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Tiniest Of Space Bodies To Get Close Examination


       Tiniest of space bodies to get close examination
       June 2, 1998

       A NASA/Marshall Space Flight Center news release

       As astrophysicists turn their telescopes to probe the origins
       of stars and planets, they will start giving more attention
       to the smallest of astronomical bodies - dust particles -
       which both make them and also obscure the view.

       "We're developing an experimental method to measure
       scattering and extinction cross sections for dust particles
       in the solar system," said Dr. James Spann of NASA's Marshall
       Space Flight Center. Spann is leading development of the
       Dusty Plasmas Laboratory. In it, a single grain of dust is
       suspended by static electricity while it is bombarded with
       electrons and light and its reactions measured.

       Dust might seem like a lowly object to receive such
       attention, but it's an important factor in the vacuum between
       planets and stars. Dust particles drift through space where
       they absorb and scatter light.

       How rapidly they extinguish light over the millions or
       billions of miles of "empty" space determines how visible the
       source will be.

       "We think we can devise an experiment that replicates the
       environment of these particles in planetary or preplanetary
       atmospheres," Spann said.

       The observations planned by Spann and another Marshall
       scientist, Dr. Mian Abbas, will balance between two well
       known areas of optics, Rayleigh scattering and geometrical
       optics. Rayleigh scattering, where an object is much smaller
       than a wavelength of light, is why the sky is blue.
       Geometrical optics, where an object is much larger than a
       wavelength of light, is why lenses bend light.

       Between these two is the Mie theory covering light scattered
       by objects that are about the same size as a wavelength of
       light.

       "It's a very beautiful theory," Spann said. "It's incredibly
       fascinating for a lot of reasons."

       One of those reasons is how infrared light is scattered by
       dust grains which are much larger than visible light, but
       about the size of longer-wavelength infrared.

       Little work has been done in this area - it's mostly
       extrapolated from visible light observations or from the bulk
       properties of dust. The work won't be easy.

       "Part of the challenge in this experiment is that these
       grains are irregularly shaped," Spann explained. "Unless
       you're dealing with liquid droplets, which are spherical, the
       orientation of the grain is important." Thus, a grain may be
       larger than a wavelength of light across its length, but much
       smaller across its width.

       Interplanetary dust particles range from 5 to 100 microns in
       length; 30 microns is typical. They can be spherically or
       irregularly shaped, and made of silicate or carbonaceous
       materials. In total, it's a complex range of particles that
       Spann and Abbas will try to measure in detail.

       With the Dusty Plasmas Laboratory, Spann and Abbas will be
       able to make unique measurements of how dust particles
       polarize light - convert its vibrations so they are all in
       one plane - and the angles at which the light is reflected.

       "We can make significant contributions to planetary
       missions," Spann said.

       "All planetary atmospheres have dust, aerosols and grains
       hanging in the atmosphere." Even Mars with its tenuous
       atmosphere has months-long dust storms that obscure the
       surface.

       Results from the Dusty Plasmas Laboratory will also help in
       understanding what is seen in the thick dust clouds in deep
       space where planets are slowly condensing. Infrared
       telescopes can see little of what is happening because the
       view is obscured by the very dust that eventually will become
       planets, comets, asteroids, or just the dust that, as in our
       solar system, reflects sunlight back to give the sky a slight
       glow along the plane of the planets.

       Knowledge of the distribution of interplanetary and
       circumsolar dust particles and their physical and optical
       characteristics provides valuable information about many
       issues dealing with the origin and formation of the solar
       system. Interplanetary dust particles (IDPĚs) are considered
       to have their origin in cometary, asteroidal, and meteoritic
       sources, along with possible contributions from planets and
       the pre-solar molecular cloud.

       Dust particles in the interplanetary medium are produced by a
       variety of sources and have a diverse size range. Particles
       ranging from 5 to 100 microns contribute to most to the
       zodiacal light, with a typical particle size of 30 microns.
       The major constituents of the spherical or irregularly shaped
       circumsolar and IDPĚs are believed to be silicates and
       carbonaceous materials, as indicated by analyses of
       stratospheric dust particles of interplanetary origin.

       An experimental technique being developed in the laboratory
       at Marshall Spaced Flight Center for measurements of
       scattering and extinction cross sections of some commonly
       known interplanetary and circumsolar dust particles will be
       presented. This technique is based on irradiating a single
       charged dust particle suspended by electrodynamic balance in
       a cavity and measuring the scattered radiation as a function
       of angle. Comparison with Mie theory calculations leads to
       simultaneous determination of the particle radius, the
       complex refractive index, and the scattering and extinction
       cross sections.

       An application of this technique will also be discussed for
       investigation of rotational bursting phenomena whereby large
       size cosmic and interplanetary particles are believed to
       fragment into smaller dust particles.