Subject: Re: Sound bites on asteroid searches

From: Nick Kaiser

Submitted: Wed, 26 Feb 2003 09:16:32 -1000 (HST)

Message number: 84 (previous: 83, next: 85 up: Index)

On Wed, 26 Feb 2003, David Morrison wrote:

> In the real world, impacts are deterministic: There either is or is 
> not an asteroid that will collide with Earth during this century.

But our knowledge is probabilistic.  Based on our current
knowledge there is some probability per unit time that we will be 
struck by an object of some energy, composition etc.  The correct way
to quantify the utility of future NEO searches is to compute how that
probability will be reduced if, as is most likely, no objects are
found.

That this is hardly a "sound-byte".  Moreover, a common reaction, even 
from technical folks, on hearing this is to exclaim that "just making an 
observation cannot change the risk".  The problem is that people often
forget that a probability is a statement of our knowledge about the
world, rather than some objective reality.  I understand then why you
don't like to use the P-word, but it is really the right way to look
at this, so we have a communication/education problem. 

> 
> Many science objectives can be met with statistical samples, but 
> planetary defense requires a complete survey.

But no survey will be complete.  That's why we need to explain 
the probabilities, even though this is evidently a challenge.

> 
> The public does not care if the annual risk of dying from an impact 
> is one in a million or one in two million -- they want to know if it 
> will happen to them.
> 
> When you cross a busy intersection, you don't ask what the statistics 
> are on pedestrian accidents -- you look to see if a car or truck is 
> headed toward you.

That seems a bit condescending. A good analogy is screening for
cancer.  Surely people understand that this is probabilistic, and
can weigh up the costs and benefits (or at least their policy making 
representatives can).  I don't see that NEO searches are any different.

> 
> There is no last-minute warning system: Either we discover an 
> asteroid and predict its impact decades or centuries in advance, or 
> else it strikes without warning -- the first you will know is when 
> the sky lights up and the ground starts to shake.

Well, one could operate an LSST in such a mode as to detect "death-plunge"
objects.  It would dictate a very different observing strategy and would
tilt the balance towards distributed apertures, since one would want to
go very broad.  We looked into this for Pan-STARRS (and our review panel
wants us to reaxamine this) but we concluded that the utility was small.
As it happens tough, about 10% of the incoming collision risk comes from
the trailing "sweet-spot", so there is some chance of detecting incoming
stuff.  One would get about a month warning of a 30m impactor.

Cheers
Nick



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