Cosmic Muck Rake is the nickname 
for this magnetic contrivance devised 
by researchers at the University of 
Washington for the collection of 
cosmic spheres from the sea floor. 
The rake is usually towed through 
the water at depths of about five 
kilometers, picking up tens of 
thousands of spheres in a day. 
Leo Pilachowski 
though most meteoroids in space are 
believed to be of cometary origin and 
are probably composed of weak and 
fragile materials. 
Fortunately, the strength problem, 
or the ability of materials to resist 
pressure, is less severe for dust par- 
ticles than for large meteoroids. Small 
particles of less than 100 microns in 
diameter decelerate from their cosmic 
velocity of about fifteen kilometers 
per second high in the atmosphere 
to speeds of about one centimeter per 
second near altitudes of one hundred 
kilometers. There the air is so thin 
that the maximum pressure generated 
on the particles is a thousand times 
smaller than the pressure experienced 
by bigger objects lower in the atmos- 
phere. The fact that many of the ex- 
traterrestrial dust particles that have 
been collected are more porous and 
fragile than conventional meteorites 
is proof that the strength problem is 
not severe for cosmic dust. 
The heating of interplanetary dust 
as it enters the atmosphere is also 
a function of size. Most of the particles 
smaller than fifty microns decelerate 
in very thin air where the rate of fric- 
tional heating is so low that the par- 
ticles are not heated to their melting 
points. Dust particles that survive en- 
try without any melting are called “mi- 
crometeorites,” as defined in the 
1950s by the American astronomer 
Fred Whipple. Most of the particles 
larger than 100 microns do not slow 
down until they reach high enough 
air densities to cause melting. Big par- 
ticles larger than a few millimeters 
can have fairly cool interiors even 
though their exteriors melt, but as they 
melt, surface tension pulls them into 
the shape of spheres, and they solidify 
within a time span of seconds and 
at very high altitudes. Some rare par- 
ticles larger than 100 microns can es- 
cape melting only if they enter the 
atmosphere at an oblique angle. 
To investigate the true physical na- 
ture of interplanetary dust, it is an 
enormous advantage to capture indi- 
vidual particles that can then be stud- 
ied in the laboratory. Space would 
seem to be the obvious place to collect 
interplanetary dust. Unfortunately, 
this is impossible to do in a non- 
destructive way because of the enor- 
mously high velocity of the particles 
relative to any collector that might 
be put into space. A cosmic dust par- 
ticle striking a spacecraft decelerates 
from a velocity of fifteen kilometers 
per second to zero in only a billionth 
of a second. The impact vaporizes the 
particle and a tiny crater (and possibly 
some debris) is the only remaining 
evidence that the particle ever existed. 
With this problem in mind, it is clear 
that the earth itself is perhaps the 
ideal cosmic dust collector. In the thin 
air of the upper atmosphere, incoming 
particles are decelerated relatively 
gently, and the “collision” with the 
earth’s atmosphere can be nondestruc- 
tive. 
The problem with the earth is that 
while it is a good cosmic dust catcher, 
the actual recovery of individual dust 
samples for laboratory study is very 
difficult. Paradoxically, although ex- 
traterrestrial dust is everywhere 
around us in our terrestrial environ- 
ment, it is almost impossible to find. 
The situation is different for large me- 
teorites, of course. If a 100-kilogram 
meteorite fell today in New York City 
it would almost certainly be found 
since large rocks falling out of the 
sky are unusual and conspicuous ob- 
jects. This is not the case for ten- 
micron cosmic dust particles. While 
it is true that on the average a square 
meter of New York will collect one 
cosmic dust particle every day, it will 
also collect up to ten billion other par- 
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