Apophis: Risk Forecast and Communication Issues about a Possible Asteroid Strike in 2036

1. Poster Presentation #13, 31st Hazards Research & Applications Workshop, 9-12 July 2006, Boulder, Colorado USA Clark R. Chapman Southwest Research Inst., 1050 Walnut St., Boulder CO 80302 cchapman@boulder.swri.edu The Timeline for 2004 MN4/Apophis TAKE-AWAY MESSAGE: A thousand-foot-wide asteroid, Apophis, has a very small chance of striking the Earth on 13 April 2036. If it does, it will hit somewhere along a path stretching from Siberia, across the Pacific to Central America, and then across the Atlantic to the Cape Verde Islands, probably causing a tsunami as big as the Indian Ocean tsunami of 2004. There is only 1-chance-in-37,000 that it will hit, thanks to radar echoes obtained last month that refined the asteroid’s orbit. But there was a day in December 2004 when the then-available data implied a 1-in-20 chance of a strike in 2029; uncertainties in estimates of its size meant there was >1% chance that a >1 km asteroid would strike, threatening the end of civilization as we know it on that date in 2029. The path-of-risk in 2029 stretched across Europe, Iran, the Ganges River Valley, and the Philippines. This path was kept secret by NASA scientists to avoid panicking the public. Should it have been released? If so, when? A few days later, more observations of the asteroid became available and it was announced that it would actually miss the Earth by 5 Earth diameters. However, the observational errors were misunderstood and a month later it was realized that it will miss the Earth by only half that distance and risks passing through a 600-meter wide “keyhole” that would bring it back to strike in 2036. How should astronomers, and NASA, communicate with the public about such continually evolving, low-probability risks of an enormous disaster a couple of decades in the future? • 19 & 20 June 2004: Asteroid discovered at Kitt Peak Observatory using non-standard, experimental search method. Designated 2004 MN4. • Summer 2004: As often happens, MN4 was not seen again, became lost. • 18 December 2004: MN4 rediscovered by Spaceguard Survey observer in Australia; data linked to June data by the Minor Planet Center. • 20 December 2004: JPL Sentry calculates 1-in-5000 chance of impact on 13 April 2029. Kitt Peak observers reanalyze their June observations. • 22 December 2004: Reanalyzed June data plus new observations during last few nights result in 1-in-250 chance of impact. • 23 December 2004: JPL and Univ. of Pisa scientists jointly announce their semi-independent calculations of a 1-in-170 chance of impact by MN4, estimated to be 460 meters in diameter, with potential force of thousands of megatons TNT equivalent (still on 13 April 2029). This is first-ever case of Torino Scale = 2 prediction. Urgent observations continue. • 24 December 2004: New observations indicate 1-chance-in-60 of impact, Torino Scale = 4! News media mostly inattentive due to Christmas holiday. • 24 December 2004: NASA scientist emails colleagues that he has calculated the path-of-risk for the 2029 impact; he says it is “sobering” and that it would be “inflammatory” to release it within the next year. • 25 December 2004: New observations indicate 1-chance-in-40 of impact. • 26 December 2004: Indian Ocean tsunami disaster occurs; all news media turn attention to the unfolding catastrophe. • 27 December 2004 (morning): New observations indicate 1-chance-in-20 of impact. But an over-the-holiday-weekend search for low-probability pre-discovery observations has an unexpected success: faint images taken on 15 March 2004 by the Spacewatch telescope were missed by automatic detection software in March, but are real detections and are now measured for positions. • 27 December 2004 (afternoon): Heavily influenced by March data, JPL announces 0% chance of a strike: MN4 will miss the Earth by 40,000 miles. • 30 Dec. 2004 – 8 Jan. 2005: Email discussion between asteroid and tsunami experts. Consensus is that MN4 would cause a tsunami comparable to the Indian Ocean tsunami. • 11 January 2005: R. Binzel (MIT) reports spectral observations of MN4, indirectly indicating that its diameter is about 300 meters. • 3 February 2005: Arecibo radar detection of MN4 drastically reduces miss distance in 2029 from 40,000 to 22,000 miles (below height of communications satellites). It is soon realized that there are “keyholes” that MN4 might pass through in 2029, resulting in small chances of actual impacts in later years, between 2034 and 2054. • June 2005: Chance of an impact in 2036 is about 1-in-12,000. • July 2005: As 2004 MN4 disappears from easy telescopic observation, the latest sightings indicate a 2036 impact probability of 1-in-8,300. • 7 August 2005: Another radar detection of MN4, now named “Apophis”, is successful. Impact probability in 2036 is now 1-in-5,500. • 6 May 2006: Final radar opportunity is successful. Impact in 2036 is now just 1-chance-in-37,000. There are 6 other years extending to 2077 in which there is a tiny chance of impact. Except marginally, Apophis will not be visible again to optical telescopes or radar until 2012. Apophis: Risk Forecast and Communication Issues about a Possible Asteroid Strike in 2036 The Near-Earth asteroid Itokawa, imaged by the Japanese spacecraft Hayabusa last autumn. It is 535 meters long and 210 meters wide, about the size of Apophis. Why the Impact Probability Relentlessly Increased, then Disappeared Credit: Stan Ward How to Estimate and Communicate about Errors & Uncertainties One of the most difficult issues in the relationship between scientists and the public concerns uncertainties, error-bars, etc. Astronomers are used to calculating formal statistical errors about measurements of distant objects in space. But astronomers rarely have to deal with the practical consequences of their calculations. The asteroid/comet impact hazard – a low-probability, high-consequence hazard – is a rare example for astronomers where uncertainties are both difficult to estimate and have potentially serious consequences for policy-makers and the general public. In many cases, the chances of an asteroid impacting are much smaller than the chances that the astronomer will make an erroneous calculation! Yet these tiny chances are important because the potential catastrophe is so great. More often than making an actual error, astronomers fail to appreciate systematic errors and other biases that influence their judgments about potential impacts. The public often misunderstands when astronomers obtain more observations, which help to refine knowledge of an asteroid’s path, resulting in changes to estimates of impact probability. When astronomers eventually determine that an asteroid is not going to hit the Earth after all, some people conclude that the original prediction was a “mistake.” It was not. Just as with forecasts of where a hurricane will strike and how powerful it will be, astronomers are continually updating their knowledge of an asteroid’s orbit and how big the asteroid is. Nevertheless, the case of Apophis revealed actual mistaken judgments about errors. Both the observations that discovered Apophis in June 2004 and the pre-discovery images from March 2004 that were found in December were obtained in non-standard ways, unlike most observations obtained by the Spaceguard Survey. As a result, the estimate that Apophis would miss the Earth in 2029 by 40,000 miles was badly in error, as revealed by subsequent radar detections. The actual miss distance of about 22,000 miles is far outside the “error bars” of the 40,000-mile estimate. It turns out that the assumed precision of the March and June 2004 data points was greatly and erroneously exaggerated. Astronomers also failed to appreciate the uncertainty of the other quantity used to determine the Torino Scale value: the size of the asteroid. The original estimate of 460 meter diameter was little more than a guess, based on the apparent brightness of Apophis. Many asteroids with that brightness could be around 250 meters in size, but many others could be well over 1 kilometer in size, large enough to threaten a global climate disaster, which would put our civilization at risk. Indeed, the size of Apophis has not yet been measured directly. The shape of its spectrum, from which its size has been indirectly estimated, is unusual (or perhaps erroneous). Currently, it seems more likely that Apophis is between 200 and 400 meters across than larger, but we really can’t be sure. Astronomers need to adopt “meta-error-bars” that take qualitative uncertainties into account, and to use Bayesian statistics. Once astronomers get their errors calculated correctly, it is time to discuss the predictions and uncertainties with the larger public. This difficult issue has been addressed in several case studies in Prediction: Science, Decision Making, and the Future of Nature (eds. Sarewitz et al. 2000). It is all the more difficult when confronting a hazard that has never been witnessed by modern mankind and which involves very small probabilities, which people have great difficulty relating to. It is essential that astronomers do a better job in order to develop and maintain credibility. “I made a plot of the impact points, superimposed on the globe, which is a sobering study in geopolitics. The problem is that this information does not provide the proper tenor for what we hope will continue to be a fairly moderated public reaction. Indeed, the details of the impact zone could prove to be highly inflammatory….If you have a press contact [who] wants to know where it would hit, please refer them to us for properly evasive responses.” – NASA scientist, 24 December 2004. Path-of-Risk in 2029: Tell the Public or Keep it Secret? The Character of the Impact Hazard The threat from >1 km asteroids will be decreased by 90% by 2009, thanks to the Spaceguard Survey. The remaining threat to life is chiefly due to impacts on land by objects 50 – 200 meters in size, which happen every few centuries. The remaining threat to infrastructure is from objects 200 – 500 meters in size that cause infrequent, devastating tsunamis. Are These Risk-Communication Guidelines Relevant to discussing a threat decades in the future, when the prospects are good that the threat will vanish after weeks, months, or a few years of more observations? Shades of the DHS terrorism threat-level scale? The color-coded scale above, adopted by asteroid astronomers at a meeting in Torino, Italy, in 1999, attempts to simply describe the seriousness of an asteroid impact prediction. The words are the scale; the diagram at the bottom shows how the TS value is calculated from two numbers: (a) the megatonnage of the threatening impact (related to the diameter of the asteroid) and (b) the probability that the collision will actually happen. Preliminary calculation by Rusty Schweickart, B612 Foundation For many years, we have asked how the asteroid impact hazard fits within the national “all-hazards” disaster reduction planning (see 2001 newsclip to the left). There still is little-or-no awareness of this hazard, or of its similarities and unique attributes compared with other hazards, within FEMA or DHS. In 2005 Congress passed, and the President signed, a law amending the Space Act (NASA’s charter) requiring NASA to find 90% of near-Earth asteroids larger than 140 meters by 2020. It also required NASA to report back to Congress by 31 December 2006 what options were available for detecting and characterizing near-Earth asteroids and mitigating their threats. At a meeting held two weeks ago in Vail, Colorado, NASA received input from many scientists and engineers. NASA reported that they were also consulting with the NSF and DOE, but that they were not consulting with FEMA or DHS. Why not? (See stack of Vail White Papers below.) Meteor Crater, Ariz. What could we do if an asteroid really were going to strike? With years or decades of advanced warning, we could send out a “Gravity Tractor” spacecraft, which would hover above the asteroid and – without touching it – gradually drag it off its Earth-bound trajectory. [Concept by Ed Lu and Stan Love, Nature, Nov. 2005; Artwork: Dan Durda]