Microgravity Science Standards Science Primer - NASA · 2013-06-27 · Microgravity Science Primer ... of solids, liquids, ... composition. Such intermingling occurs in many situations,

Microgravity — A Teacher’s Guide with Activities in Science, Mathematics, and Technology,EG-1997-08-110-HQ, Education Standards Grades 5–8 (∆), 9–12 (❏)

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MicrogravityScience PrimerWe experience many manifestations of gravity ona day to day basis. If we drop something, it fallstoward Earth. If we release a rock in a container ofwater, the rock settles to the bottom of the con-tainer. We experience other effects of gravityregularly, although we may not think of gravity asplaying a role.

Consider what happens when a container of wateris heated from below. As the water on the bottomis heated by conduction through the container, itbecomes less dense than the un-heated, coolerwater. Because of gravity, the cooler, more densewater sinks to the bottom of the container and theheated water rises to the top due to buoyancy. Acirculation pattern is produced that mixes the hotwater with the colder water. This is an example ofbuoyancy driven (or gravity driven) convection.The convection causes the water to be heatedmore quickly and uniformly than if it were heatedby conduction alone. This is the same densitydriven convection process to which we refer whenwe state matter-of-factly that”hot air rises.”

In addition to mixing, density differences can alsocause things to differentially settle through aprocess called sedimentation. In this process, themore dense components of mixtures ofimmiscible fluids or solid particles in fluids sett’eto the bottom of a container due to gravity. If youfill a bucket with very wet mud, and then leave thebucket sitting on the ground, over time the moredense soil particles will sink to the bottom of thebucket due to gravity, leaving a layer of water ontop. When you pick up a bottle of Italian saladdressing from the grocery store shelf, you seeseveral different layers in the bottle. The densesolids have settled to the bottom, the vinegarforms a middle layer, and the least dense oil is ontop.

Science Standards

∆ o Physical Science∆ o Unifying Concepts and Processes

Heat transfer occurs through one of three processes or acombination of the three. Conduction is the flow of heat througha body from an area of higher temperature to an area of lowertemperature. Molecules in the hot region increase theirvibrational energy as they are heated. As they collide withmolecules with lower vibrational energy (cooler ones). some ofthe vibrational energy is passed to the cooler ones, their energy isincrcased. and heat is passed on.

Heat transfer by convection is the movement of heat by motionof a fluid. This motion can he the result of some force, such as apump circulating heated water. and is referred to as forcedconvection. If the motion is the result of difterences in density(thermal or compositional). the convection is referred to asbuoyancy-driven, density-driven. or natural convection.

Radiation is the emission of energy trom the surface of a body.Energy is emitted in the form of electromagnetic waves orphotons (packets of light). The character (wavelength. energy ofphotons, etc.) of the radiation depends on the temperature.surface area. and characteristics of the body emitting the energy.Electromagnetic waves travel with the speed of light throughempty space and are absorbed (and/or reflected) by objects theyfall on, thus transferring heat. An excellent example of radiativeheating is the sun’s heat that we experience on Earth.

Mathemathics Standards

∆ o Mathematical Conneclions

Science Standards

∆ o Earth and Space Science∆ o Physical Science∆ o Unifying Concepts and Processes

The mass of a body divided hy its volume is its average density.

Science Standards


When two or more liquids are immiscible they do not mixchemically.


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Density Table Units (kg/m3)

Interstellar space 10-21 - 10-18

Atmosphere at normal altitudeof Space Shuttle in orbit 1-4x10-11

Air at 0°C and 1 atm 1.3Carbon Dioxide 1.9Balsa 110-140Bone 170-200Cork 220-260The Larch 500-560Lithium 530Applewood 660-840Peat Blocks 840Ice 920Olive Oil 920Sodium 970Water at 0°C and 1 atm 1000Rock Salt 2180Graphite 2300-2700Alunıinum 2700Basalt 2400-3100Talc 2700-2800Dolomite 2830Diamond 3010-3520Average density of Earth 5520Iron 7860Lead 11340Irdium 22400Osmium 22500Uranium nucleus 3x1017

Neutron star (center) 1017-1018

Mathematics Standards

∆ o Algebrao Functions

∆ Geometryo Geometry from a Synthetic Perspective

∆ o Mathematical Connections∆ o Mathematics as Communication∆ o Mathematics as Problem Solving∆ Measurement

o Trigonometry

Science Standards

∆ o Physical Science∆ o Science and Technology∆ o Science in Personal and Social Perspectives∆ o Unifying Concepts and Processes

Gravity can also mask some phenomena thatscientists wish to study. An example is theprocess of diffusion. Diffusion is the interminglingof solids, liquids, and gases due to differences incomposition. Such intermingling occurs in manysituations, but diffusion effects can be easilyhidden by stronger convective mixing. As anexample, imagine a large room in which all aircirculation systems are turned off and in which agroup of women are spaced ten feet apartstanding in a line. If an open container ofammonia were placed in front of the first womanin line and each woman raised her hand when shesmelled the ammonia, it would take aconsiderable amount of time before everyoneraised her hand. Also, the hand raising wouldoccur sequentially along the line from closest tothe ammonia to furthest from the ammonia. If thesame experiment were performed with a fancirculating air in the room, the hands would beraised more quickly, and not necessarily in thesame order. In the latter case, mixing of theammonia gas with the air in the room is due toboth diffusion and convection (forced convectiondue to the fan) and the effects of the twoprocesses cannot be easily separated. In a similarmanner, buoyancy driven convection can maskdiffusive mixing of components in scientificexperiments.

Some behavior of liquids can also be masked bygravity. If you pour a liquid into a container onEarth, the liquid conforms to the bottom of thecontainer due to gravity. Depending on the shapeof the container and on the properties of thecontainer and the liquid, some of the liquid maycreep up the walls or become depressed alongthe walls due to the interrelated phenomena ofsurface tension, adhesion, cohesion, andcapillarity.

The resulting curved surface may be familiar toanyone who has measured water in a smalldiameter glass container (the water cupsupward) or has looked at the level of mercury in aglass thermometer (the mercury cupsdownward). The distance the contact


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line between the liquid and the container movesup or down the container wall is affected bygravity.

Experiments performed on Earth often takeadvantage of the effects of gravity discussed. Formany experiments, however, these effects tend tomake the execution of experiments or the analysisof experimental results difficult and sometimeseven impossible. Therefore, many researchersdesign experiments to be performed undermicrogravity conditions. The different scientificresearch areas that are studied in microgravityinclude biotechnology, combustion science, fluidphysics, fundamental physics, and materialsscience. Each of these areas, or disciplines, isdiscussed below. The discipline is defined, someof the specific effects of gravity that illustrate thebenefits of microgravity research are discussed,and some examples of current research arepresented. In addition, a brief discussion of themicrogravity environment of orbiting spacecraft isprovided as is an introduction to the applicationof microgravity research to the exploration anddevelopment of space.

The MicrogravityEnvironment ofOrbiting SpacecraftWhile freefall reduces the effects of gravity, beingin an orbiting laboratory introduces otheraccelerations that cause effects that areindistinguishable from those due to gravity. Whena spacecraft is in orbit around Earth, the orbit isactually defined by the path of the center of massof the spacecraft around the center of Earth. Anyobject in a location other than on the linetraversed by the center of mass of the spacecraftis actually in a different orbit around Earth.Because of this, all objects not attached to thespacecraft move relative to the orbiter center ofmass. Other relative motions of unattachedobjects are related to aerodynamic drag on the

Capillarity can be defined as the attraction a fluid has for itselfversus the attraction it has for a solid surface (usually the fluid’scontainer). Thc surface tension σ in a liquid-liquid or liquid-gassystem is the fluids’ tendency to resist an increase in surface area.Surface tension is temperature dependent. Surface tension,capillarity, adhesion, and cohesion work together to drive thecontact angle θ between a solid-liquid interface and liquid-liquidinterface when a small diameter tube is dipped into a liquid.When the contact angle θ=0, the liquid “wets” the tubccompletely. When θ<90° (an acute angle), the liquid rises in thetube; when θ>90° (an obtuse angle). the liquid is depressed in thetube and does not wet the walls. The distance belween the liquidsuri’ace in the container and in the tube is h=2<σcosθ/rρg wherer is the radius of the tube (D/2), ρ is the density of the liquid, andg is the acceleration due to gravity.


o Functions∆ Geometry

o Geometry from a Synthetic Perspective

Science Standards

∆ o Science and Technology∆ o Science in Personal and Social Perspectives∆ o Unifying Concepts and Processes

Something that is concave is curved inward like the inner surfaceof a sphere. Something that is convex is curved like the outersurfiace of a sphere. A variety of concave and convex lenses andmirrors are used in the design of eyeglasses, magnifying glasses,cameras, microscopes, and telescopes. In the cxample in the text,water cupping upward produces a concave surface; mercurycupping downward produces a convex surface.

θh

D

ρ

h

θ

ρ

D


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∆ Computation and Estimation∆ o Mathematical Connections∆ o Mathematics as Communication∆ Measurement

Science Standards

Grades 5-8 (∆); Grades 9-12 (o)

∆ o Physical Science∆ o Science and Technology∆ o Unifying Concepts and Processes

Quasi-steady accelerations in spacecraft are related to theposition in the spacecraft, aerodynamic drag, and vehiclerotation. For the Space Shuttle Orbiters, these accelerations areon the order of lx10-6 g and vary with the orbital frequency.


∆ Computation and Estimation∆ o Mathematical Connections∆ o Mathematics as Communication∆ Measurement

Science Standards


g-jitter indicates the vibrations expenenced by microgravityexpenments (for example on parabolic aircraft and the SpaceShuttle) that cause effects similar to those that would be causedby a time-varying gravitational field.

The quasi-steady microgravity environment on the OrbiterColumbia shows the effiects of variations in Earth’s atmosphericdensity. The primary contribution to the variation is the day/nightdiffierence in atmospheric density. The plot shows that the dragon the Orbiter varies over a ninety minute orbit.

vehicle and spacecraft rotations. A spacecraft inlow-Earth orbit experiences some amount of dragdue to interactions with the atmosphere. Anobject within the vehicle, however, is protectedfrom the atmosphere by the spacecraft itself anddoes not experience the same deceleration thatthe vehicle does. The floating object andspacecraft therefore are moving relative to eachother. Similarly, rotation of the spacecraft due toorbital motion causes a force to act on objectsfixed to the vehicle but not on objects freelyfloating within it. On average for the SpaceShuttles, the quasi-steady accelerationsresulting from the sources discussed above(position in the spacecraft, aerodynamic drag,and vehicle rotation) are on the order of 1x1 0-6 g,but vary with time due to variations in theatmospheric density around Earth and due tochanges in Shuttle orientation.

In addition to these quasi-steady accelerations,many operations on spacecraft cause vibrationsof the vehicle and the payloads (experimentapparatus). These vibrations are often referred toas g-jitter because their effects are similar tothose that would be caused by a time-varyinggravitational field. Typical sources for vibrationsare experiment and spacecraft fans and pumps,motion of centrifuges, and thruster firings. With acrew onboard to conduct experiments, additionalvibrations can result from crew activities.

The combined acceleration levels that result fromthe quasi-steady and vibratory contributions aregenerally referred to as the microgravityenvironment of the spacecraft. On the SpaceShuttles, the types of vibration-causingoperations discussed above tend to create acumulative background microgravity environmentof about 1x10-4 g, considering contributions forall frequencies below 250 Hz.

BiotechnologyBiotechnology is an applied biological sciencethat involves the research, manipulation, and


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manufacturing of biological molecules, tissues,and living organisms. With a critical andexpanding role in health, agriculture, andenvironmental protection, biotechnology isexpected to have a significant impact on oureconomy and our lives in the next century.Microgravity research focuses on three principalareas—protein crystal growth, mammalian celland tissue culture, and fundamentalbiotechnology.

Gravity significantly influences attempts to growprotein crystals and mammalian cell tissue onEarth. Initial research indicates that proteincrystals grown in microgravity can yieldsubstantially better structural information thancan be obtained from crystals grown on Earth.Proteins consist of thousands—or in the case ofviruses, millions—of atoms, which are weaklybound together, forming large molecules. OnEarth, buoyancy-induced convection andsedimentation may inhibit crystal growth. Inmicrogravity, convection and sedimentation aresignificantly reduced, allowing for the creation ofstructurally better and larger crystals.

The absence of sedimentation means that proteincrystals do not sink to the bottom of their growthcontainer as they do on Earth. Consequently, theyare not as likely to be affected by other crystalsgrowing in the solution. Because convective flowsare also greatly reduced in microgravity, crystalsgrow in a much more quiescent environment,which may be responsible for the improvedstructural order of space-grown crystals.Knowledge gained from studying the process ofprotein crystal growth under microgravityconditions will have implications for proteincrystal growth experiments on Earth.

Research also shows that mammalian cells—particularly normal cells—are sensitive toconditions found in ground-based facilities usedto culture (grow) them. Fluid flows caused bygravity can separate the cells from each other,

Protein crystals grown in microgravity can haveregular, simple shapes and a more highly orderedinternal structure than those grown on Earth.

1 g µ g


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severely limiting the number of cells that willaggregate (come and stay together). But tissuesamples grown in microgravity are much largerand more representative of the way in whichtissues are actually produced inside the humanbody. This suggests that better control of thestresses exerted on cells and tissues can play animportant role in their culture. These stresses aregreatly reduced in microgravity.

Protein Crystal GrowthThe human body contains over 100,000 differentproteins. These proteins play important roles inthe everyday functions of the body, such as thetransport of oxygen and chemicals in the blood,the formation of the major components of muscleand skin, and the fighting of disease. Researchersin this area seek to determine the structures ofthese proteins, to understand how a protein’sstructure affects its function, and ultimately todesign drugs that intercede in protein activities(penicillin is a well-known example of a drug thatworks by blocking a protein’s function).Determining protein structure is the key to thedesign and development of effective drugs.

The main purpose in growing protein crystals isto advance our knowledge of biological molecularstructures. Researchers can use microgravity tohelp overcome a significant stumbling block inthe determination of molecular structures: thedifficulty of growing crystals suitable forstructural analysis. Scientists use X-raydiffraction to determine the three-dimensionalmolecular structure of a protein. They cancalculate the location of the atoms that make upthe protein based on the intensity and position ofthe spots formed by the diffracted X-rays. Fromhigh resolution diffraction data, scientists candescribe a protein’s structure on a molecularscale and determine the parts of the protein thatare important to its functions. Using computeranalysis, scientists can create and manipulatethree-dimensional models of the protein andexamine the intricacies of its structure to create adrug that”fits” into a protein’s active site, likeinserting a key into a lock to “turn off” the

Crystallized protein lysozyme after dialysis to remove smallmolecule contaminants.


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protein’s function. But X-ray diffraction requireslarge, homogeneous crystals (about the size of agrain of table salt) for analysis. Unfortunately,crystals grown in Earth’s gravity often haveinternal defects that make analysis by X-raydiffraction difficult or impossible. Space Shuttlemissions have shown that crystals of someproteins (and other complex biological moleculessuch as viruses) grown on orbit are larger andhave fewer defects than those grown on Earth.The improved data from the space-grown crystalssignificantly enhance scientists’ understanding ofthe protein’s structure and this information can beused to support structure-based drug design.

Scientists strive for a better understanding of thefundamental mechanisms by which proteins formcrystals. A central goal of microgravity proteincrystal growth experiments is to determine thebasic science that controls how proteins interactand order themselves during the process ofcrystallization. To accomplish this goal, NASA hasbrought together scientists from the proteincrystallography community, traditional crystalgrowers, and other physical scientists to form amultidisciplinary team in order to address theproblems in a comprehensive manner.

Mammalian Cell and Tissue CultureMammalian cell tissue culturing is a major area ofresearch for the biotechnology community. Tissueculturing is one of the basic tools of medicalresearch and is key to developing future medicaltechnologies such as ex vivo (outside of thebody) therapy design and tissue transplantation.To date, medical science has been unable to fullyculture human tissue to the mature states ofdifferentiation found in the body.

The study of normal and cancerous mammaliantissue growth holds enormous promise forapplications in medicine. However, conventionalstatic tissue culture methods form flat sheets ofgrowing cells (due to their settling on the bottomof the container) that differ in appearance andfunction from their three-dimensional counterparts

Science Standards


A substance that is homogeneous is uniform in structure and/orcomposition.

Three different types of protein c rystals grown on theSpace Shuttle Columbia in 1995: satellite tobacco mosaicvirus, lysozyme, and thaumatin.


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Science Standards

∆ o Life Science∆ o Unifying Concepts and Processes

Differentiation is the process (or the result of that process) bywhich cells and/or tissues undergo a progressive specialization ofform or function.


o Algebrao Conceptual Underpinnings of Calculuso Geometry from an Algebraic Perspective

∆ o Mathematical Connections∆ o Mathematics as Problem Solving

Science Standards


The forces acting on a surface can be separated into componentsperpendicular (normal) to and tangential to the surface. Thenormal force causes a normal stress and the tangential force isresponsible for a tangential, or shear, stress acting on the surface.Shear forces cause contiguous parts of a structure or liquid toslide relative to each other.

growing in a living body. In an effort to enhancethreedimensional tissue formation, scientists havedeveloped a ground-based facility for cell andtissue culture called a bioreactor. This instrumentcultures cells in a slowly rotating horizontalcylinder, which produces lower stress levels onthe growing cells than previous Earth-basedexperimental environments. The continuousrotation of the cylinder allows the sample toescape much of the influence of gravity, butbecause the bioreactor environment tends to berather passive, it is sometimes difficult for thegrowing tissue to find the fresh media (foodsupply) it needs to survive.

Another reason normal mammalian cells aresensitive to growth conditions found in standardbioreactors is that fluid flow causes shear forcesthat discourage cell aggregation. This limits boththe development of the tissue and the degree towhich it possesses structures and functionssimilar to those found in the human body. Tissuecultures of the size that can be grown in thesebioreactors allow tests of new treatments oncultures grown from cells from the patient ratherthan on patients themselves. In the future, thistechnology will enable quicker, more thoroughtesting of larger numbers of drugs andtreatments. Ultimately, the bioreactor is expectedto produce even better results when used in amicrogravity environment.

In cooperation with the medical community, thebioreactor design is being used to prepare bettermodels of human colon, prostate, breast, andovarian tumors. Cells grown in conventionalculture systems may not differentiate to form atumor typical of cancer. In the bioreactor,however, these tumors grow into specimens thatresemble the original tumor. Similar results havebeen observed with normal human tissues aswell. Cartilage, bone marrow, heart muscle,skeletal muscle, pancreatic islet cells, liver cells,and kidney cells are examples of the normaltissues currently being grown in rotatingbioreactors by investigators. In addition,

A bioreactor vessel thatflew on the Space Shuttle Discovery inJuly 1995.


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laboratory models of heart and kidney diseases,as well as viral infections (including Norwalk virusand Human Immunodeficiency Virus (HIV)) arecurrently being developed using a modified NASAbioreactor experiment design with slightvariations in experimental technique and someadjustments to hardware. Continued use of thebioreactor can improve our knowledge of normaland cancerous tissue development. NASA isbeginning to explore the possibility of culturingtissues in microgravity, where even greaterreduction in stresses on growing tissue samplesmay allow much larger tissue masses to develop.A hioreactor is in use on the Russian SpaceStation Mir in preparation for the InternationalSpace Station.

Fundamental BiotechnologyElectrophoresis has been studied on a dozenSpace Shuttle flights and has led to additionalresearch in fluid physics in the area ofelectrohydrodynamics. Phase partitioningexperiments, which use interfacial energy (theenergy change associated with the contactbetween two different materials) as the means ofseparation, have flown on six missions.

Combustion Science

Combustion, or burning, is a rapid, self-sustaining chemical reaction that releases asignificant amount of heat. Examples of commoncombustion processes are burning candles, forestfires, log fires, the burning of natural gas in homefurnaces, and the burning of gasoline in internalcombustion engines. For combustion to occur,three things must normally be present: a fuel, anoxidizer, and an ignition stimulus. Fuels can besolid, liquid, or gas. Examples of solid fuelsinclude filter paper, wood, and coal. Liquid fuelsinclude gasoline and kerosene. Propane andhydrogen are examples of gaseous fuels.Oxidizers can be solid (such as ammoniumperchlorate, which is used in Space Shuttle boosterrockets), liquid (like hydrogen peroxide), or gaseous

Science Standards

∆ o Physical Science∆ o Science and Technology∆ o Sciences in Personal and Social Perspectives∆ o Unifying Concepts and Processes

Electrophoresis is the separation of a substance based on theelectrical charge of the molecule and its motion in an appliedelectric field.

Science Standards

∆ o Physical Science∆ o Science and Technology∆ o Science in Personal and Social Perspectives∆ o Unifying Concepts and Processes

An exception to the standard combustion process is hypergoliccombustion. In this situation, a fuel and an oxidizerspontaneously react on contact without the need for an ignitionstimulus. The jets used to maintain and change the Shuttle’sorientation when in orbit are powered by hypergolic reactions.


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(like oxygen). Air, which contains oxygen, is aparticularly common oxidizer. An electrical sparkis an example of an ignition stimulus.

Combustion is a key element in many of modernsociety’s critical technologies. Electric powerproduction, home heating, ground transportation,spacecraft and aircraft propulsion, and materialsprocessing are all examples in which combustionis used to convert chemical energy to thermalenergy. Although combustion, which accounts forapproximately 85 percent of the world’s energyusage, is vital to our current way of life, it posesgreat challenges to maintaining a healthyenvironment. Improved understanding ofcombustion will help us deal better with theproblems of pollutants, atmospheric change andglobal warming, unwanted fires and explosions,and the incineration of hazardous wastes. Despitevigorous scientific examination for over a century,researchers still lack full understanding of manyfundamental combustion processes.

Some objectives of microgravity combustionscience research are to enhance ourunderstanding of the fundamental combustionphenomena that are affected by gravity, to useresearch results to advance combustion scienceand technology on Earth, and to address issues offire safety in space. NASA microgravitycombustion science research combines theresults of experiments conducted in ground-based microgravity facilities and orbitinglaboratories and studies how flames ignite,spread, and extinguish (go out) undermicrogravity conditions.

Research in microgravity permits a new range ofcombustion experiments in whichbuoyancyinduced flows and sedimentation arevirtually eliminated. The effects of gravitationalforces often impede combustion studiesperformed on Earth. For example, combustiongenerally produces hot gas (due to the energyreieased in the reaction), which is less dense thatthe cooler gases around it. In Earth’s gravity, the

The familiar shape of a candle flame on Earth iscaused by buoyancy-driven convection. Inmicrogravity, a candle flame assumes a sphericalshape as fresh oxidizer reaches it by diffusionprocesses.


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hot gas is pusched up by the denser surroundinggases. As the hot gas rises, it creates buoyancy-induced flow that promotes the mixing of theunburned fuel, oxidizer, and combustionproducts.

The ability to significantly reduce gravity-drivenflows in microgravity helps scientists in severalways. One advantage is that the “quieter” andmore symmetric microgravity environment makesthe experiments easier to model (describemathematically), thus providing a better arenafor testing theories. In addition, eliminatingbuoyancy-induced flows allows scientists to studyphenomena that are obscured by the effects ofgravity, such as the underlying mechanisms offuel and heat transport during combustionprocesses. Because buoyancy effects are nearlyeliminated in microgravity, experiments of longerduration and larger scale are possible, and moredetailed observation and examination ofimportant combustion processes can occur.

Scientists often desire an even mixture of thecomponent parts of fuels so that modelsdeveloped for their experiments can usesimplified sets of equations to represent theprocesses that occur. Sedimentation affectscombustion experiments involving particles ordroplets because, as the components of greaterdensity sink in a gas or liquid, their movementrelative to the other particles creates anasymmetrical flow around the dropping particles.This can complicate the interpretation ofexperimental results. On Earth, scientists mustresort to mechanical supports, levitators, andstirring devices to keep fuels mixed, while fluidsin microgravity stay more evenly mixed withoutsticking together, colliding, or dispersingunevenly.


∆ Computation and Estimationo Discrete Mathematics

∆ o Mathematical Connections∆ o Mathematics as Communication∆ o Mathematics as Problem Solving∆ o Mathematics as Reasoning

Science Standards

∆ o Physical Science∆ o Science as Inquiry∆ o Science and Technology∆ o Unifying Concepas and Processes

The creation and use of mathematical models is a key elementof science, engineering, and technology. Modeling begins withidentifying the physical and chemical phenomena involved in anexperiment. Associated mathematical equations such as equationsof motion are then identified. These governing equations aresolved in order to predict important aspeces of the experimentbehavior, using appropriate values of experiment parameters suchas density, composition, temperature, and pressure. Simplemathematical models can be solved hy hand, while morecomplex experiments are generally modeled using sophisticatedalgorithms on high speed computers.

In microgravity research, scientists use modeling in preparationfor flight experiments and in analysis of the results. Models andexperiment procedures are fine-tuned based on comparisonsbetween model predictions and the results of ground-basedmicrogravity experiments (for example, drop facilities andparabolic aircraft flights). This preliminary work allowsresearchers to best take advantage of space flight opportunities.


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To date, combustion science researchers havedemonstrated major differences in the structuresof various types of flames burning undermicrogravity conditions and under 1 g conditions.In addition to the practical implications of theseresults in combustion efficiency, pollutant control,and flammability, these studies establish thatbetter understanding of the individual processesinvolved in the overall combustion process can beobtained by comparing results from microgravityand Earth gravity tests. One clear example of theadvantage of these comparison tests is in the areaof fire safety. Most smoke detectors have beendesigned to detect soot particles in the air, but thesizes of soot particles produced in 1 g aredifferent from those produced in microgravityenvironments. This means that smoke-detectingequipment must be redesigned for use onspacecraft to ensure the safety of equipment andcrew.

Comparisons of research in microgravity and in1 g have also led to improvements in combustiontechnology on Earth that may reduce pollutantsand improve fuel efficiency. Technologicaladvances include a system that measures thecomposition of gas emissions from factorysmoke stacks so that they can be monitored. Inaddition, a monitor for ammonia, which is onegas that poses dangers to air quality, is alreadybeing produced and is available for industrial use.Engineers have also designed a device that allowsnatural gas appliances to operate more efficientlywhile simultaneously reducing air pollution. Thismay be used in home furnaces, industrialprocessing furnaces, and water heaters in thefuture. Another new technology is the use ofadvanced optical diagnostics and lasers to betterdefine the processes of soot formation so thatsoot-control strategies can be developed. Deviceshave also been developed to measure percentagesof soot in exhausts from all types of engines andcombustors, including those in automobiles andairplanes.

Transmission Electron Microscope image of laser-heated soot.


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The combustion science program supportsexperiments in the following research areas:

Premixed Gas FlamesIn premixed gas flame research, the fuel andoxidizer gases are completely mixed prior toignition. Scientists are interested in flame speed(the rate at which the flame zone travels awayfrom the ignition source and into the unreactedmixture) as a function of both the type of fuel andoxidizer used and the oxidizer-to-fuel ratio. Withsufficiently high or low ratios, the flame does notmove into the unreacted mixture; these criticalratios are referred to as lower and upperflammability limits and are of considerableinterest in terms of both safety and fundamentalscience. Gravity can strongly affect both flamespeed and flammability limits, chiefly throughbuoyancy effects. Scientists in this area are alsoresearching gravity’s effects on the stability,extinction, structure, and shape of premixed gasflames.

Gaseous Diffusion FlamesIn this area of research, the fuel and oxidizergases are initially separate. They tend to diffuseinto each other and will react at their interfaceupon ignition. The structure of these flames undermicrogravity conditions is quite different than onEarth because of buoyancy-induced flows causedby Earth’s gravity. Scientists study flammabilitylimits, burning rates, and how diffusion flamestructure affects soot formation. Within this area,results of studies of the behavior of gas-jet flamesin a microgravity environment, both in transitionand in turbulent flows, are being used to developmodels with potential applications in creatingeffective strategies to control soot formation inmany practical applications.

Liquid Fuel Droplets and SpraysIn this research area, scientists study thecombustion of individual liquid fuel dropletssuspended in an oxidizing gas (air, for example).For these experiments, investigators commonlyuse fuels

Candle flame energy flow. Adapted from “The Science of Flames”poster, National Energy Foundation, Salt Lake City, Utah.


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Ultraviolet images of OH radiation taken at hnIf:vecnndinterval.s during a drrvp tower test of the DropletCombustion Experiment. The diameter of the flame producedby burning a heptane droplet decreases in freefall.

such as heptane, kerosene, and methanol. Gravityhinders fundamental studies of dropletcombustion on Earth due to flows induced byhigh-density droplets that sink andbuoyancyinduced upward acceleration of hotcombustion products relative to the surroundinggas. These flows cause drops to burn unevenly,making it difficult for scientists to drawmeaningful conclusions from their experiments.

This area of study also includes the investigationof the combustion of sprays and ordered arraysof fuel droplets in a microgravity environment foran improved understanding of interactionsbetween individual burning droplets in sprays.Knowledge of spray combustion processesresulting from these studies should lead to majorimprovements in the design of combustors usingliquid fuels.

Fuel Particles and Dust CloudsThis area is particularly important in terms of firesafety because clouds of coal dust have thepotential to cause mine explosions and grain-dustclouds can cause silos and grain elevators toexplode. It is particularly difficult to study thefundamental combustion characteristics of fuel-dust clouds under normal gravity because initiallywelldispersed dust clouds quickly settle due todensity differences between the particles and thesurrounding gas. Because particles stick togetherand collide during the sedimentation process,they form nonuniform fuel-air ratios throughoutthe cloud. In microgravity, fuel-dust cloudsremain evenly mixed, allowing scientists to studythem with much greater experimental control witha goal of mitigating coal mine and grain elevatorhazards.

Flame Spread Along SurfacesAn important factor in fire safety is inhibiting thespread of flames along both solid and liquidsurfaces. Flame spread involves the reactionbetween an oxidizer gas and a condensed-phasefuel or the vapor produced by the “cooking” of


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such a fuel. Research has revealed majordifferences in ignition and flame-spreadingcharacteristics of liquid and solid fuels undermicrogravity and normal gravity conditions.Material flammability tests in 1 g, which arestrongly affected by buoyancy-induced flows, donot match results obtained in microgravity. It istherefore useful to study both flame spread andmaterial flammability characteristics inmicrogravity to ensure fire safety in environmentswith various levels of gravity. The knowledgegained from these studies may also lead to betterunderstanding of dangerous combustionreactions on Earth. Microgravity experimentseliminate complexities associated with buoyancyeffects, providing a more fundamental scenariofor the development of flame-spreading theories.

Smoldering CombustionSmoldering combustion is a relatively slow,nonflaming combustion process involving anoxidizer gas and a porous solid fuel. Well-knownexamples of smoldering combustion are“burning” cigarettes and cigars. Smolderingcombustion can also occur on much larger scaleswith fuels such as polyurethane foam. When aporous fuel smolders for a long period of time, itcan create a large volume of gasified fuels, whichare ready to react suddenly if a breeze or someother oxidizer flow occurs. This incites the fuel tomake the transition to full-fledged combustion,often leading to disastrous fires (like thoseinvolving mattresses or sofa cushions). Sinceheat is generated slowly in this process, the rateof combustion is quite sensitive to heat exchange;therefore, buoyancy effects are particularlyimportant. Accordingly, smoldering combustion isexpected to behave quite differently in theabsence of gravity.

Combustion SynthesisCombustion synthesis, a relatively new area ofresearch, involves creating new materials througha combustion process and is closely tied to workin materials science. One area of particularinterest is referred to as self-deflagrating high-

View looking down at a piece qf ashless filter paper with a 1centimeter grid on it. On the USMP-3 Shuttle mission, a radiantheater (two concentric rings exposed at the center of the image)was used to ignite samples to study flame spread and smolderingin weak air f1ows under microgravity conditions. In this image,areas where the grid is not seen have been burned, with thecracking and curling edges of the burning paper leaving a cuspedappearance. The flame started at the heater site and propagatedtoward the right where a fan provided a source of fresh air.Charred paper around the burnt area is a darker grey than theuraffected paper. White areas to the right of the heater rings aresoot zones.


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Science Standards


A fluid is something that flows. Highly compressible fluids areusually considered gases; essentially incompressible fluids areusually considered liquids. Fluids tend to conform to the shape ofa container. On Earth’s surface, liquids tend to fill the bottom ofan open or closed container and gases tend to fill closedcontainers.

temperature synthesis. This occurs when twomaterials— usually two solids—are mixedtogether, are reactive with one another, and createa reaction that gives off a large amount of heat.Once the reaction is started, the flame willpropagate through a pressed mixture of theseparticles, resulting in a new material. Much of theinitial research in this groundbreaking areainvolves changing variables such as composition,pressure, and preheat temperature. Manipulatingthese factors leads to interesting variations in theproperties of materials created through thesynthesis process.

Flame processes are also being used to createfullerenes and nanoparticles. Fullerenes, a newform of carbon, are expensive to produce at thistime and cannot be produced in large quantities,but scientists predict more uses for them will bedeveloped as they become more readily available.Nanoparticles (super-small particles) are also ofgreat interest to materials scientists due to thechanges in the microstructure of compactedmaterials that can be produced by sintering,which results in improved properties of the finalproducts. These nanoparticles can thus be usedto form better pressed composite materials.

Fluid PhysicsA fluid is any material that flows in response toan applied force; thus, both liquids and gases arefluids. Some arrangements of solids can alsoexhibit fluid-like behaviors; granular systems(such as soil) can respond to forces, like thoseinduced by earthquakes or floods, with a flow-likeshift in the arrangement of solid particles and theair or liquids that fill the spaces between them.Fluid physicists seek to better understand thephysical principles governing fluids, includinghow fluids flow under the influence of energy,such as heat or electricity; how particles and gasbubbles suspended in a fluid interact with andchange the properties of the fluid; how fluidsinteract with solid boundaries; and how fluidschange phase, either from fluid to solid or from onefluid phase to another. Fluid phenomena studied


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range in scale from microscopic to atmosphericand include everything from the transport of cellsin the human body to changes in the compositionof the atmosphere.

The universal nature of fluid phenomena makestheir study fundamental to science andengineering. Understanding the fluid-like behaviorof soils under stress will help civil engineersdesign safe buildings in earthquake-prone areas.Materials engineers can benefit from a bettergrasp of how the structure and properties of asolid metal are determined by fluid behaviorduring its formation. And knowledge of the flowcharacteristics of vapor-liquid mixtures is usefulin designing power plants to ensure maximumstability and performance. The work of fluidphysics researchers often applies to the work ofother microgravity scientists.

Complex FluidsThis research area focuses on the uniqueproperties of complex fluids, which includecolloids, gels, magneto-rheological fluids, foams,and granular systems.

Colloids are suspensions of finely divided solidsor liquids in fluids. Some examples of colloidaldispersions are aerosols (liquid droplets in gas),smoke (solid particles in gas), and paint (solid inliquid). Gels are colloidal mixtures of liquids andsolids in which the solids have linked together toform a continuous network, becoming veryviscous (resistant to flow). Magneto-rheologicalfluids consist of suspensions of colloidalparticles. Each particle contains many tiny,randomly oriented magnetic grains and anexternally applied magnetic field can orient themagnetic grains into chains. These chains mayfurther coalesce into larger-scale structures in thesuspension, thereby dramatically increasing theviscosity of the suspension. This increase,however, is totally reversed when the magneticfield is turned off.

Side views of water and airflowing through a clearpipe. At Ig,the air stays on top. In microgravity, the air canform a coredown the center of the pipe.

1 g

µ g


30

Science Standards


Rheology is the scientific study of the deformation and flow ofmatter.

USML-2 Payload Commander Kathryn C. Thomton works at theDrop Physics Module, used to investigate liquid drop behavior inmicrogravity.

A foam is a nonuniform dispersion of gas bubblesin a relatively small volume of liquid that containssurface-active macromolecules, or surfactants(agents that reduce the surface tension ofliquids). Foams have striking properties in thatthey are neither solid, liquid, nor vapor, yet theyexhibit features of all three. Important uses forcustomdesigned foams include detergents,cosmetics, foods, fire extinguishing, oil recovery,and many physical and chemical separationtechniques. Unintentional generation of foam, onthe other hand, is a common problem affectingthe efficiency and speed of a vast number ofindustrial processes involving the mixing oragitation of multicomponent liquids. It alsooccurs in polluted natural waters and in thetreatment of wastewater. In all cases, control offoam rheology and stability is required.

Examples of granular systems include soil andpolystyrene beads, which are often used aspacking material. Granular systems are made upof a series of similar objects that can be as smallas a grain of sand or as large as a boulder.Although granular systems are primarilycomposed of solid particles, their behavior can befluid-like. The strength of a granular system isbased upon the friction between and geometricinterlocking of individual particles, but undercertain forces or stresses, such as those inducedby earthquakes, these systems exhibit fluidicbehavior.

Studying complex fluids in microgravity allowsfor the analysis of fluid phenomena often maskedby the effects of gravity. For example, researchersare particularly interested in the phase transitionsof colloids, such as when a liquid changes to asolid. These transitions are easier to observe inmicrogravity. Foams, which are particularlysensitive to gravity, are more stable (and cantherefore be more closely studied for longerperiods of time) in microgravity. In magneto-rheological fluids, controlling rheology induced bya magnetic field has many potential applications,from shock absorbers and clutch controls for carsto robotic joint controls. Under the force


31

of Earth’s gravity, the magnetic particles in thesefluids often fall out of suspension due tosedimentation, but in microgravity this problem iseliminated. Investigations of the behavior ofgranular systems, which have previously beenhampered by Earth’s gravity, are more feasible inmicrogravity because they do not settle as theydo on Earth.

Multiphase Flow and Heat TransferThis research area, which has applications in theengineering of heat transfer systems and gaspurification systems, focuses on complexproblems of fluid flow in varying conditions.Scientists are seeking to add to their currentlylimited knowledge of how gravity-dependentprocesses, such as boiling and steamcondensation, occur in microgravity. Boiling isknown to be an efficient way to transfer largeamounts of heat, and as such, it is often used forcooling and for energy conversion systems. Inspace applications, boiling is preferable to othertypes of energy conversion systems because it isefficient and the apparatus needed to generatepower is smaller.

Another of the mechanisms by which energy andmatter move through liquids and gases isdiffusive transport. The way atoms and moleculesdiffuse, or move slowly, through a liquid or gas isdue primarily to differences in concentration ortemperature. Researchers use microgravity tostudy diffusion in complex systems, a processthat would normally be eclipsed by the force ofgravity.

Understanding the physics of multiphase flow andheat transfer will enable scientists to extend therange of human capabilities in space and willenhance the ability of engineers to solve problemson Earth as well. Applications of this researchmay include more effective air conditioning andrefrigeration systems and improvements in powerplants that could reduce the cost of generatingelectricity.


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Interfacial PhenomenaResearch in this area focuses on how aninterface, like the boundary between a solid and aliquid, acquires and maintains its shape. Interfacedynamics relate to the interaction of surfaces inresponse to heating, cooling, and chemicalinfluences. A better understanding of this topicwill contribute to improved materials processingand other applications.

Interfacial phenomena, such as the wetting andspreading of two immiscible liquids or thespreading of fluid across a solid surface, areubiquitous in nature and technology. Duckfeathers and waterproof tents repel water becausethe wetting properties of the surfaces of theirfibers prevent water from displacing the air in thegaps between the fibers. In contrast, waterspontaneously displaces air in the gaps of asponge or filter paper. Technologies that rely ondousing surfaces with fluids like agriculturalinsecticides, lubricants, or paints depend on thewetting behavior of liquids and solids. Wetting isalso a dominant factor in materials processingtechniques, including film and spray coating,liquid injection from an orifice, and crystalgrowth. Interfaces dominate the properties andbehavior of advanced composite materials, wherewetting of the constituent materials dictates theprocessing of such materials. Understanding andcontrolling wetting and spreading pose bothscientific and technological challenges.

In reduced gravity, wetting determines theconfiguration and location of fluid interfaces, thusgreatly influencing, if not dominating, thebehavior of multiphase fluid systems. Thisenvironment provides scientists with an excellentopportunity to study wetting and surface tensionforces that are normally masked by the force ofEarth’s gravity. This research also providesinformation that can help improve the design ofspace engineering systems strongly affected bywetting, including liquid-fuel supply tanks, two-phaseheat transfer and/or storage loops, and fluidsmanagement devices for life support purposes.

Comparison of thermocapillary flows on Earth (top) and inmicogravity (bottom). The flow pattern (indicated by the whiteareas) in the Earth-based experiments is only evident on the fluid’ssurface, while the flow pattern in microgravity encompasses theentire fluid.


33

Dynamics and StabilityThis broad area of research includes dropdynamics, capillarity, and magneto/electro-hydrodynamics.

Drop dynamics research deals with the behaviorof liquid drops and gas bubbles under theinfluence of external forces and chemical effects.Research in drop dynamics ranges from the studyof rain in the atmosphere to the investigation ofchemical processes. A potential application ofthese studies is in the realm of materialsprocessing. In forming solid materials fromliquids in space, it is usually important to createpure and/or uniform solids—gas bubbles anddrops of foreign liquids are undesirable. Yet dueto the microgravity environment, these bubblesand drops of substances of lower densities wouldnot “rise to the top” the way they would if theywere on the ground, which makes extraction ofthe bubbles difficult. Researchers are attemptingto resolve this problem in order to facilitate bettermaterials processing in space.

Scientists are also interested in studying singlebubbles and drops as models for other naturalsystems. The perfect spheres formed by bubblesand drops in microgravity (due to the dominanceof surface tension forces) are an easy fit totheoretical models of behavior—feweradjustments need to be made for the shape of themodel. Investigators can manipulate the sphericaldrops using sound and other impulses, creatingan interactive model for processes such as atomfissioning.

Capillarity refers to a class of effects that dependon surface tension. The shape a liquid assumes ina liquid-liquid or liquid-gas system is controlledby surface tension forces at the interface. Smalldisturbances in the balance of molecular energiesat these boundaries or within the bulk of the liquidcan cause shifts in the liquid’s position and shapewithin a container (such as a fuel tank) or in acontaining material (such as soil). These changes,or capillary effects, often occur in liquids on Earth,

This sequential photo shows a liquid bridge undergoing aseries of shape changes. Liquid bridge investigations on theShuttle have tested theories of electrohydrodynamics.

In materials science research, float 20ne samples aresometimes usedfor crystal growth. For a 87oat-zonesampler the surface tension of the melt keeps the samplesuspended between two sample rods in afurnace. Athorough understanding of the capillarity and surfacetension effects in a molten sample allows betterexperiment control and results prediction.


34

Science Standards


Joule heating occurs when electric current flows through amaterial. This is how an electric toaster works .

Researchers observe thefloat package and data rack of a superfluidhelium experiment on a parabolic aircraft fight.

but are to some degree masked or minimized bythe stronger force of gravity. In microgravity,however, capillary effects become prominent. Thestudy of capillary phenomena in microgravity willenable researchers to better understand andpredict fluid configurational changes both onEarth and in low-gravity environments.

Microgravity fluid physics researchers also studythe effects of magnetic and electric fields on fluidflows, or magneto/electrohydrodynamics.Promising microgravity research subjects in thisarea include weak fluid flows, such as thosefound in poorly conducting fluids in a magneticfield, and Joule heating. In Earth’s gravity, Jouleheating causes buoyancy-driven flows which, inturn, obscure its effects. In microgravity, however,buoyancy-driven flows are nearly eliminated, soresearchers are not only able to study the effectsof Joule heating, but they can also observe otherprocesses involving applied electric fields, suchas electrophoresis.

Fundamental Physics

Physics is a major part of fundamental sciencewhere the ultimate goal is to establish a unifieddescription of the basic laws that govern ourworld. At present fundamental physics includeslow temperature physics, condensed matterphysics (the study of solids and liquids), lasercooling and atomic physics, and gravitational andrelativistic physics. A unifying characteristic ofthese research areas is that they addressfundamental issues which transcend theboundaries of a particular field of science.

The majority of experiments in fundamentalphysics are extensions of investigations inEarthbased laboratories. The microgravityexperiment in these cases presents anopportunity to extend a set of measurementsbeyond what can be done on Earth, often byseveral orders of magnitude. This extension canlead either to a more precise confirmation of ourprevious understanding of a problem,


35

or it can yield fundamentally new insight ordiscovery. The remainder of fundamental physicsresearch involves tests of the fundamental lawswhich govern our universe. Investigations aim atenhancing our understanding of the most basicaspects of physical laws, and as such may wellhave the most profound and lasting longrangeimpact on mankind’s existence on Earth and inspace.

There are many examples of how fundamentalscience has had an impact on the average person.Basic research in condensed matter physics toexplain the behavior of semiconductors led to thedevelopment of transistors which are now used incommunication devices, and which produce evermore prevalent and capable computer technology.Research in low temperature physics to explorethe properties of fluids at very low temperaturesled to advanced magnetic resonance techniquesthat have brought extremely detailed magneticresonance imaging to the medical doctor, sotoday much exploratory surgery can be avoided.A less widely appreciated part played byfundamental science in today’s world has beenthe need to communicate large quantities of datafrom physics experiments to collaborators atmany locations around the world. Satisfying thisneed was instrumental in the development of theInternet and the World Wide Web.

Fundamental physics research benefits from boththe reduction in gravity’s effects in Earth-orbit andfrom the use of gravity as a variable parameter. Incondensed matter physics, the physics of criticalpoints has been studied under microgravityconditions. This field needs microgravity becausethe ability to approach a critical point in theEarthbound laboratory is limited by the uniformityof the sample which is spoiled by hydrostaticpressure variations. One of the important issuesin condensed matter physics is the nature of theinterface between solids and fluids. The boundaryconditions at this interface have an influence onmacroscopic phenomena, including wetting. Themicroscopic aspects of the system near the

Science Standards


The critical point is the temperature at which the differencesbetween liquids and gases disappear. Above that temperature, theliquid smoothly transforms to the gaseous state; boilingdisappears.


∆ o Mathematical Connections∆ o Mathematics as Communication∆ o Mathematics as Problem Solving

Science Standards


Hydrostatic pressure is the result of lhe weighl of a materialabove the point of measurement.


36


∆ o Mathematical Connections∆ o Mathematics as Communication∆ o Mathematics as Problem Solving∆ o Measurement

Science Standards

∆ o History and Nature of Science∆ o Physical Science∆ o Science and Technology∆ o Science as Inquiry∆ o Unifying Concepts and Processes

There are three temperature scales commonly used in the world.The Kelvin scale, the Celsius temperature scale, and theFahrenheit scale. The SI unit for temperature is the Kelvin. Inmost scientific laboratories, temperatures are measured andrecorded in Kelvin’s or degrees Celsius. The Celsius scale is usedfor weather reporting in most of the world. The United States andsome other countries use the Fahrenheit scale for weatherreporting.

The Kelvin scale is defined around the triple point of water (solidice, liquid water, and water vapor coexist in thermal equilibrium)which is assigned the temperature 273.16 K. This is equal to0.01°C and 32.02°F. Absolute zero, the coldest anything can get,is 0 K, -273.15°C, and -459.67°F.

Questions for Discussion• How do you convert between these different temperature

scales?• What are the boiling and freezing points of water on all these

scales, at 1 atm pressure?

boundary are difficult to study. However, when thefluid is near a critical point, the boundary layeradjacent to the solid surface acquires amacroscopic thickness. Research undermicrogravity conditions permits the study of notonly the influence of the boundaries onthermodynamic properties, but also transportproperties such as heat and mass transport. Oneof the most dramatic advancements in atomicphysics over the last decade has been thedemonstration that laser light can be used to coola dilute atomic sample to within micro- or evennano-degrees of absolute zero. At these lowtemperatures, the mean velocity of the atomsdrops from several hundred m/s to cm/s or mm/s, a reduction by four to five orders of magnitude.When atoms are moving this slowly,measurements of atomic properties can be mademore precisely because the atoms stay in a givenpoint in space for a longer time. In this regime,the effects of gravity dominate atomic motion soexperiments performed in a microgravityenvironment would allow even more precisemeasurements.

Among the most important goals of suchresearch is the improvement of ultra-highprecision clocks. These clocks not only providethe standard by which we tell time, but are crucialto the way we communicate and navigate onEarth, in the air, and in space. Laser cooled atomshave significantly improved the accuracy andprecision of clocks because these atoms movevery slowly and they remain in a givenobservation volume for very long times. However,observation times in these clocks are still affectedby gravity. Because of the effects of gravity, theatoms used in these clocks ultimately fall out ofthe observation region due to their own weight.Increased observation times are possible inmicrogravity and can result in furtherimprovements in precision of at least one or twoorders of magnitude.

Indeed, clocks are central to the study of generalrelativity and in questions concerning the verynature of gravity itself. The motivation for space


37

based clocks is not only tied to the improvedperformance expected in a microgravityenvironment but also these clocks will haveaccess to different positions in space than areavailable on Earth. An important example of thisphysics is revealed in the comparison of an Earth-based clock with a space-based clock. Thiscomparison provides a direct measurement of thegravitational redshift. Tests of Einstein’s theoriesof relativity and of other theories of gravitationserve as a foundation for understanding howmatter and space-time itself behave at largelength scales and under extreme conditions. Thefreefall environment of orbit, the use of lowtemperature techniques, and the use of highprecision frequency standards offer opportunitiesto perform improved tests of these theories.Direct tests of gravitation theories and otherfundamental theories, including the Law ofUniversal Gravitation, can be performed in amicrogravity environment.

Materials ScienceMaterials science is an extremely broad field thatencompasses the study of all materials. Materialsscientists seek to understand the formation,structure, and properties of materials on variousscales, ranging from the atomic to microscopic tomacroscopic (large enough to be visible).Establishing quantitative and predictiverelationships between the way a material isproduced (processing), its structure (how theatoms are arranged), and its properties isfundamental to the study of materials.

Materials exist in two forms: solids and fluids.Solids can be subdivided into two categorie—-crystalline and noncrystalline (amorphous)—based on the internal arrangement of their atomsor molecules. Metals (such as copper, steel andlead), ceramics (such as aluminum oxide andmagnesium oxide), and semiconductors (such assilicon and gallium arsenide) are all crystallinesolids because their atoms form an ordered

Many materials scientists use a triangle such as this todescribe the relationship between structure, processing,and properties. Microgravity can play an important rolein establishing the relationships in a quantitative andpredictive manner.


38

Science Standards


A semiconductor is a substance, such as germanium and silicon,that is a poor electrical conductor at room temperature but isimproved by minute additions of certain substances (dopants) orby the application of heat, light, or voltage; a material with aforbidden energy gap less than 3 eV.

Schematic of the Electromagnetic Containerless Processing Facility(TEMPUS) used on Shuttle missions STS-65 and STS-83.

internal structure. Most polymers (such asplastics) and glasses are amorphous solids,which means that they have no long rangespecifically ordered atomic or moleculararrangement.

One principal objective of microgravity materialsscience research is to gain a better understandingof how gravity-driven phenomena affect thesolidification and crystal growth of materials.Buoyancy-driven convection, sedimentation, andhydrostatic pressure can create defects(irregularities) in the internal structure ofmaterials, which in turn alter their properties.

The virtual absence of gravity-dependentphenomena in microgravity allows researchers tostudy underlying events that are normallyobscured by the effects of gravity and which aretherefore difficult or impossible to studyquantitatively on Earth. For example, inmicrogravity, where buoyancy-driven convectionis greatly reduced, scientists can carefully andquantitatively study segregation, a phenomenonthat influences the distribution of a solid’scomponents as it forms from a liquid or gas.

Microgravity also supports an alternativeapproach to studying materials calledcontainerless processing. Containerlessprocessing has an advantage over normalprocessing in that containers can contaminate thematerials being processed inside them. Inaddition, there are some cases in which there areno containers that will withstand the very hightemperatures and corrosive environments neededto work with certain materials. Containerlessprocessing, in which acoustic, electromagnetic,or electrostatic forces are used to position andmanipulate a sample, thereby eliminating the needfor a container, is an attractive solution to theseproblems.

Furthermore, microgravity requires much smallerforces to control the position of containerlesssamples, so the materials being studied are not


39

disturbed as much as they would be if they werelevitated on Earth.

Materials science research in microgravity leadsto a better understanding of how materials areformed and how the properties of materials areinfluenced by their formation. Researchers areparticularly interested in increasing theirfundamental knowledge of the physics andchemistry of phase changes (when a materialchanges from liquid to solid, gas to solid, etc.).This knowledge is applied to designing betterprocess-control strategies and productionfacilities in laboratories on Earth. In addition,microgravity experimentation will eventuallyenable the production of limited quantities ofhigh-quality materials and of materials that exhibitunique properties for use as benchmarks.

Microgravity researchers are interested instudying various methods of crystallization,including solidification (like freezing water tomake ice cubes), crystallization from solution (theway rock candy is made from a solution of sugarand water), and crystal growth from the vapor(like frost forming in a freezer). These processesall involve fluids, which are the materials that aremost influenced by gravitational effects.Examining these methods of transforming liquidsor gases into a solid in microgravity givesresearchers insight into other influentialprocesses at work in the crystallization process.

Electronic Materials Electronic materials play an important role in theoperation of computers, medical instruments,power systems, and communications systems.Semiconductors are well-known examples ofelectronic materials and are a main target ofmicrogravity materials science research.Applications include creating crystals for use inX-ray, gamma-ray, and infrared detectors, lasers,computer chips, and solar cells. Each of thesedevices epends on the ability to manipulate thecrystalline and chemical structure (perfection) of

Schematic diagram of a multizone furnace used to growsemiconductor materials on the Shuttle. A mechanism moves anexisting c crystal through the temperature zones, melting thesample then cooling it so that it solidifies. In other furnacedesigns, the heating mechanism moves and the sample isstationary. What are the advantages and disadvantages of eachapproach?


40


∆ o Mathematical Connections∆ Patterns and Functions∆ Geometry

o Geometry from a Synthetic Perspective

Science Standards

∆ o Earth and Space Science∆ o Physical Science∆ o Science and Technology∆ o Unifying Concepts and Processes

Questions for Discussion• What is an ordinary drinking glass made from?• What different things are added to glass to change its

properties?• What natural processes produce glasses?• What are the differences between how glasses and crystalline

solids fracture?

the material, which can be strongly influenced bygravity as crystals are formed.

The properties of electronic materials are directlyrelated to the degree of chemical and crystallineperfection present in the materials. However,perfect crystals are not normally the ultimategoal. For example, the presence of just a fewimpurities in some electronic materials canchange their ability to conduct electricity by overa million times. By carefully controlling crystallinedefects and the introduction of desirableimpurities to the crystals, scientists andengineers can design better electronic deviceswith a wide range of applications.

Glasses and CeramicsA glass is any material that is formed without along range ordered arrangement of atoms. Somematerials that usually take crystalline forms, likemetals, can also be forced to form as glasses byrapidly cooling molten materials to a temperaturefar below their normal solidification point. Whenthe material solidifies, it freezes so quickly that itsatoms or molecules do not have time to arrangethemselves systematically.

Ceramics are inorganic nonmetallic materials thatcan be extraordinarily strong at very hightemperatures, performing far better than metallicsystems under certain circumstances. They willhave many more applications when importantfundamental problems can be solved. If a ceramicturbine blade, for example, could operate at hightemperatures while maintaining its strength, itwould provide overall thermodynamic efficienciesand fuel efficiencies that would revolutionizetransportation. The problem with ceramics is thatwhen they fail, they fail catastrophically, breakingin an irreparable manner.

Glasses and ceramics are generally unable toabsorb the impacts that metals can; instead, theycrack under great force or stress (whereas metalsgenerally bend before they break). An importantpart of ceramics and glass research in

Schematic of silicon dioxide tetrahedra. The topview is of a crystalline ordered structure. Thebottom view is of a disordered glassy solid.


41

microgravity involves controlling the minute flawsthat govern how these materials fail. Frominformation obtained through microgravityresearch, scientists hope to be able to control theprocessing of ceramics so that they can, duringprocessing, prevent the formation ofimperfections that lead to catastrophic failure.

Applications for knowledge obtained throughresearch in these areas include improving glassfibers used in telecommunications and creatinghigh-strength, abrasion-resistant crystallineceramics used for gas turbines, fuel-efficientinternal combustion engines, and bioceramicartificial bones, joints, and teeth.

Metals and AlloysMetals and alloys constitute an importantcategory of engineered materials. These materialsinclude structural materials, many types ofcomposites, electrical conductors, and magneticmaterials. Research in this area is primarilyconcerned with advancing the understanding ofmetals and alloys processing so that structureand, ultimately, properties, can be controlled asthe materials are originally formed. By removingthe influence of gravity, scientists can moreclosely observe influential processes in structureformation that occurs during solidification. Theproperties of metals and alloys are linked to theircrystalline and chemical structure; for example,the mechanical strength and corrosion resistanceof an alloy are determined by its internalarrangement of atoms, which develops as themetal or alloy solidifies from its molten state.

One aspect of the solidification of metals andalloys that influences their microstructures is theshape of the boundary, or interface, that existsbetween a liquid and a solid in a solidifyingmaterial. During the solidification process, as therate of solidification increases under the samethermal conditions, the shape of the solidifyinginterface has been shown to go through a seriesof transitions. At low rates of growths theinterface is planar (flat or smoothly curved

Science Standards


An alloy is a combination of two or more metals.

Magnification of a sample of an aluminum-indium alloy. Whenthe sample is melted then controllably solidifies in the AGHF; theindium forms in cylindrica lfibers within a solid aluminummatrix.


42


∆ Geometryo Geometry from an Algebraic Perspectiveo Geometry from a Synthetic Perspective

∆ o Mathematical Connections∆ o Mathematics as Communication

Science Standards


One of the important characteristics of a solid is its shape. On avisible scale, the function of some solids may depend on theability to sit in a stable manner on a surface or to fit tightly intosome configuration. On a smaller scale, the structures ofcrystalline solids are defined by the ordered placement of atoms.The basis of understanding crystalline structure and the shapes ofsolids is a knowledge of the definitions of two-dimensionalshapes (polygons) and three-dimensional solids (polyhedra).

A simple k-sided polygon is defined by connecting k points in aplane with line segments such that no edges intersect except atthe defining points (vertices). The sum of the angles in anypolygon equals 2x(k-2)x90°. Specific names given to somesimple polygons are given below.

Name # of Sides (k)triangle 3quadrilateral 4pentagon 5hexagon 6heptagon 7octagon 8nonagon 9decagon 10undecagon 11dodecagon 12

Regular polygons are those for which all the sides are the samelength and all the angles are the same. The angles of a regularpolygon are defined by θ=(k-2)x 180°/k.

Questions for Discussion• Discuss special cases of triangles and quadrilaterals such as

isosceles triangles, parallelograms, trapezoids.• What is the common name for a regular triangle? For a regular

quadrilateral?• Is there a general equation for the area of any polygon’?

on a macroscopic scale). As the rate of growthincreases, the interface develops a corrugatedtexture until three dimensional cells (similar inshape to the cells in a beehive but much smaller)form in the solid. A further increase in the rate ofgrowth causes the formation of dendrites. Thedevelopment of these different interface shapesand the transition from one shape to another iscontrolled by the morphological stability (shapestability) of the interface. This stability isinfluenced by many factors. Gravity plays animportant role in a number of them. In particular,buoyancy-driven convection can influence thestability and, thus, the shape of the solidifyinginterface. Data obtained about the conditionsunder which certain types of solidificationboundaries appear can help to explain theformation of the crystalline structure of amaterial.

Another area of interest in metals and alloysresearch in microgravity is multiphasesolidification. Certain materials, which are knownas eutectics and monotectics, transform from asingle phase liquid to substances of more thanone phase when they are solidified. When thesematerials are processed on Earth, the resultantsubstances have a structure that was influencedby gravity either due to buoyancy-drivenconvection or sedimentation. But when processedin microgravity, theory predicts that the endproduct should consist of an evenly dispersed,multiphase structure.

Eutectic solidification is when one liquid, ofuniform composition, forms with two distinctsolid phases. An example of such a material is thealloy manganese-bismuth. Solidifying liquid Mn-Bi results in two different solids, each of whichhas a chemical composition that differs from theliquid. One solid (the minor phase) is distributedas rods, particles, or layers throughout the othersolid (a continuous matrix, or major phase).

Monotectics are similar to eutectics, except that amonotectic liquid solidifies to form a solid and a


43

liquid (both of which are different in compositionfrom the original liquid). Al-In is a monotectic thatstarts out as indium dissolved completely inaluminum, but when the alloy is solidified underthe appropriate conditions, it forms a solidaluminum matrix with long thin “rods” of liquidindium inside it. As the system cools, the rods ofliquid indium freeze into solid rods. The indiumrods are dispersed within the structure of thesolidified material.

PolymersPolymers are macromolecules (very largemolecules) made up of numerous small repeatingmolecular units called monomers. They appearnaturally in wool, silk, and rubber and aremanufactured as acrylic, nylon, polyester, andplastic. Polymers are typically composed of longchains of monomers, appearing on the molecularscale as if they had a spine of particular elementssuch as carbon and nitrogen. The bondingbetween individual polymer molecules affects thematerial’s physical properties such as surfacetension, miscibility, and solubility. Manipulation ofthese bonds under microgravity conditions maylead to the development of processes to producepolymers with more uniform and controlledspecific properties. Important optoelectronic andphotonic applications are emerging for polymers,and many of the properties needed are affected bythe polymers’ crystallinity. This crystallinity,which is the extent to which chains of moleculesline up with each other when the polymer isformed, may be more easily understood andcontrolled when removed from the influence ofgravity.

Growing polymer crystals is more difficult thangrowing inorganic crystals (such as metals andalloys) because the individual polymer moleculesweigh more and are more structurally complex,which hinders their ability to attach to a growingcrystal in the correct position. Yet in microgravity,the process of polymer crystal growth can bestudied in a fundamental way, with specialattention to the effects of such variables as

Regular polyhedra (or the Platonic Solids) are listed and shownbelow.

Name Formed Bytetrahedron 4 trianglescube 6 squaresoctahedron 8 trianglesdodecahedron 12 pentagonsicosahedron 20 triangles

The Five Regular Polyhedra or Platonic Solids Tnp-Tetrahedron;second row left-Cube: second row right- Octahedron; third rowleft-Dodecahedron; third row right-Icosahedron.

Questions for Discussion• What do you think of as a cylinder and cone?• What are the general definitions cylinder and cone’?• What shapes are some mineral samples you have in your

classroom?• Investigate the crystalline structure of halite (rock salt),

fluorine, quartz, diamond, iron.


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NASA’s Enterprise for the HumanExploration and Development of Space

The goals of this Enterprise are to• Increase human knowledge of nature’s processes using the

space environment,• Explore and settle the Solar System,• Achieve routine space travel,• Enrich life on Earth through people living and working in

space.

Microgravity research will contribute to the areas of cryogenicfuel management, spacecraft systems, in-situ resource utilization,power generation and storage, life support, fire safety, spacestructures, and science exploration.

Elemental Percent Weight on Earth and Moon

Earth’s Crust Lunar HighlandSoils

O 47 45 Fe 5 5

Si 28 21Mg 2 4Ca 4 11Al 8 13Na 3 0K 3 0

temperature, compositional gradients, and thesize of individual polymer units on crystal growth.In addition, just as microgravity enables thegrowth of larger protein crystals, it may allowresearchers to grow single, large polymer crystalsfor use in studying properties of polymers anddetermining the effects of crystal defects on thoseproperties.

Microgravity Research andExploration

There is one endeavor for which microgravityresearch is essential. That is the goal of exploringnew frontiers of space and using the Moon andMars as stepping stones on our journey. Toachieve these goals, we must design effective lifesupport systems, habitation structures, andtransportation vehicles. To come up withworkable designs, we must have a thoroughunderstanding of how the liquids and gases thatwe need to sustain human, plant, and animal lifecan be obtained, transported, and maintained; ofhow structural materials can be formed in-situ(on site); and of what types of fuels and fueldelivery systems would allow us to get aroundmost efficiently. Microgravity research canprovide the insight needed to get us on our way.

The ability to use extraterrestrial resources is akey element in the exploration of the solarsystem. We believe that we can use the Moon asa research base to develop and improveprocesses for obtaining gases and water forhuman life support and plant growth; for creatingbuilding materials; and for producing propellantsand other products for transportation and powergeneration. Oxygen extracted from lunar rocksand soils will be used for life support and liquidoxygen fuel. A byproduct of the extraction ofoxygen from lunar minerals may be metals andsemiconductors such as magnesium, iron, andsilicon. Metals produced on the Moon andmaterial mined from the surface will then be usedfor construction of habitats, successiveprocessing plants, and solar cells.


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Current research in the areas of microgravityscience will guide our path as we develop themeans to use the Moon as a stepping stone toMars. Research into how granular materialsbehave under reduced gravity conditions will beimportant when we design equipment to mine andmove large amounts of lunar material. The abilityto extract gases and metals from mineralsrequires an understanding of how gases, liquids,and solids of different densities interact in lunargravity. Building blocks for habitats and otherstructures can be made from the lunar regolith.Research into sedimentation and sintering underreduced gravity conditions will lead to appropriatemanufacturing procedures. Experiments havealready been performed on the Space Shuttle todetermine how concrete and mortar mixes andcures in microgravity. An understanding of fluidflow and combustion processes is vital for all thematerials and gas production facilities that will beused on the Moon and beyond.

Science Standards

∆ o Earth and Space Science∆ o Physical Science

Regolith is a layer of powder-like dust and loose rock that restson bedrock. In the case of the moon, fragmentation of surfacerocks by meteorite bombardment created much of the regolithmaterial.

Microgravity Science Standards Science Primer - NASA · 2013-06-27 · Microgravity Science Primer ... of solids, liquids, ... composition. Such intermingling occurs in many situations,

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