting. The drop in numbers from those of a few hundred metres in diameter to those of a few tens of metres is not yet explained, but is perhaps due to the transition, at around 200 metres diameter, from 'rubble pile' structure among larger asteroids, which are less resistant to disruption by collisions, to monolithic bodies in the smaller size range, which are more resistant to further collisional breakup. Thus, although continuing surveys for the sole purpose of risk reduction may be of diminishing value, the scientific rewards will remain high, and we can hope that ever more powerful surveys will continue in the future.

Alan Harris is a senior research scientist with the Space Science Institute, 4603 Orange Knoll Avenue, La Canada, California 91011-3364, USA.

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NEO News (now in its fourteenth 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, Ames Research Center, the International Astronomical Union, or any other organization. To subscribe (or unsubscribe) contact dmorrison@arc.nasa.gov. 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.