Sooner or later, it's bound to happen. Sooner or later,
scientists who study Earth-crossing asteroids say, astronomers
will find one that has a significant chance of striking the
planet. Unlike several recently discovered asteroids that were
first given very long odds for a collision, this time more
precise orbital calculations won't eliminate the possibility.
This one will be an asteroid ''with our name on it,'' in the
words of David Morrison, a scientist at the NASA Ames Research
Center and one member of a small community of astronomers,
physicists, engineers and other scientists who think a lot about
such an unthinkable event. It is not clear what would happen
then, though Dr. Morrison and others are trying to awaken
governments and the public to the need to at least think about
developing a way to respond. ''Eventually we will discover
something,'' Dr. Morrison said, though maybe not in this century
or even this millennium. ''Society should start planning for
that unexpected but potentially tragic possibi