Creating Microgravity

Drop Towers and Tubes

Drop towers accommodate large experiment packages, generally using a drop shield to contain the package and isolate the experiment from aerodynamic drag during free fall in the open environment. NASA's Glenn Research Center in Cleveland, Ohio has a 145-meter drop tower facility that begins on the surface and descends into Earth like a mine shaft. The test section of the facility is 6.1 meters in diameter and 132 meters deep. Beneath the test section is a catch basin filled with polystyrene beads. The 132-meter drop creates a microgravity environment for a period of 5.2 seconds. To begin a drop experiment, the experiment apparatus is placed in either a cylindrical or rectangular test vehicle that can carry experiment loads of up to 450 kilograms. The vehicle is suspended from a cap that encloses the upper end of the facility. Air is pumped out of the facility until a vacuum of 10 -2 torr is achieved. (Atmospheric pressure is 760 torr.) By doing so, the acceleration effects caused by aerodynamic drag on the vehicle are reduced to less than 10 -5 g. During the drop, cameras within the vehicle record the action and data is telemetered to recorders.

A smaller facility for microgravity research is located at the NASA Marshall Space Flight Center in Huntsville, Alabama. It is a 100-meter-high, 25.4-centimeter-diameter evacuated drop tube that can achieve microgravity for periods of as long as 4.5 seconds. The upper end of the tube is fitted with a stainless steel bell jar. For solidification experiments, an electron bombardment or an electromagnetic levitator furnace is mounted inside the jar to melt the test samples. After the sample melts, drops are formed and fall through the tube to a detachable catch fixture at the bottom of the tube (Figure 2). Additional drop facilities of different sizes and for different purposes are located at the NASA Field Centers and in other countries. A 490-meter-deep vertical mine shaft in Japan has been converted to a drop facility that can achieve a 10 -5 g environment for up to 11.7 seconds.

Aircraft

A typical flight lasts 2 to 3 hours and carries experiments and crewmembers to a beginning altitude about 7 km above sea level. The plane climbs rapidly at a 45-degree angle (pull up), traces a parabola (push-over), and then descends at a 45-degree angle (pull out) (Figure 4). During the pull up and pull out segments, crew and experiments experience between 2g and 2.5g. During the parabola, at altitudes ranging from 7.3 to 10.4 kilometers, net acceleration drops as low as 10 -3 g. On a typical flight, 40 parabolic trajectories are flown. The gut-wrenching sensations have earned the aircraft the nickname of "vomit comet." NASA also operates a Learjet for low-gravity research out of the NASA Glenn Research Center. Flying on a trajectory similar to the one followed by the KC-135, the Learjet provides a low-acceleration environment of 5x10 -2 g to 75x10 -2 g for up to 20 seconds.

Rockets

Small rockets provide a third technology for creating microgravity. A sounding rocket follows a suborbital trajectory and can produce several minutes of free fall. The period of free fall exists during its coast, after burn out, and before entering the atmosphere. Acceleration levels are usually at or below 10 -5 g. NASA has employed many different sounding rockets for microgravity experiments. The most comprehensive series of launches used SPAR (Space Processing Application Rocket) rockets for fluid physics, capillarity, liquid diffusion, containerless processing, and electrolysis experiments from 1975 to 1981. The SPAR could lift 300 kg payloads into freefall parabolic trajectories lasting four to six minutes (Figures 5, 6).

Orbiting Spacecraft

Although airplanes, drop facilities, and small rockets can be used to establish a microgravity environment, all of these laboratories share a common problem. After a few seconds or minutes of lowg, Earth gets in the way and the free fall stops. In spite of this limitation, much can be learned about fluid dynamics and mixing, liquid-gas surface interactions, and crystallization and macromolecular structure. But to conduct longer term experiments (days, weeks, months, and years), it is necessary to travel into space and orbit Earth. Having more time available for experiments means that slower processes and more subtle effects can be investigated.

To see how it is possible to establish microgravity conditions for long periods of time, it is first necessary to understand what keeps a spacecraft in orbit. Ask any group of students or adults what keeps satellites and Space Shuttles in orbit and you will probably get a variety of answers. Two common answers are: "The rocket engines keep firing to hold it up." and "There is no gravity in space."

Although the first answer is theoretically possible, the path followed by the spacecraft would technically not be an orbit. Other than the altitude involved and the specific means of exerting an upward force, there would be little difference between a spacecraft with its engines constantly firing and an airplane flying around the world. In the case of the satellite, it would just not be possible to provide it with enough fuel to maintain its altitude for more than a few minutes.

The second answer is also wrong. In a previous section, we discussed that Isaac Newton proved that the circular paths of the planets through space was due to gravity's presence, not its absence.

Newton expanded on his conclusions about gravity and hypothesized how an artificial satellite could be made to orbit Earth. He envisioned a very tall mountain extending above Earth's atmosphere so that friction with the air would not be a factor. He then imagined a cannon at the top of that mountain firing cannonballs parallel to the ground. As each cannonball was fired, it was acted upon by two forces. One force, the explosion of the black powder, propelled the cannonball straight outward. If no other force were to act on the cannon ball, the shot would travel in a straight line and at a constant velocity. But Newton knew that a second force would act on the cannonball: the presence of gravity would cause the path of the cannonball to bend into an arc ending at Earth's surface (Figure 7).

Newton demonstrated how additional cannonballs would travel farther from the mountain if the cannon were loaded with more black powder each time it was fired. With each shot, the path would lengthen and soon, the cannonballs would disappear over the horizon. Eventually, if a cannonball were fired with enough energy, it would fall entirely around Earth and come back to its starting point. The cannonball would begin to orbit Earth. Provided no force other than gravity interfered with the cannonball's motion, it would continue circling Earth in that orbit.

Orbiting spacecraft provide ideal laboratories for microgravity research. As on airplanes, scientists can fly with the experiments that are on the spacecraft. Because the experiments are tended, they do not have to be fully automatic in operation. A malfunction in an experiment conducted with a drop tower or small rocket means a loss of data or complete failure. In orbiting spacecraft, crewmembers can make repairs so that there is little or no loss of data. They can also make on-orbit modifications in experiments to gather more diverse data.

Perhaps the greatest advantage of orbiting spacecraft for microgravity research is the amount of time during which microgravity conditions can be achieved. Experiments lasting for more than two weeks are possible with the Space Shuttle. When the International Space Station becomes operational, the time available for experiments will stretch to months. The International Space Station will provide a manned microgravity laboratory facility unrivaled by any on Earth (Figure 8).