SP-401 Skylab, Classroom in Space

 [76] Part II - Student Experiments Chapter 6: Weight and Weightlessness.

[77] Mass is the quantity of matter in any object. Gravity is a force that tends to draw massive objects together. The magnitude of this force is proportional to the product of their masses. A "gravitational constant" relates this force to the masses of the objects and their separation distance.

The gravitational force between an object and the Earth is called weight, which is a result of the Earth's gravity acting upon the object's mass. In orbit, mass measurement is important for measuring food quantities, changes in weight of astronauts, and the results of scientific experiments. However, an object orbiting Earth effectively counteracts the pull of Earth's gravity. A space station such as Skylab, orbiting 270 miles above Earth, is held in orbit by the equilibrium between Earth's gravity and the "centrifugal force" associated with the space station's path and velocity. Unrestrained objects, then, are for practical purposes as weightless as Skylab itself.

Two of the Skylab student investigations were concerned with mass properties. One proposed a method for determining mass in the weightless space environment, while the other suggested a means of calculating the gravitational constant in that environment.

Mass Measurement

Even though objects in Skylab were apparently weightless, their mass properties were unchanged.

Mass is the quantity of matter. A 1-inch cube of platinum has approximately the same mass as a 2-inch cube of aluminum.

[78] Measurement of mass is therefore an acceptable alternative to measurement of weight. An experiment to measure mass was proposed by Vincent B. Converse of Harlem High School, Rockford, III. His idea was to measure the periods of oscillation of a spring mass system to which various masses were attached.

The experiment consisted of a cantilevered beam, firmly attached to a frequency counter but free to vibrate, six test masses, a motion picture camera, and a data table. The table listed results of a theoretical calculation, against which the measurements made in Skylab could be compared.

The masses were attached to the free end of the beam, one at a time. The weighted beam was deflected from its rest position, storing energy in it, which acted as a spring. When the end of the beam was released' the stored energy caused it to vibrate. A strain-gauge sensed the oscillation and provided a signal to a frequency counter, which had a visible readout of the vibration period in seconds. The camera was programed to operate at 24 frames per second for 50 oscillations after the initial beam deflection, and to photograph the readout.

On August 27, Owen K. Garriott, scientist pilot on the second mission, operated Converse's experiment with excellent results. Oscillation periods for five different masses were obtained, and the entire operation was recorded on 16-mm movie film and on videotape. Each mass was calculated from the frequency of the beam oscillations, which were then compared with the ground-based calculations. There was a 3- to 4-percent difference between the ground-based data and the flight-measured data, but it could be attributed to an inexact knowledge of the beam's physical properties. After completing his final report, Converse recommended the use of a centrifugal device for the measurement of loose material while in zero gravity.

The Skylab devices that measured the mass of the astronauts, residual food, and other objects....

 . Vincent B. Converse proposed an experiment aboard Skylab to measure mass in a weightless environment. He is shown, above right, with his science adviser Robert Head checking out equipment used in his investigation.

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The experiment developed by Converse used a simple but precision scientific instrument.

Scientist Pilot Owen K. Garriott performed Converse's experiment during the second Skylab mission by attaching masses to the end of its cantilevered beam and deflecting it so the beam could oscillate freely.

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 . Left: James E. Healy worked with NASA engineers to determine the long-term effects of astronauts' movements within a space vehicle upon its dynamics. Right: Healy discusses his experiment with his science adviser Harry Coons.

....operated on the same spring-mass oscillation principle. These devices provided accurate mass measurements of the astronauts' weight, intakes, and body wastes throughout each mission.

Universal Gravity

James Healy, a student at St. Anthony's High School, Bayport, Long Island, N.Y., suggested a repetition in space of an experiment performed on Earth in 1798 by Henry Cavendish, the famous English chemist and physicist. Healy proposed to duplicate Cavendish's experiment to measure the force of gravity between two masses by means of a modified Cavendish balance. But because the forces involved were so very small, such an experiment would have been very difficult to construct and to conduct within the environment of Skylab. Since the huge space station had to maneuver slightly in order to remain in a stable position in orbit, forces would have been created that were greater than those that could have been [81] measured. Thus, Healy's experiment could not be implemented for Skylab.

As an alternative project, he became affiliated with Bruce Conway, a principal investigation for another experiment. Conway, an engineer at NASA's Langley Research Center, had instruments aboard Skylab to measure the effects of various crew movements on the dynamics of the space station. While Conway's analysis of data was to be concerned with the short-term effects of such movements, Healy's analysis would consider the long-term effects. Thus, what he learned would be of assistance to designers of future large manned spacecraft.