Contributed editorial appearing in
Scientific Computing & Instrumentation 19:3, February 2002, pg. 16.
On a recent family snow-tubing trip, my kindergartner asked about the snow making machines. After I acknowledged them as such, he proudly stated, “Oh yeah. That’s what they use to crunch up ice cubes and shoot onto the grass so we can have snow.” I modified his description a bit by saying it was much easier to spray liquid water into the cold night air and let it freeze into snowflakes as it falls to the ground. He then concluded it would also make the snow “much fluffier.” Exactly.Snow is one of many processes made possible by the physics of free- fall. While permitted to change velocity at or near the acceleration of gravity, processes such as crystal growth occur much differently than when held in a stationary frame of reference. Though often used to imply “small” as in “microscope,” the “micro-” prefix in “microgravity” is increasingly being used in its true engineering form to mean an effective force due to the acceleration within a gravitational field that is one one-millionth of “g” (9.8 m/s/s). Under conditions of micro-gravity, the effect of buoyancy-driven convection in fluids is minimized and permits processes such as diffusion and surface tension to dominate the system. Semiconductor crystals can have fewer defects and superior electrical properties when harvested in microgravity. Protein structure elucidation using x-ray diffraction can be obtained by growing protein crystals in microgravity when convection and sedimentation prevent crystal growth under normal laboratory conditions. Combustion is markedly different in microgravity and is the subject of intense study yielding advances in practical applications and developments in combustion theory.
The least-expensive way to study a system under conditions of one of the reduced force of gravity is to drop it and study it while it plummets to the earth. The major drawback is that inevitably the earth gets in the way. For each second of free fall, 9.8 m/s/s of breaking acceleration must be applied to the falling object over the span of a few hundred milliseconds to bring it back to rest. The experimental payload in the 2.2-second drop tower at the NASA Glenn Research Center experiences an acceleration of 25 g’s (245 m/s/s) as it comes to rest. In NASA-Glenn’s Dropping in Microgravity Environment (DIME) program for university students, an 11.4-kg payload is interfaced to a Valitec, Inc. AD2000 ReadyDAQ stand-alone data acquisition system. This data logger is powered by a 9-V alkaline battery and acquires data from eight analog and three digital input channels at a resolution of 8-bits and at a maximum rate of 500 Hz. The values are stored in 2 Mbytes of internal memory and downloaded via an RS-232 serial port after the drop. The ReadyDAQ, a backlight, and a video acquisition system go along for the 2.2-second ride in an experimental rig that is encased in a drag-reducing shield. All of the components must withstand the 25-g acceleration at the conclusion of the drop.
When the Center of Applied Space Technology and Microgravity (ZARM) at the University of Brernen approached the excimer laser manufacturer, Lambda Physik, in the mid 1990s about using their lasers to excite a laser-induced fluorescence (LIF) visualization system in their 4.7-second drop tower, it was obvious that the laser system would not survive the 30-gs experienced at the end of the drop. Instead, the excimer laser was mounted at the top of the drop tower and only the 256x256 pixel CCD detector and optics went along for the ride. Even though the rig was not tethered during the drop, an exactly straight drop through the 120-m evacuated drop shaft could not be obtained due to the Coriolis force from to the earth’s rotation and small vibrations transferred through the release mechanism. The excimer beam was carefully aligned through an aperture in the top of the drop rig onto fast-response beam- steering optics that compensated for movements every 10 milliseconds while a total of 250 images were acquired and stored in RAM during the drop.
To avoid the abrupt acceleration and extend the duration of the microgravity conditions, experimenters also can utilize the NASA KC-135 aircraft that flies in a parabolic trajectory providing 20-25 seconds of reduced gravity. The NASA Space Shuttle can provide microgravity for greater than two weeks, while NASA Spartan Satellites can support unattended experiments for up to 12 months. The International Space Station (ISS) is an excellent venue for long-duration microgravity experiments. The Space Acceleration Measurement System-II (SAMS-II) and Microgravity Acceleration Measurement System (MAMS) aboard the United States Laboratory Module measure the exact acceleration experienced by experiments from high-frequency vibrations generated by the crew and mechanical equipment and from low-frequency acceleration due to aerodynamic drag, vehicle rotation, and station venting. Hopefully these experiments will elucidate the chaos mathematics responsible for crystal growth so that I’m ready for the time when my son asks why all snowflakes are different.
