On Fri, 2002-06-21 at 01:30, David Weinshenker wrote: > Randall Clague wrote: > > it's never too early to fully fund The Watch. > > Hmmm... doctors say that you should not subject a patient to tests which > will have no benefit in makeing treatment decisions; and that's a fairly > sensible position. Programmers have been known to say, "Never test for > an error condition that you're not prepared to handle." - that has a certain > ironic fatalism to it... it may do you no good to see it coming, if it's too > late (or if you don't have the means) to duck. > > But then on the other hand (was this the premise of the stories you mention?) > if you see a really big one far enough out that you've got time to start > building a response...
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. 1950 DA is probably the most famous of these and the only currently known body with a hazard level greater than the background level, with a 1/300 chance of impacting on March 16, 2880, which would result in an energy flux of 100 GT (yes, gigatons). That would be very bad, but at least it's not a planet killer. NeoDys at <http://newton.dm.unipi.it/cgi-bin/neodys/neoibo> has some pretty good information about the known PHAs. Unfortunately, it turns out that we've only discovered a tiny fraction of the PHAs that must be out there to fit the current model of background impact risk. And there are a lot of currently known asteroids that aren't even being checked for possible impacts because their orbits are not characterized well enough to classify them as NEAs even if there are possible orbits that they could have based on observations to make them NEAs. "The Watch" or Skyguard survey would need to run for centuries before it got a good fraction of the PHAs out there. And starting one in a few centuries would have the same issues. The likelihood of such a project's catching something on its actual approach is incredibly small. So it really is never too soon to start something like this. It would be a shame to be wiped in a couple centuries by something we could only have seen had we been looking in this century. -- Sean R. Lynch KG6CVV http://www.chaosring.org/~seanl/
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