Apophis: Risk Forecast and Communication Issues about a ...
Apophis: Risk Forecast and Communication Issues about a Possible Asteroid Strike in 2036
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
The Timeline for 2004 MN4/Apophis
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.
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 asteroids 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?
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
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 prediscovery 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
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.
Credit: Stan Ward
3 February 2005: Arecibo radar detection of MN4 drastically reduces
miss distance in 2029 from 40,000 to 22,000 miles (below height of
How to Estimate and Communicate about Errors & Uncertainties
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.
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.
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.
The public often misunderstands when astronomers obtain more observations, which help to refine knowledge of an asteroids 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 asteroids orbit and how big the asteroid is.
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
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.
there is a tiny chance of impact. Except marginally, Apophis will not be
visible again to optical telescopes or radar until 2012.
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 cant be sure.
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
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.
Path-of-Risk in 2029: Tell the Public or Keep it Secret?
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 colorcoded 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
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]
Meteor Crater, Ariz.
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.
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 (NASAs 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.)
Question Facilitation Tools. Dr. Mark Morton. Centre for Teaching Excellence. University of Waterloo. [email protected] www.markmorton.ca. Learning Outcomes for this Workshop. ... Grab an iClicker and get read to use it! Clickers.
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