From: jscotti@lpl.arizona.edu (Jim Scotti x2717)
Newsgroups: sci.astro
Subject: Re: meteor over Italy = Killer meteorite ?
Keywords: NEOs, Spaceguard, Meteors, Meteorites
Date: 26 Jan 93 01:41:09 GMT
Organization: Lunar & Planetary Laboratory, Tucson AZ.

In article <93022.124235K3032E0@ALIJKU11.BITNET> K3032E0@ALIJKU11.BITNET writes:
>
>  [ Stuff about the meteor seen in Europe and the reported deaths
>    possibly caused by a meteorite impact deleted...]
>
>Nontheless, I think not even the largest telescopes on earth with the most
>sensetive CCDs could have detected this small object before it entered the
>earths atmosphere - not to mention to search for unknown bodies of this
>size| After all, we are not able to detect most of the much larger near-earth
>objects. 

  I beg to differ with you on these points.  As a discoverer of many
Near Earth Objects (NEO's), and as part of the Spacewatch Team which uses 
CCD's and a 0.91 meter (36 inch) telescope, I'd like to comment on this.  
We have discovered and obtained orbits for 3 objects whose estimated sizes
are under 10-15 meters diameter, and another 5 NEO's whose estimated sizes
are under about 50 meters diameter.  We can (in principle) detect a 5-10
meter object at 0.01 AU from the Earth and can track them out to 0.04 AU
or so.  We have detected one object in Earth orbit whose brightness,
if it had been an NEO, would result in an estimated size of just 0.5 meters
diameter!  And we do this with only a 36 inch telescope!  Of course, we
cover a very tiny part of the sky, so the probability of detecting this
particular object is very small, but they CAN BE DETECTED.  Incidentally,
we detected the Galileo spacecraft when it was 8 million kilometers from
the Earth!
  As to the second point, we are not able to detect most of the larger
NEO's only because we do not have resources at the moment to do the search.
As has been discussed in this group and elsewhere, a modest proposal to
build 6 telescopes of 2-3 meter aperture and outfit them with mosaics of
CCD's would allow the nearly complete cataloging of the roughly 2000 NEO's
larger than 1 km diameter and a sizable sampling of smaller objects in
only about 20 years time!  If the money were allocated now, this survey
could start now.  BTW, this survey has been called the "Spaceguard
Survey", after such a hypothetical survey described by Arthur C. Clarke
in "Rendezvous with Rama".
  Spacewatch is working on upgrades which we hope will increase the rate
discovery of NEO's by about a factor of 5-10 within a few years.  We have
already more or less doubled the discovery rate of a few years ago and
more efficient techniques and emulsions have almost doubled the rate of
detection of the photographic observers in the same time as well.

>          I suppose some catastrophe like the Tunguska-event, which distroyed
>a wide area in Siberia in 1908, would happen just as unexpected in our days.

  Without more comprehensive sky coverage with state of the art CCD's and
telescopes, this is still true and is likely to remain true even with a
system of telescopes described above.  In any case, without looking toward
the daytime side of Earth, you can still get blind-sided be small NEO's
moving away from perihelion.

>There's not much change (sic) of discovering near-earth-objects a long time before
>they eventually impact on the surface on earth, if they are discovered at all.

  With a fully implemented Spaceguard survey, the largest most globally
dangerous objects will be detected in less than a lifetime and the orbits
of those objects will be able to be projected far enough into the future
to plan, at least on a century timescale, for possible collisions.

>However, impacts which cause major damages don't happen very often, though
>a few falls in this century could have caused much more damage if they had
>impacted in more populated areas. Examples are the Sikhote-Aline-meteorite
>shower, when approx. 100t of iron meteorites rained dowm in 1947, the
>1.77t Jilin-meteorite (which missed the city of Jilin, China, only by a few
>kilometers in the 1970's) and, of course, the Tunguska-event.

  The Spacewatch discoveries of objects smaller than about 100 meters
have allowed us to characterize the probability of small objects hitting
the Earth.  We find that objects about 10 meters diameter should hit
Earth about 1-10 times each year.  Fortunately, most of those objects
will not survive atmospheric entry.  Also, it appears that the majority
of these small objects also have relatively low eccentricity orbits and
correspondingly lower impact velocities with the Earth.
  Tunguska sized events are probably once per century to once per thousand
years - remember that the Tunguska impactor is estimated to have been only
about 50 meters in diameter!

>Don't worry, be happy, and keep looking UP|
>  Herbert

  Don't worry - I AM looking UP & hunting for those potentially dangerous 
little beasties!

Jim.

---------------------------------------------
Jim Scotti 
{jscotti@lpl.arizona.edu}
Lunar & Planetary Laboratory
University of Arizona
Tucson, AZ 85721 USA
---------------------------------------------
Newsgroups: sci.astro
From: chris@chrism.demon.co.uk (Chris Marriott)