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What's Shaking in Space?

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What's Shaking in Space?

Science on Flight Day 15 of MSL-1

return to MSL-1 science home pageJuly 15, 1997 10 a.m. CDT

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MMA logo - animatedAnyone who has leaned to stay upright while taking a turn in a car has imitated part of how engineers are measuring the quality of the ride that experiments are getting aboard the Microgravity Sciences Laboratory 1 (MSL-1) mission.

"The force that you have to exert to keep yourself upright is equivalent to the force that pushes you off center," said Andreas Schutte, an engineer with Daimler-Benz Aerospace AG in Bremen, Germany.

Schutte is part of the Microgravity Measurement Assembly (MMA) team, one of four teams that have equipment aboard Space Shuttle Columbia to measure vibrations in the spacecraft. (The team logo, above, left, illustrates the operation of the microgravity measurement instruments.)

The work does not get as much attention as the other experiments aboard MSL-1 - no flame balls or bouncing samples of molten gold - but it is vitally important to ensuring that the other experimenters can do their work. NASA's Lewis Research Center supplied three of the facilities aboard Columbia: the Orbital Acceleration Research Experiment (OARE), Quasi-Steady Acceleration Measurement (QSAM), and the Space Acceleration Measurement System (SAMS); the MMA is provided by the European Space Agency (ESA). Dr. Maurizio Nati of the European Space Technology and Engineering Center (ESTEC) in Noordwijk, The Netherlands, is the project manager.

While everyone is familiar with astronauts floating in space, few people realize that they are not really in zero gravity.

"The first thing we should do is explain that gravity and acceleration are the same things," said Giancarlo Scaramelli, senior systems engineer with Vitrociset, a company under contract to ESA.

"Any time the Shuttle moves or vibrates, it creates a small gravity or acceleration, There's no absolute zero-g, but there's very, very, very small gravity."

The Shuttle and its crew and payload are still influenced by Earth's gravity, otherwise they would fly away. What they experience is variously called weightlessness, free fall, or microgravity. Everything in the Shuttle is falling together, endlessly, around the world, so the effect is like gravity being canceled. (read more about free fall in our Microgravity Primer!)

Well, almost canceled. The truth is, the equipment it takes to operate the Shuttle and Spacelab, and the crew itself, causes small accelerations which affect the experiments.

MSL-1 Rack ConfigurationIf you are growing a crystal or lighting off a droplet of fuel, then you need to know if one of those accelerations has affected your results. On the Life and Microgravity Sciences (LMS) mission flown in 1996, for example, scientists running a bubble dynamics experiment saw the bubble fly off the tip of its needle.

"The scientists were a bit puzzled by this," Schutte said. "They called us and asked us for our data and we were able to tell them that 'One minute ago there was a crew impact near this facility,' so they were able to continue their experiments."

On MSL-1, the MMA team has helped the TEMPUS electromagnetic furnace and the Droplet Combustion Experiment teams. (The diagram above, right shows the locations of the instruments in the Spacelab module.)

"On several occasions the TEMPUS team had very delicate samples and we measured the accelerations for them, Scaramelli said. "At least once they halted their experiment and waited for a better time" after DCE had finished some work. (3D rendering at left shows the MMA instrument. Click image for a 111KB jpeg.)

The principle behind measuring acceleration aboard the Shuttle traces all the way back to Sir Isaac Newton's second law of motion: a body at rest (or in motion) tends to stay at rest (or in motion) unless acted upon by an outside force.

(It should be noted that the accelerometer on your car is really a speedometer. Acceleration is the rate at which speed changes, and that is what affects materials science experiments. 60 mph is a measure of speed. 60 mph or 17,500 mph, the Shuttle's orbital speed, is fine. 0 to 60 in 5 seconds is acceleration and that, or smaller changes, is what causes materials to shift in their containers.)

Both the spatial 3-axis electrostatic accelerometer (ASTRE, in French) and the microgravity sensor package are based on this, although they use it in different ways.

ASTRE is the one that follows the example of a person leaning in the car.

A gold-coated "proof mass," about the size of an artist's eraser (1x4x4 cm) is floated inside a chamber lined with contacts that generate a static electric charge. The electrodes repel the proof mass so the mass stays centered in the chamber. If the Shuttle moves, the chamber moves so the mass is closer to one wall then the other. ASTRE's electronics measure the change in position and adjust the electrostatic field to keep the mass centered.

In effect, the force needed to center the mass is equal to the force trying to offset it, just as you can feel the sharpness of the car's turn by how much force you need to stay upright.

"If you measure one and change the sign [reverse the direction], you know the acceleration," Schutte said.

The microgravity sensor package uses a different approach. It has three sensors (one for each dimension) built on silicon chips by the same technology used in photo-etching computer chips. A small arm, with a weight on the tip, extends across an empty space on the chip. If the chip is moved sideways the arm flexes. Capacitors on each side of the empty space measure the change in position. Three sensors are needed, mounted at right angles to each other, to measure full movement.

With both accelerometers, and with the NASA-Lewis accelerometers, if there is no motion, there is no reading. But, there's always motion.

Data plots (like those above, updated in real-time, online at Kennedy Space Center) shown by Scaramelli and Schutte show what appears to be seismograph plot. These movements would not measure on the Richter scale, but many experiments are sensitive to them: refrigerator pumps cycling off and on, crew members exercising on the bicycle ergometer, valves opening and closing, and so on. That also means that most of the accelerations are not steady, like a car picking up speed, but have cycles, like a motor spinning.

Most stay below 0.000001 g, or 1/100,000 the Earth's gravity. Some go above, and experiments have to be scheduled to avoid interference.

TEMPUS, for example, will resonate 4.5 times per second (4.5 Hertz). Most other vibrations dampen out, but activities like crew exercise that generate 4.5 Hertz noise will echo in the TEMPUS furnace and cause problems.

"There are many factors involved" in the sensitivity of each facility, Scaramelli said.

Even if the Shuttle could be completely dampened and made perfectly quiet, some acceleration would remain. One of the long-term goals of ASTRE, Scaramelli explained, is to measure the so-called quasi-static component, accelerations that are always there.

These are caused by the thin traces of the Earth's atmosphere dragging on the Shuttle, which direction the Shuttle is pointing, and even the gravitational pulls of the sun and moon (the same pulls that produce the tides on Earth).

"The signal is there," Schutte said, pointing to one of the data plots, "but it's not easy to visualize. We're looking for the right filter [computer code to remove noise] to see it."


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Author: Dave Dooling
Curator: Bryan Walls
NASA Official: John M. Horack