Tiniest Of Space Bodies To Get Close Examination

May 29, 1998

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.

NASA/Marshall Space Flight Center--Space Sciences Laboratory

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