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

1
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 cchapman_at_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 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 gt1 chance that a gt1 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.

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 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. 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 cant 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 gt1 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 (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.)
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