HOW do you 鈥渨eigh鈥� something that鈥檚 floating around a spacecraft in zero-g? For astronauts trying to do experiments in orbit, the simple problem of measuring mass is well nigh impossible. Until now, that is. Dutch scientists have developed a set of space scales that gauge the mass of weightless objects simply by shaking them back and forth.
On Earth, an object鈥檚 mass can be deduced from its weight, but this is not possible in the microgravity conditions of orbit. An alternative is to see how much force is needed to accelerate the object at a particular rate. Using Newton鈥檚 second law, F = ma, then allows you to calculate m, the object鈥檚 mass, from F and a, the force and acceleration.
That鈥檚 the principle behind the zero-g scales, developed by Marline Classens and a team of engineers at the Delft University of Technology in the Netherlands. The scales consist of a moving chamber mounted between springs within a rigid frame (see Graphic). The test object is placed in the chamber and a motor moves the frame back and forth over a known distance.
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A computer can then calculate the mass of the object simply by measuring the forces in the springs and the rate of acceleration of the chamber.
The Delft team has already tested the scales on a parabolic flight on a 鈥渧omit comet鈥� aircraft organised by the European Space Agency. The aircraft flies on a parabolic path just like that of a thrown ball, providing a 20-second period of weightless free fall in which to test the scales.
And even on their first test, the scales measured masses accurately to within 2 per cent of their known value. Accuracy was worst with very light masses, because the mass of the chamber was so much higher than the mass being measured. Lighter chambers need to be developed, says Classens. Performing small mass measurements in space is important because a key area of microgravity research involves 鈥渁stroculture鈥� experiments. In these, various nutrient fluids are used to grow, for instance, soya plants, to see which produce soybeans with improved oil, protein or carbohydrate content. Knowing the mass of the beans is useful.
At the moment the scales can only handle small inanimate objects or liquids up to 500 grams in mass. 鈥淭he test chamber moves too fast to put living things in, but with improvements, it may be possible to measure the mass of small animals,鈥� says Classens.
In the past, NASA has used an oscillating device to measure the body mass of astronauts. The scales consisted of a gliding chair attached between two springs, but it gave only very rough approximations.
Classens says the problem with the oscillating device is that the astronauts鈥� internal organs also shake and move around, interfering with the readings. Computer models of human organs that take account of their motion could one day make such measurements more accurate.