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Frank,

> There's no 'a priori' guarantee that
> such objects will be detected twenty
> years in advance.

Of course, there is no a priori guarantee.
But my understanding is that something they really do.
(I did not follow the subject after that lecture long ago).

> The big unknown is
> long-period comets which are frequently
> discovered as little as one year before
> they reach the vicinity of the
> Earth's orbit.

He was speaking of asteroids (of the size threatening
the whole life on Earth). Long period comets cannot
be detected so early, but as I understand the danger
of a comet is much smaller.

> All we have to do is compile an almanac
> an almanac of all of the exceedingly
> faint asteroids that happen to be in
> orbits near the Earth's orbit.

Yes, and this they do using automatic telescopes that
scan the sky and photograph it continuously.
Comparison of two photographs of the same
piece of the sky made in some time interval shows whether
anything has changed.

A.


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