Re: [ASTRO] Minicomets bombarding Earth

Jim Scotti (jscotti@LPL.Arizona.EDU)
Sun, 6 Jul 1997 03:31:40 -0700 (MST)

interpretation of his observations.  First a brief summary of what his
observations are.  First, back in the early 1980s, he observed that there
were dark spots on UV observations of the Earth's atmosphere which he
interpreted as being small icy bodies about 100 meters in diameter
bombarding the Earth.  They supposedly are torn apart - originally he
proposed that tidal stress broke them up around 3,000 to 10,000 kilometers
above the atmosphere so that they spread out to be a few 10s of kilometers
in diameter by the time they hit the atmosphere, thereby making them very
low density by the time they get to low Earth orbital altitudes.  He later
changed his breakup mechanism to electrostatic disruption when it was
pointed out that tidal disruption would not cause the water cloud to
spread out so much in the short time between breakup and impact.  More
recently, he has downgraded the small comets to about 10 meters diameter
and suggested that they are very low albedo objects (less than 2% albedo)
and low density (between 0.1 and 0.01 g/cm^3). 
   Satellite impact rates are probably one of the weakest arguments
against the small comet hypothesis - even at such a high impact rate,
there is still a lot of room between satellites and I think it was
calculated that for the shuttle the impact rate is something like 1 in
3,000 missions, and even then, the water cloud is supposed to be
kilometers across and so low density that it would hardly be noticed. 
   More importantly, these objects are not being detected by us here at
Spacewatch.  We actually see objects in the size range that Frank expects
his small comets to be - around 10 meters - and we see them out beyond the
distance of the moon.  From the rate we detect these objects, we estimate
the impact rate at about 10 per year.  That's about 1 million times less
frequent that Frank's estimated impact rate.  We should see these objects
at rates close to 10,000 per night or one every several seconds.  We would
be swamped by seeing trails on our computer screen in real time.  Frank
argues that these objects are very dark in order to make them too faint
for us to detect.  Magic on his part, if you ask me.  He also claims that
observations made at our telescope by one of his colleagues, Clayne
Yeates, back in 1986 and 1987 confirm his objects.  The trouble is, we
were using an RCA CCD in those days which had a radioactive substrate and
the reduction analysis made no attempt to model the cosmic ray rate, using
only a simple poisson noise model.  Every one of us involved in Spacewatch
who looked at his images thought he was observing noise. 
   Another bit of evidence against the small comet hypothesis is the lack
of detections by the Apollo Seismic experiments.  Frank originally argued
that tidal breakup worked in the moon's vicinity and reduced his mini
comets to a vapor cloud, but that was shown to not work, and his more
recent electrostatic breakup mechanism does not work near the moon, so his
objects hit the moon intact.  Here's were some more of his magical
particals come into play - they are supposedly very low density so that
their impacts result in basically a bounce off the moon without disturbing
the lunar regolith signifantly.  There are several obvious problems with
this still.  First, the lunar atmosphere was measured and there's nowhere
near enough water present.  If the objects impact the way higher density
objects should, the comet should vaporize and cause craters about 100
meters in diameter and there would have to be a huge water atmosphere
induced by the collision rate of about 1 per minute.  Even if they bounce
elastically, they impart their momentum (times 2!) into the moons surface
and should fly away from the moon as an expanding vapor cloud.  The moon
should have a substantial coma which has not been observed.  Frank
estimates completely unrealistic seismic parameters in order for his
objects not to have been detected by the Apollo seismometers. 
   This topic has been addressed in great detail both in the literature
and online.  There were a series of letters in the GRL back in 1986 and
later with attacks on Frank's hypothesis and his replies to those
arguments (followed by replies to those replies and so on...).  Dessler
(1991, Rev. Geophys. vol. 29, 355-382) and Frank and Sigwarth (1993, Rev.
Geophys. vol. 31, 1-28) give the best summaries of both sides of the
   Sorry this has gotten so long, but I hope it helps to address the
problem and points you to sources to go into more detail if you like. 


On Sat, 5 Jul 1997, Steve Walter wrote:

> I have read what I could on the mini-comets' bombardment of Earth, and
> (without my doing any real scientific analysis) I have to say that I still
> have a hard time believing such a wide-spread amount of conglomerated
> matter is doing such a thing.
> I make this statement based on the VERY high number of man-made satellites
> up in orbit, and the knowledge of what happens to one of them when even a
> tiny micro-meteor is believed to hit something like a large solar array
> panel -- there's a VERY noticable attitude disturbance on the spacecraft!
> >From my experience -- very few instances of meteors hitting spacecraft have
> been recorded, despite 30-years of experience.  I would expect this number
> to be MUCH larger hazard for a mini-comet-like condensation of material
> constantly sweeping toward Earth.
> My feelings on this do not explain what Dr. Frank believes he's seeing, nor
> the high-altitude bursts discussed by Paul Rybski.  Whatever is going on,
> though, needs to be consistent with real-life observations from satellite
> control.
> Steve Walter
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Jim Scotti                              
Lunar & Planetary Laboratory 
University of Arizona                520/621-2717 
Tucson, AZ 85721 USA