November 5, 1996
Interferograms
- images built by having light waves interfere with each other - compare
how warm fluid flows on the ground (bottom) and doesn't in space (top).
Click to view a larger image and explanation.
How do you prove that you can hold a space experiment better than rock-steady?
You do it with mirrors.
A major research area for NASA is using the weightlessness of space to grow
advanced materials -- such as crystals of proteins -- that are difficult
or impossible to grow on Earth. Yet, even in orbit a spacecraft has small
vibrations that disturb crystals (for research on new electronics devices
or for medical research) that grow as slow as an inch a week (3.8 mm/day).
To smooth the ride, Marshall developed the Suppression of TrAnsient Events
by LEvitation (STABLE) system which uses electromagnets to suspend equipment
inside a rack and act as shock absorbers.
STABLE was developed jointly by NASA's Marshall Space Flight Center and
by McDonnell Douglas Corp. in Huntington
Beach, Calif.
But how to prove that STABLE works as advertised? You can see part of the
answer by dropping ice into a glass of warm water. Different fluids usually
have different indices of refraction, meaning they bend light differently.
That index of refraction will change when a fluid changes temperature, so
light bends where warm and cold layers meet. It's like injecting a tracer
without disturbing the experiment.
To turn that view of hot and cold liquid into an accurate picture they could
measure, Marshall scientists built a device, nicknamed Chuck, using mirrors
to combine images of two identical fluid cells into one image. The cells,
4 cm square and 1 cm thick (1.6 x 0.4 in.), were filled with a halocarbon
fluid (related to Teflon) that would react to heat more slowly than water.
Chuck's
hardware was small enough to fit in a briefcase - its footprint is less
than 24 by 21 cm (9.3 x 8.3 in.) and height is about 6 cm (2.4 in.). As
an extra challenge, the experiment was developed in less than six months
so it would be ready in time for the STABLE mission.Click
to view a larger image.
One fluid cell had a small heater and four temperature monitors, and each
cell was placed in front of a mirror. A beam-splitter (a half-silvered mirror
diagonally across a cube of glass) split a laser beam and sent half to each
cell. The mirrors behind the cells reflected the light back to the beam-splitter
which recombined the two beams into one beam focused on a TV camera.
Light behaves as waves that combine to build a higher peak if they are in
step, or to level each other out if they are out of step. Any change in
the two light beams will cause the waves to interfere with each other and
form light and dark bands called an interference pattern. Thus, adding an
image of the test cell (with heater) to an image of the control cell (no
heater) produces interference patterns that depict any change in the test
cell.
This kind of interferometer was invented by astronomer A. A. Michelson in
1881 to prove that light is carried through space by a fluid called ether
(by 1887, Michelson and E. W. Morley wound up proving that ether does not
exist). Since then, Michelson interferometers have been used in countless
experiments requiring measurements so sensitive that light itself must be
the yardstick.
And that's how Chuck worked inside STABLE aboard the STS-73 Space Shuttle
mission in October 1995. On Earth, when the heater was turned on the warm
fluid would float to the top, as shown
in the interferogram at the top of this page. In space, warm fluid would
shift (float is not exactly correct) in response to any residual accelerations
that got through while STABLE was on or off. Chuck produced an image that
can be translated into measurements of the vibration.
Even the pictures themselves clearly show a difference between tests on
Earth -- a stream of warm liquid rises to the top -- and in space -- the
warm fluid just forms a bubble within the colder fluid.
Computer
simulations depict the differences in fluid flow on the ground and in space.
Click for an explanation and a larger image.
Results from the mission are under study, but scientists are pleased with
Chuck's performance. It was the first experiment focused on how heat moves
through a fluid under extremely low vibrations, and it provided a benchmark
for assessing math models used in materials experiments.
More details on Chuck are available in a recent paper:
Ramachandran, N., Baugher, C. R., Rogers, J., Peters, P., Roark, W., and
Pearcy, G. "Thermal diffusion experiment Chuck - Payload of STABLE."
Proceedings
of SPIE Conference on Space Processing of Materials, Denver, Aug.
4-9, 1996. , Ed. N.Ramachandran. SPIE 2809: 367-378. (Note:
Chuck is paper 2809-49).
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Headlinesreturn to Space Sciences Laboratory Home
Author: Dave
Dooling
Curator: Bryan Walls
NASA Official: John M. Horack