56 ASTRONOMY t FEBRUARY 2014Tiny particles in the solar system may answer some of its biggest questions.
by Meenakshi WadhwaTHROUGH THE MICROSCOPE
M
ost of us have had the
wondrous experience of
seeing shooting stars in
the night sky. It is incred-
ible to realize that the
objects creating these bright streaks of light
are mostly the size of a grain of sand or
smaller. In fact, it is this dust, originating
mostly from comets and asteroids in our
solar system, that makes up much of the 50
to 100 tons of material astronomers esti-
mate falls to Earth every day.
Scientists have made several efforts to
collect extraterrestrial particles after their
arrival on Earth. For example, researchers
have recovered such particles from deep-
sea sediments or from large quantities of ice
that were melted in Greenland and Antarc-
tica. But these particles change quite sig-
nificantly because of their fiery entry
through Earth’s atmosphere and from sub-
sequent weathering on its surface.
To obtain pristine extraterrestrial dust,
NASA uses high-f lying aircraft and space-
craft. Why go to such extraordinary effort
to collect these tiny particles? And what
have we learned from studying them?Collecting cosmic dust
Astronomers call the thousands of tons of
extraterrestrial particles that fall to Earth
annually cosmic dust. The grains typically
measure less than 100 microns, about the
width of a human hair. While frictional
heating with air destroys some of them, our
atmosphere slows a majority sufficiently so
that they fall gently to Earth’s surface.
For more than three decades, NASA has
used ER-2 and WB-57 high-f lying aircraft
to collect these particles at an altitude of
about 12 miles (19 kilometers). Only about
10 percent of this material is cosmic dust.
The rest is of terrestrial origin.
Sticky surfaces (typically Lexan plates
coated with silicone oil) beneath the wings
of the aircraft collect the particles. Once the
collectors arrive at the Cosmic Dust Labo-
ratory, established in 1981 at NASA’s John-son Space Center in Houston, the curator
examines them through a microscope.
The particles are then extracted from
the collectors, rinsed in solvents to remove
the silicone oil, and imaged and docu-
mented with a scanning electron micro-
scope. The curator publishes information
about the particles for the scientific com-
munity, and researchers can then request
particular grains to study.
Such studies have revealed that a large
fraction of cosmic dust is material not pres-
ent in any of the known types of meteorites,
most of which originated from asteroids.
Some of these particles represent dust from
comets and are perhaps the most primitive,
least-altered material in the solar system.
They have not changed due to heating or by
interaction with water, processes that have
occurred on many asteroidal bodies.What are we learning from
Here’s what
all the fuss is
about. This
aggregate in-
terplanetary
dust particle
can reveal much
about conditions
in our early solar
system. It measures about
10 microns across. ALL IMAGES: NASANASA’s Stardust spacecraft obtained this false-
color composite image of Comet 81P/Wild 2 on
January 2, 2004, by combining a long exposure
showing the jets of material streaming from the
comet’s nucleus and a short exposure showing
details of the nucleus itself.