[ExI] Planetary defense (PDC2011 summary)
Anders Sandberg
anders at aleph.se
Fri May 13 12:55:49 UTC 2011
Ok, a quick summary of what I learned at the 2011 IAA Planetary Defense
Conference in Bucharest.
Microsummary: we are on the right track!
The latest estimates of the NEO size distribution by Alan W Harris
retain the shape of previous estimates, a power law with a pretty steep
exponent and an unexplained 'dimple' where there are too few 10-100 m
NEOs. Donald Yeomans showed that the impact flux from comets is small
compared to NEAs (less than a percent). Satelite surveys have produced a
flood of data (check out this animation
http://vimeo.com/groups/skysthelimit/videos/15166379
or my graphs at
http://www.flickr.com/photos/arenamontanus/sets/72157626697547832/with/5712742374/
) and we have good reason to think we have already seen a sizeable
fraction of the NEAs that would be serious GCRs - and they are in orbits
that are safe for at least the next century. This has really reduced the
expected risk. The mandate from the US congress wants a 90% completion
by 2020 for the big (>140) NEAs, which might be tough unless there is
some extra push (ideally a Venus orbit satelite or more time at the
Large Synoptic Survey).
The LSS will be able to catch a lot of 45 m objects 1-3 months out, and
people are starting to seriously look at finding smaller objects on
their "death plunge" just before they hit Earth. They pose just local
risk (if any at all) but as the GCR risk is retired their relative risk
go up. Mark Boslough showed that some of them might produce pretty
destructive airbursts. This domain also deals with the "we are going to
be so successful that we are going to put ourselves out of business"
problem - we are 10 times more likely to save lives by including
imminent impactors in the next surveys, although the number of lives
saved might be smallish.
Obama's call for a manned NEO mission is a tough challenge, mainly
because the best objects from a mission technical perspective (low
deltav etc) might be smaller than the spaceship! Overall, NEO missions
are maturing but the Japanese seem to be a decade ahead with Hayabusa
and Hayabusa II.
Deflecting asteroids with kinetic impactors looks pretty good, but a lot
hinges on the porosity of the asteroid. A fluffy asteroid just absorbs
the impact, while a "hard" asteroid will eject a plume of debris that
gives an extra push. Unfortunately we have no way of measuring the
porosity, so plenty of talks investigated models and ways of estimating it.
Gravity tractors are wimpy, but seem to be fairly close to a realistic
technology. They are pretty useless for deflecting an asteroid away from
Earth, but enough for preventing it from going through a keyhole. This
makes them a pretty ideal supplement for any mission. Flotillas of
tractors can be more effective than single tractors. Paul W. Chodas also
showed that there are "Jabbas", robust states of an orbit that are hard
to budge.
Nuclear deflection looks like it is workable, but it is definitely a
last resort and mainly useful for imminent hits. Some serious issues
about how to avoid dispersing loosely held together impactors. The main
problem is getting the warheads to the impactor in time and to have them
detonate at the right standoff distance. Erick Ball described a real
"Armageddon" scenario where a 5 km long periodic comet discovrered ~290
days out could be deflected if mankind really got its act together -
something we all felt was doubtful. Several other deflection methods
(laser ablation, robotic rockthrowing, ion beam shepherds, painting to
cause Yarkovsky effect deviations...) are investigated but not ready
from prime time. Another problem is that NEOs seem to be quite different
from each other, and methods that work for one type might need to be
tweaked for other types.
Organisationally, things are moving forward. NEO study and defense is
becoming more and more organised in the big space agencies, the UN and
the US government. Some interesting notes from Frans von der Dunk on the
legal aspects: check out the report "Legal Aspects of NEO Threat
Response and Related Institutional Issues"
http://swfound.org/media/40426/Legal%20Aspects%20of%20NEO%20Response%20and%20Institutional%20Issues%20-%20Final%20Report.pdf
In particular, the 'responsibility to proetect' might apply here,
requiring states to have capabilities to deal with NEO risks. There are
also an interesting possible tradeoff between sharing information and
doing missions in an open manner and avoiding liability: if damage
occurs in the course of a NEO response states might not be held liable
as long as the mission is confirmed to parameters set by proper mandate
international community groups.
My own talk was about the issue of cognitive bias and rationality in
impact mitigation. Basically, our biases are interfering with both the
public, decisionmakers and the research community, and overcoming them
is an important part in public relations, explaining the situation and
doing the research. Not all of them are bad for the impact risk
community: the preference for hard numbers really helps it in the
"competition" with softer risks. But availability bias (it has never
happened, never will) and scope neglect (a million dead are just
statistics) make many decisions rather irrational. There are also
problems with planetary defence being a public good, being long-term (it
might be rational to put off doing things for a while, since tech is
advancing - but this easily leads to putting off doing things too long;
the "sweet spot" might be a decade, about a political lifetime) and
discounting the future too heavily.
However, I think the impact community are an example to all of us
dealing with other xrisks. They are doing a pretty good job. They have
managed to 1) demonstrate the existence of a risk and quantify it, 2)
convinced enough decisionmakers to fund preliminary investigation, 3)
built a lively interdisciplinary community devoted to the risk and
mitigation (with an inflow of new students carrying on and developing
the thinking). Other risk communities may do well to study how they did it.
And finally, a great motto: "There ain't no such thing as a free launch".
--
Anders Sandberg,
Future of Humanity Institute
Philosophy Faculty of Oxford University
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