[ExI] Planetary defense (PDC2011 summary)

[Anders Sandberg]

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