On 21 Jun 2002, Sean R. Lynch KG6CVV wrote:
> It's not a matter of seeing the object "as it's approaching," but of
> discovering PHAs and characterizing their orbits well enough to be able
> to find potential impacts...
There is one class of PHAs which will have to be seen as they approach:
long-period comets simply don't come around the Sun repeatedly in any
reasonable amount of time. They are a particularly ugly case: short
warning times, awkward orbits which render rendezvous-based deflection
methods unworkable, and some possibility that one could have an orbit
which would put it near or behind the Sun in our sky until it was quite
close.
> "The Watch" or Skyguard survey would need to run for centuries before it
> got a good fraction of the PHAs out there.
This really isn't true. Quite a modest effort (tens of millions, not
billions, per year) could fairly quickly (a decade or two) nail down
essentially all the short-period comets and NEAs which are big enough to
threaten the entire planet or large portions of it, and over a somewhat
longer period could do a respectable job on the ones that are big enough
to have wide-area effects. Tunguska-sized things would take a lot longer,
yes, but they are not the big worry. The level of resources currently
invested in this stuff is really ridiculously small.
Henry Spencer
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