hardy678-ga,
Your questions are fascinating. Thanks for bringing them to Google
Answers, and allowing us an opportunity to look into this topic for
you.
Let me give you some brief, direct answers to your specific questions,
and then elaborate a bit with some more in-depth information.
>>If a very large asteroid that we have not discovered yet were bound
to hit earth, would we know about it before it hit?<<
Scientists are actively in the process of cataloging the largest
asteroids. As a rule of thumb, the larger the asteroid, the easier it
is to spot as it nears Earth (and by astronomically 'near', it can
still be decades away from possible impact). So, for a very large
asteroid, the odds are quite good that would know about a possible
impact well in advamce -- enough time to devise a defensive strategy.
However, though the odds are good, they are not 100%. A very large
asteroid could, conceivably, sneak through our observation net.
>>Is this still true for small asteroids or are they much harder to see?<<
The ease with which we can see an asteroid depends largely on its
brightness. A very large, very distant asteroid might have the same
overall brightness as a much smaller, much closer asteroid. So...in
one sense...both asteroids are equally easy to see. However, we would
be able to see the larger asteroid years or decades before any
potential impact. But for the smaller ones, they may only be spotted
months -- or even only days -- prior to their intercept with Earth.
>>What is the latest we would find out about a world-threatening
asteroid before it hit the earth?<<
An event like this is extremely rare. Presumably, the last time it
may have occurred on Earth was when the dinosaurs became extinct 65
million years ago. In all likeliehood, we would be alerted to the
possibility of a collision course with a very large object many years
or decades ahead of any actual impact. However, there is always a
chance -- however small -- that a large asteroid could remain
undetected in its approach to Earth, finally to be seen with only a
few months advanced warning.
No one really denies the possible destructive power of an asteroid
collision, up to the possibility that a large enough impact could
pretty much wipe out human life on the planet. At the same time, the
odds of something this catastrophic occuring are remote in the
extreme...what the scientists, with their gift for understatement,
have dubbed a "Low Probability, High Consequence Event".
As I said earlier, it's been at least 65 million years since the last
such event may have taken place, and could easily be tens or hundreds
of millions years before the next one.
You're much better off taking care of yourself by looking both ways
before crossing the street, as that's where the real, everyday dangers
lie.
I promised some additional information on this topic, and to provide
it, I'd like to turn to an in-depth report that was prepared a few
years ago by NASA. The report examines what may be needed in terms of
the next generation of NEO observation systems:
http://neo.jpl.nasa.gov/neo/neoreport030825.pdf
Study to Determine the Feasibility of Extending the Search for
Near-Earth Objects to Smaller Limiting Diameters
August 22, 2003
I've included some relevant excerpts from this report, below, along
with a few of my own comments, which are in brackets:
[NASA hopes to have 90% of larger asteroids catalogued soon...which
obviously means that at least 10% will remain uncatalogued, and could
be the source of a surprise visit]
...In 1998, NASA formally embraced the goal of finding and cataloging,
by 2008, 90% of all near-Earth objects (NEOs) with diameters of 1 km
or larger that could represent a collision risk to Earth...The 1 km
diameter metric was chosen after considerable study indicated that an
impact of an object smaller than 1 km could cause significant local or
regional damage but is unlikely to cause a worldwide catastrophe
[Presently, we're only at the 50% level of having catalogued the large
asteroids, though presumably those that pose the largest near-term
danger are the ones that are the easiest to spot]
...The impact of an object much larger than 1 km diameter could well
result in worldwide damage up to, and potentially including,
extinction of the human race. The NASA commitment has resulted in the
funding of a number of NEO search efforts that are making considerable
progress toward the 90% by 2008 goal. At the current epoch, more than
50% of the expected population included in the goal has been
discovered and the subject objects continue to be discovered at
impressive rates.
[The thrust of the NASA report is that the cumulative impact of
smaller, more frequent impacts deserve more attention than they've
been getting, since most attention to date has been on the larger
NEOs]
...Given the steeply increasing population of near-Earth objects with
decreasing diameter, it is much more likely that civilization will
experience the impact of an object smaller than 1 km than experience
an impact from a larger one. Indeed, the significance of small
impactors is beginning to be appreciated by the broad public and by
scientists alike.
[Cataloging gives us a long-range, plenty-of-time alert system, while
'warnings' apply to objects that appear relatively suddenly]
...The direct benefit of a search program comes from two sources,
cataloging and warning...by discovering and cataloging NEOs and by
verifying that none will impact within the next century, we reduce the
potential risk to life and property on Earth. If an object is actually
discovered on a threatening trajectory there will presumably be many
years, even decades, in which to execute a plan to deflect or disrupt
the impactor.
...The term ?warning? describes a situation where an impactor is first
detected and recognized some days to months before the event. This
?warning period? would afford civil authorities an opportunity to take
actions that would mitigate the impact effects, but there would be
insufficient time to avert the collision. In such a scenario the
warning benefit is largely comprised of causalities avoided through
the evacuation of affected areas.
[A large asteroid can do a heck of a lot of damage]
...At the time of the Spaceguard Survey Report (Morrison 1992), the
lower limit to the size of an impactor that could lead to global
environmental consequences was estimated to be around 1.5-2 km. Such
events are estimated to occur about once in a million years and could
lead to a billion or more deaths. Thus, the annualized risk is on the
order of a thousand deaths per year.
...A final touch point is the impact that led to the extinction of the
dinosaurs, which is estimated to have been an object about 10 km in
diameter (108 MT). By this size, most of the world's population would
almost certainly perish.
...The risk of a global environmental catastrophe resulting from the
impact of an asteroid larger than 1 km in diameter is what motivated
the original Spaceguard Survey. The risk associated with such an event
is so large that the residual risk from even the small fraction of the
population that will remain undiscovered by the present surveys is
substantial.
[But even smaller ones can pack a wallop, though not necessarily on a
global scale. Figure 3-1 in the report shows that a high-speed impact
from a meteor 25 meters in radius would have a destructive impact over
an area the size of Washington DC (note, though, that this is not to
say it could destroy an area this large, but that at least some
destruction could be expected over this large an area). An impact
from a meteor with a radius of about 100 meters could affect an area
the size of Connecticut.
Figure 3-1. Radius of Destruction versus Impactor Radius
I trust this is the sort of information you were looking for.
However, if there's anything else you'd like to know on this topic,
don't hesitate to ask. Just post a Request for Clarification, and I'm
at your service.
Cheers,
pafalafa-ga
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