Microgravity Teacher's Guide
First, What is Gravity?
What is Microgravity?
How to Use This Material
As opportunities for extended space flight have become available, microgravity research in physical and biological sciences has grown in importance. Using the Space Shuttle and soon the International Space Station, scientists are able to add long term control of gravity's effects to the short list of variables they are to manipulate in their experiments. Although most people are aware of the floating effects of astronauts and things in orbiting spacecraft, few understand what causes microgravity much less how it can be utilized for research.
The purpose of this curriculum supplement guide is to define and explain microgravity and show how microgravity can help us learn about the phenomena of our world. The front section of the guide is designed to provide teachers of science, mathematics, and technology at many levels with a foundation in microgravity science and applications. It begins with background information for the teacher on what microgravity is and how it is created. This is followed with information on the domains of microgravity science research; biotechnology, combustion science, fluid physics, fundamental physics, materials science, and microgravity research geared toward exploration. The background section concludes with a history of microgravity research and the expectations microgravity scientists have for research on the International Space Station.
Following the background information are classroom activities that enable students to experiment with the forces and processes microgravity scientists are investigating today. The activities employ simple and inexpensive materials and apparatus that are widely available in schools. The activities emphasize hands-on involvement, prediction, data collection and interpretation, teamwork, and problem solving. Activity features include objectives, materials and tools lists, management suggestions, assessment ideas, extensions, instructions and illustrations, student work sheets, and student readers. Because many of the activities and demonstrations apply to more than one subject area, a matrix chart relates activities to national standards in science and mathematics and to science process skills.
Finally, the guide concludes with a suggested reading list, NASA educational resources including electronic resources, and an evaluation questionnaire. We would appreciate your assistance in improving this guide in future editions by completing the questionnaire and making suggestions for changes and additions. The evaluation can be sent to us by mail or electronically submitted through the Internet site listed on the form.
Note on Measurement and Standards
In developing this guide, metric units of measurement were employed. In a few exceptions, notably within the "Materials and Tools" lists, British units have been listed. In the United States, metric-sized parts such as screws and wood stock are not as accessible as their British equivalents. Therefore, British units have been used to facilitate obtaining required materials.
Subjects relating to mathematics, physical science, and technology are hypertext links. Definitions, questions for discussion, and examples are provided in these links. These links first list applicable Mathematics and Science Content Standards, indicated by grade level: Grades 5-8, Grades 9-12.
Space flight is important for rnany reasons. Space flight carries scientific instruments and human researchers high above the ground, permitting us to see Earth as a planet and to study the complex interactions of atmosphere, oceans, land, energy, and living things. Space flight lifts scientific instruments above the filtering effects of the atmosphere, making the entire electromagnetic spectrum available and allowing us to see more clearly the distant planets, stars, and galaxies. Space flight permits us to travel directly to other worlds to see them close up and sample their compositions. Finally, space flight allows scientists to investigate the fundamental states of matter--solids, liquids, and gases--and the forces that affect them in a microgravity environment.
The study of the states of matter and their interactions in microgravity is an exciting opportunity to expand the frontiers of science. Areas of investigation include biotechnology, combustion science, fluid physics, fundamental physics, materials science, and ways in which these areas of research can be used to advance efforts to explore the Moon and Mars.
Microgravity is the subject of this teacher's guide. This publication identifies the underlying mathematics, physics, and technology principles that apply to microgravity. Supplementary information is included in other NASA educational products.
Gravitational attraction is a fundamental property of matter that exists throughout the known universe. Physicists identify gravity as one of the four types of forces in the universe. The others are the strong and weak nuclear forces and the electromagnetic force.
More than 300 years ago the great English scientist Sir Isaac Newton published the important generalization that mathematically describes this universal force of gravity. Newton was the first to realize that gravity extends well beyond the domain of Earth. The basis of this realization stems from the first of three laws he formulated to describe the motion of objects. Part of Newton's first law, the law of inertia, states that obiects in motion travel in a straight line at a constant velocity unless acted upon by a net force. According to this law, the planets in space should travel in straight lines. However, as early as the time of Aristotle, scholars knew that the planets travelled on curved paths. Newton reasoned that the closed orbits of the planets are the result of a net force acting upon each of them. That force, he concluded, is the same force that causes an apple to fall to the ground--gravity.
Newton's experimental research into the force of gravity resulted in his elegant mathematical statement that is known today as the Law of Universal Gravitation. According to Newton, every mass in the universe attracts every other mass. The attractive force between any two objects is directly proportional to the product of the two masses being considered and inversely proportional to the square of the distance separating them. If we let F represent this force, r represent the distance between the centers of the masses, and m 1 and m 2 represent the magnitudes of the masses, the relationship stated can be written symbolically as:
From this relationship, we can see that the greater the masses of the attracting objects, the greater the force of attraction between them. We can also see that the farther apart the objects are from each other, the less the attraction. If the distance between the objects doubles, the attraction between them diminishes by a factor of four, and if the distance triples, the attraction is only one-ninth as much.
The eighteenth-century English physicist Henry Cavendish later quantified Newton's Law of Universal Gravitation. He actually measured the gravitational force between two one kilogram masses separated by a distance of one meter. This attraction was an extremely weak force, but its determination permitted the proportional relationship of Newton's law to be converted into an equality. This measurement yielded the universal gravitational constant, G. Cavendish determined that the value of G is 6.67 x 10 -11N m 2/ kg 2. With G added to make the equation, the Law of Universal Gravitation becomes:
The presence of Earth creates a gravitational field that acts to attract objects with a force inversely proportional to the square of the distance between the center of the object and the center of Earth. When we measure the acceleration of an object acted upon only by Earth's gravity at the Earth's surface, we commonly refer to it as one g or one Earth gravity. This acceleration is approximately 9.8 meters per second squared (m/ s 2). The mass of an object describes how much the object accelerates under a given force. The weight of an object is the gravitational force exerted on it by Earth. In British units (commonly used in the United States), force is given in units of pounds. The British unit of mass corresponding to one pound force is the slug.
While the mass of an object is constant and the weight of an object is constant (ignoring differences in g at different locations on the Earth's surface), the environment of an object may be changed in such a way that its apparent weight changes. Imagine standing on a scale in a stationary elevator car. Any vertical accelerations of the elevator are considered to be positive upwards. Your weight, W, is determined by your mass and the acceleration due to gravity at your location.
If you begin a ride to the top floor of a building, an additional force comes into play due to the acceleration of the elevator. The force that the floor exerts on you is your apparent weight, P, the magnitude of which the scale will register. The total force acting on you is F=W+P=mae, where ae, is the acceleration of you and the elevator and W=mg. Two example calculations of apparent weight are given in the notes . Note that if the elevator is not accelerating then the magnitudes W and P are equal but the direction in which those forces act are opposite (W=-P). Remember that the sign (positive or negative) associated with a vector quantity, such as force, is an indication of the direction in which the vector acts or points, with respect to a defined frame of reference. For the reference frame defined above, your weight in the example in the margin is negative because it is the result of an acceleration (gravity) directed downwards (towards Earth).
Imagine now riding in the elevator to the top floor of a very tall building. At the top, the cables supporting the car break, causing the car and you to fall towards the ground. In this example, we discount the effects of air friction and elevator safety mechanisms on the falling car. Your apparent weight P=m(ae-g). =(60 kg)(-9.8 m/s 2 -(- 9.8 m/s2)) = 0 kg m/s2; you are weightless. The elevator car, the scale, and you would all be accelerating downward at the same rate, which is due to gravity alone. If you lifted your feet off the elevator floor, you would float inside the car. This is the same experiment that Galileo is purported to have performed at Pisa, Italy, when he dropped a cannonball and a musketball of different mass at the same time from the same height. Both balls hit the ground at the same time, just as the elevator car, the scale, and you would reach the ground at the same time.
For reasons that are discussed later, there are many advantages to performing scientific experiments under conditions where the apparent weight of the experiment system is reduced. The name given to such a research environment is microgravity. The prefix micro- (m) derives from the original Greek mikros meaning small. By this definition, a microgravity environment is one in which the apparent weight of a system is small compared to its actual weight due to gravity. As we describe how microgravity environments can be produced, bear in mind that many factors contribute to the experienced accelerations and that the quality of the microgravity environment depends on the mechanism used to create it. In practice, the microgravity environments used by scientific researchers range from about one percent of Earth's gravitational acceleration (aboard aircraft in parabolic flight) to better than one part in a million (for example, onboard Earth-orbiting research satellites).
Quantitative systems of measurement, such as the metric system, commonly use micro- to mean one part in a million. Using that definition, the acceleration experienced by an object in a microgravity environment would be one-millionth (10 -6 ) of that experienced at Earth's surface. The use of the term microgravity in this guide will correspond to the first definition. For illustrative purposes only, we provide the following simple example using the quantitative definition. This example attempts to provide insight into what might be expected if the local acceleration environment would be reduced by six orders of magnitude from 1 g to 10-6 g.
If you dropped a rock from a roof that was five meters high, it would take just one second to reach the ground. In a reduced gravity environment with one percent of Earth's gravitational pull, the same drop would take 10 seconds. In a microgravity environment equal to one-millionth of Earth's gravitational pull, the same drop would take 1,000 seconds or about 17 minutes!
Researchers can create microgravity conditions in two ways. Because gravitational pull diminishes with distance, one way to create a microgravity environment (following the quantitative definition) is to travel away from Earth. To reach a point where Earth's gravitational pull is reduced to one-millionth of that at the surface, you would have to travel into space a distance of 6.37 million kilometers from Earth (almost 17 times farther away than the Moon, 1400 times the highway distance between New York City and Los Angeles, or about 70 million football fields). This approach is impractical, except for automated spacecraft, because humans have yet to travel farther away from Earth than the distance to the Moon. However, freefall can be used to create a microgravity environment consistent with our primary definition of microgravity. We discuss this in the next section.
As illustrated in the elevator examples in the previous section, the effects of gravity (apparent weight) can be removed quite easily by putting anything (a person, an object, an experiment) into a state of freefall. This possibility of using Earth's gravity to remove the effects of gravity within a system were not always evident. Albert Einstein once said, "I was sitting in a chair in the patent office at Bern when all of a sudden a thought occurred to me: 'If a person falls freely, he will not feel his own weight.' I was startled. This simple thought made a deep impression on me. It impelled me toward a theory of gravitation." Working with this knowledge, scientists involved in early space flights rapidly concluded that micro-gravity experiments could be performed by crew members while in orbit.
The use of orbiting spacecraft is one method used by NASA to create microgravity conditions. In addition, other methods of creating such conditions are introduced here and we give examples of situations where the student can experience microgravity.
Researchers use high-tech facilities based on the elevator analogy to create micro-gravity conditions. The NASA Lewis Research Center has two drop facilities. One provides a 132 meter drop into a hole in the ground similar to a mine shaft. This drop creates a reduced gravity environment for 5.2 seconds. A tower at Lewis allows for 2.2 second drops down a 24 meter structure. The NASA Marshall Space Flight Center has a different type of reduced gravity facility. This 100 meter tube allows for drops of 4.5 second duration. Other NASA Field Centers and other countries have additional drop facilities of varying sizes to serve different purposes. The longest drop time currently available (about 10 seconds) is at a 490 meter deep vertical mine shaft in Japan that has been converted to a drop facility. Sensations similar to those resulting from a drop in these reduced gravity facilities can be experienced on freefall rides in amusement parks or when stepping off of diving platforms.
Airplanes are used to achieve reduced gravity conditions for periods of about 15 seconds. This environment is created as the plane flies on a parabolic path. A typical flight lasts two to three hours allowing experiments and crew members to take advantage of about forty periods of microgravity. To accomplish this, the plane climbs rapidly at a 45 degree angle (this phase is called pull up), traces a parabola (pushover), and then descends at a 45 degree angle (pull out). During the pull up and pull out segments, crew and experiments experience accelerations of about 2 g. During the parabola, net accelerations drop as low as 1.5x10-2 g for about 15 seconds. Due to the experiences of many who have flown on parabolic aircraft, the planes are often referred to as "Vomit Comets." Reduced gravity conditions created by the same type of parabolic motion described above can be experienced on the series of "floater" hills that are usually located at the end of roller coaster rides and when driving over swells in the road.
Sounding rockets are used to create reduced gravity conditions for several minutes; they follow suborbital, parabolic paths. Freefall exists during the rocket's coast: after burn out and before entering the atmosphere. Acceleration levels are usually around 10-5 g. While most people do not get the opportunity to experience the accelerations of a rocket launch and subsequent freefall, springboard divers basically launch themselves into the air when performing dives and they experience microgravity conditions until they enter the water.
Although drop facilities, airplanes, and rockets can establish a reduced gravity environment, all these facilities share a common problem. After a few seconds or minutes, Earth gets in the way and freefall stops. To conduct longer scientific investigations, another type of freefall is needed.
To see how it is possible to establish microgravity conditions for long periods of time, one must first 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, little difference exists between a spacecraft with its engines constantly firing and an airplane flying around the world. A satellite could not carry enough fuel to maintain its altitude for more than a few minutes. The second answer is also wrong. At the altitude that the Space Shuttle typically orbits Earth, the gravitational pull on the Shuttle by Earth is about 90% of what it is at Earth's surface.
In a previous section, we indicated that Issac Newton reasoned that the closed orbits of the planets through space were due to gravity's presence. 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. Two forces acted upon each cannonball as it was fired. One force, due to the explosion of the black powder, propelled the cannonball straight outward. If no other force were to act on the cannonball, 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: gravity would cause the path of the cannonball to bend into an arc ending at Earth's surface.
Newton considered how additional cannonballs would travel farther from the mountain each time the cannon fired using more black powder. With each shot, the path would lengthen and soon the cannonballs would disappear over the horizon. Eventually, if one fired a cannon with enough energy, the cannonball would fall entirely around Earth and come back to its starting point. The cannonball would be in orbit around Earth. Provided no force other than gravity interfered with the cannonball's motion, it would continue circling Earth in that orbit.
This is how the Space Shuttle stays in orbit. It launches on a path that arcs above Earth so that the Orbiter travels at the right speed to keep it falling while maintaining a constant altitude above the surface. For example, if the Shuttle climbs to a 320 kilometer high orbit, it must travel at a speed of about 27,740 kilometers per hour to achieve a stable orbit. At that speed and altitude, the Shuttle executes a falling path parallel to the curvature of Earth. Because the Space Shuttle is in a state of freefall around Earth and due to the extremely low friction of the upper atmosphere, the Shuttle and its contents are in a high-quality microgravity environment.