Table Of ContentEffects of Reduced Gravity
Aaron Harrinarine Persad
Contents
Introduction....................................................................................... 2
AboutGravity..................................................................................... 2
GravityonEarth............................................................................... 3
ClarifyingTerminology....................................................................... 4
EffectsofNearWeightlessnessonPhysicalProcesses.......................................... 6
FluidStability................................................................................. 7
MultiphaseFlow.............................................................................. 8
Boiling......................................................................................... 10
SurfaceTensionEffectsandNegligibleBuoyancy.......................................... 13
Aggregation................................................................................... 15
EffectsofNearWeightlessnessonLifeProcesses............................................... 16
Humans........................................................................................ 16
Plants.......................................................................................... 20
ClosingRemarks.................................................................................. 22
Cross-References................................................................................. 22
References........................................................................................ 23
Abstract
TheaccelerationduetoEarth’sgravityiscalleda1genvironment.Humansare
well adapted to living in 1 g since, through observations and experiences, they
have developed an innate intuition of how the natural world behaves. For
example, it is known that liquids will settle to the bottom of their containers,
that objects thrown will follow a parabolic path as they fall toward the ground,
and that the flame of a birthday candle will burn with familiar color and shape.
A.H.Persad(*)
AstronautsforHire,Houston,TX,USA
ThermodynamicsandKineticsLaboratory,DepartmentofMechanicalandIndustrialEngineering,
UniversityofToronto,Toronto,ON,Canada
e-mail:[email protected];[email protected]
#HerMajestytheQueeninRightofCanada2016 1
E.Seedhouse,D.Shaler(eds.),HandbookofLifeSupportSystemsforSpacecraftand
ExtraterrestrialHabitats,DOI10.1007/978-3-319-09575-2_5-1
2 A.H.Persad
However, the near-weightless environment can affect both physical and life
processes and cause them to behave in seemingly counterintuitive fashions.
This chapter focuses on near weightlessness and its effect on the behavior of
several physical and life processes that are important to life support systems in
spacecraft and habitats. Physical processes that will be discussed include fluid
configurations, thermodynamic stability, aggregation, surface tension, and
thermocapillary flow. Life sciences discussions will include human physiology,
locomotionandperception,andplantbiology.
Introduction
This chapter begins with an overview of gravity in general, followed by a brief
descriptionofEarth’sgravitationalfield.Withthiscontext,thechapterproceedswith
acloserlookatlifesupportsystems.Investigationsaremadeoftheeffectsofreduced
gravityfromtheperspectiveofphysicalandlifesciences.Attheendofthischapter,
the reader will have an appreciation of the synergy required between physical and
lifesystemstotacklethechallengesofimplementinglifesupportsystemsinreduced
gravity.
About Gravity
Itmaybesurprisingtoknowthatthereisnoscientificexplanationofgravity(Brooks
2009). Theories exist to describe how gravity behaves and how it affects physical
matter and light, but the fundamental source of gravity remains a mystery. No
attemptwillbemadeheretosolvethemysteryofgravity.Instead,thefocusinthis
sectionistodefinegravitywithinthecontextofthischapter.
Gravity is an observable, natural event where all things are attracted to one
another,regardlessoftheirsizeorofthedistancebetweenthem.In1687,SirIsaac
Newtonhypothesizedthatthegravitationforceofattractionbetweentwobodieswas
proportionaltotheproductoftheirmassesandinverselyproportionaltothesquare
ofthedistancebetweenthem:
F¼Gm1m2, (1)
2
d
whereFisforceofattraction,m andm arethemassesofthetwoobjects,disthe
1 2
distancebetweentheobjects,andGisanempiricalproportionalityconstantknown
asthegravitationalconstant.TheearliestmeasurementofthevalueofGwasmadein
1798byHenryCavendish.ThestandardvalueofGasgivenbytheNationalInstitute
of Standards and Technology (NIST) is 6.67384 (cid:2) 10(cid:3)11 m3 kg(cid:3)1 s(cid:3)2, but recent
studies have raised questions about the accuracy of this value (Gibney 2014;
Andersonetal.2015).
EffectsofReducedGravity 3
Inamajorityofcases,Newton’slawofgravityworkswellindescribingcelestial
motionsandmostofone’severydayobservationsofgravity.However,formassive
objects,orforobjectsthatmovenearthespeedoflight,Newton’slawfails.Instead
ofdescribingtheeffectsofgravitationasanattractiveforce,AlbertEinsteinin1915
attributedgravitationtothecurvatureofspaceandtime.Einstein’sgeneraltheoryof
relativity has proven to be more accurate than Newton’s law, for example, in
describing the orbit of Mercury around the Sun. However, in the nonrelativistic
limit, Einstein’s general theory of relativity simplifies to Newton’s law. With this
brief overview of gravity, the next section takes a closer look at the Earth’s gravi-
tationalenvironment.
Gravity on Earth
For the purposes of this section, Newton’s gravitation law is taken as sufficient to
describe the attractive force that Earth’s gravitational field exerts on objects. An
objectinEarth’sgravitationfieldwillaccelerate toward theEarth duetothefield’s
strength.TheaccelerationduetoEarth’sgravitydependsonseveralfactorssuchas
time,thespatialdistributionoftheplanet’smaterialcomposition,elevation,latitude,
and the shape of the planet. When the Earth is approximated as spherical, the
standardvalue ofEarth’sgravityatthesurface oftheplanet istaken as9.80665m
s(cid:3)2(symbolically,thisgravityvalueisrepresentedas1g).Undertheseassumptions,
Newton’sgravitationlawinEq.1simplifiesto:
F! ¼m!g : (2)
g
The symbol ! in Eq. 2 – indicates that the parameter is a vector: it has both
!
magnitudeanddirectioncomponents.Inradialcoordinates, gisdirectedtowardthe
center of the Earth, whereas in Cartesian coordinates, it is directed downward. By
contrast, the mass of the object, denoted by m in Eq. 2, is a(cid:1)sca(cid:1)lar since it has no
! (cid:1)! (cid:1)
dependence on direction. The magnitude of F , denoted as (cid:1)F (cid:1), is known as the
(cid:1) (cid:1)g g
(cid:1)! (cid:1)
weightofanobject.NotefromEq.2that(cid:1)F (cid:1)¼0doesnotnecessarilyimplythat
g
m=0.
!
InEq.2,F indicatestheforcethatEarth’sgravityexertsonanobjectwithamass
g
ofm.AstheobjectfallstowardtheEarth,itwilleventuallyencounterasurface,such
asatabletop,andstopfalling.Afreebodydiagram(FBD)ishelpfulinillustrating
whatismeantbysayingthattheEarth’sgravityis1g.NotethatinFig.1,theobject
!
is exerting a force of F directed downward on the tabletop, whereas the tabletop
g
! ! !
exertsanequalandupwardnormalforceofF ontheobject(whereF ¼(cid:3)F and
N N g
theupwardforce(cid:3)isindi(cid:4)catedbythenegativesign).Thus,thenetforceactingonthe
! !
object is F þ (cid:3)F ¼0 and the object remains stationary. However, if the
g g
4 A.H.Persad
Fig.1 Freebodydiagramsof
objectsinfreefalloratreston
atabletop
!
tabletopweresuddenlyremoved,thenetforceactingontheobjectwouldbeF ,and
g
the object would continue to fall toward the Earth’s surface, only stopping once it
landedontheground,andtheEarth’ssurfaceexertedanequalandoppositeforceto
the weight of the object. In both cases, whether or not the object was falling, the
object was always subjected to the Earth’s gravity of 1 g. Thus, the 1 g of gravity
“felt”onEarthisactuallyaresultoftheground(orchair,orbed,etc.)pushingback
againsttheobject’sweight;iftherewerenothingtopushagainsttheobject,itwould
experiencefreefall.
Clarifying Terminology
Strictlyspeaking“gravity”hastheunitsofacceleration.However,thetermgravityis
sometimesusedtodescribe aforce.Therefore,onemustbecareful inone’sunder-
standing of the word “gravity” when it is used in the phrase “reduced gravity.”
Considerthefollowingexample:astronautsontheInternationalSpaceStation(ISS)
willoftentalkaboutwhatitisliketofloatinsidethestationandliveinzerogravity.
Numerouspressreleasesfromspaceagenciesalsousetheterm“zerogravity,”and
someevenstatethateverythinginthespacestationfloatsbecausethereisnogravity
upthere.Fromtheperspectiveofascientist,theclaimthatthereis“zerogravity”in
spaceisfalse;theMoonorbitstheEarthbecauseofgravity;andtheEarthisheldin
orbitaroundtheSunbecauseofgravity.Iftherereallywere“zerogravity”inspace,
then the space station would not stay in orbit around the Earth; instead, it would
vanishintodeepspace.
The term “zero gravity” or “zero-g” only makes sense inthe contextof aforce;
thereisnoforcepushingback(noopposingforce)tostopthespacestationandthe
astronautsfromfallingaroundtheEarth.Infact,abettertermtousewouldbe“free
fall” or “weightlessness” since these terms do not incorrectly imply that the accel-
erationduetoEarth’sgravityiszero.
EffectsofReducedGravity 5
ISS - mean height in km
418
416
414
412
410
408
406
404
402
400
398 ©Heavens-Above.com
Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun
2014 2015
Fig.2 TheInternationalSpaceStationhastobeperiodicallyre-boostedsinceitsorbitdecaysfrom
frictionwheninlowEarthorbit(Peat2015)
However, a scientist may still raise issue about using the terms “free fall” or
“weightlessness”sincethesetermsimplythatspaceisaperfectvacuum.Inreality,
space is filled with micrometeorites and nano-sized dust particles that produce
resistance or drag to spacecraft and other objects. The drag created by these
particles in space is the reason why the space station’s orbit gradually decays
and must be periodically re-boosted, as indicated in Fig. 2. For this reason (see
chapter “▶The Microgravity Environment”), some have used the term “micro-
gravity”(10(cid:3)6gorμg)tomoreaccuratelydescribetheenvironmentonspacecraft,
since achieving a 0 g environment would not be possible. However, the term
“microgravity” again confuses acceleration and forces. Thus, more appropriate
terms to use to describe the space environment are “near free fall” or “near
weightlessness.”
Theterm“reducedgravity”isdefinedasanenvironmentwherethegravitational
fieldislessthanthatoftheEarth,forexample,ontheMoon,onMars,onasteroids,
orindeepspace.Theterm“nearweightlessness”isusedinthischaptertodescribe
the space environment that is predominantly in Earth’s gravitational field (Fig. 3).
The remainder of this chapter will focus on the effects of the near-weightless
environment.
6 A.H.Persad
Fig.3 There isgravity in space.Objects ashighas 400km abovetheEarth’s surfacearewell
withinthepulloftheEarth’sgravitationalfieldandexperience86%ofthesurface’s1gacceleration
(WikimediaCommons2009)
Effects of Near Weightlessness on Physical Processes
Itmaynotbeimmediatelyclearwhatrolephysicalscienceshastoplayinbuildinga
lifesupportsystemforspace.However,considerthatalifesupportsystemmusthave
a reservoir of water and that water must be transported from the reservoir through
tubestoanoutlet,andtheremaybeairbubblesentrappedinthetubesaffectingthe
flow of water to the outlet. In this simple example alone, a scientist must use
thermodynamics to predict how the water behaves in the storage container, and an
engineermustusemultiphasefluiddynamicsandphysicstounderstandtheflowofa
water-air mixture in the tubes. This section takes a closer look at these and other
physicalscienceprocesses.
EffectsofReducedGravity 7
Fig.4 Glasscylindersofsimilarradii(approximately30mm),butdifferentheights.Bothcontain
similaramountsofpurewater.Atroomtemperatureontheground,thebulkliquidandvaporphases
arestableandseparatedbyasingle,flatliquid-vaporinterface
Fluid Stability
Life support systems in space require that fluids such as water, liquid oxygen, and
liquidammoniabestoredinreservoirs(seechapters“▶PotableWaterSupply”and
“▶EssentialsofLifeSupportSystems:Water”).Thesereservoirscanbeflexibleor
rigid and are typically closed to the environment. The fluids can be removed or
addedtothereservoirsthroughvalvesandtubes.Ifthestoredfluidisvolatile,both
itsvaporandliquidphasesmayexistsimultaneouslyinthereservoir.Ontheground,
thereaderknowsthatliquidwaterinaclosedbottlewillalwaysbeatthebottomof
the bottle and the water vapor will be on top, with a single liquid-vapor interface
between the two phases (Fig. 4). Furthermore, the reader knows that the single-
interface configuration is stable; if one were to shake (perturb) the water bottle so
thattheliquidandvaporphasessloshedandmixed,onewouldexpecttheliquidto
eventuallysettlebackdowntothebottom.Thus,ontheground,thesingle-interface
configuration is the stable configuration for the closed water liquid-vapor system.
This makes it relatively easy for a life support system engineer to know where to
placetubestodrawouttheliquidwaterwhenontheground.
However, in near weightlessness, where should the engineer place the tubes to
drawoutliquidwaterfromthereservoir?Whatiftheengineeronlywantedtodraw
out water vapor? To answer these fundamental questions, the engineer needs to
know the stable configuration of the fluid held in the reservoir. Consider the two
glass cylinders shown in Fig. 4 which are partially filled with water and on the
ground (note that “on the ground” is a phrase used to indicate being in 1 g
environment). The cylinders have similar diameters, but different heights. There is
approximatelythesameamountofliquidwaterineachcylinder,andbothhavethe
8 A.H.Persad
single-interfaceconfiguration.Figure5indicatessomepossibleconfigurationsofthe
water in the cylinders when in near weightlessness; it may be challenging to
intuitivelyidentifywhichwillbethefinalconfigurationofthewaterineachcylinder.
Using thermodynamics, the stable equilibrium configuration of the water in
eachcylindercanbepredicted.Thedetailsofthetheorymaybefoundelsewhere
(Sasges et al. 1996; Ward et al. 2000), but the important result is that the
temperature, amount of fluid present in the container, and contact angle of the
liquid with the container’s wall are the factors that determine the equilibrium
configuration. The water in the tall cylinder in Fig. 4 is predicted to take on the
“double-interfaceconfiguration”showninFig.5,whereasthewaterinthesmaller
cylinderinFig.4ispredictedtotransitiontothe“bubbleconfiguration”indicated
in Fig. 5. In 1997, a series of water cylinder systems were flown on the Space
Shuttle mission STS-87 to test the thermodynamic prediction that the confined
waterinthetallercylinderwouldtransitiontothedouble-interfaceconfiguration
(Wardetal.2000).AsillustratedinFig.6,theexperimentalobservationsagreed
withthepredictions.
Multiphase Flow
Anotherimportantaspectofanyspacelifesupportsystemistherecyclingofwater
(see chapters “▶Potable Water Supply,” “▶Essentials of Life Support Systems:
Water,”“▶MethodsofWaterManagement,”“▶MethodsofWaterRecovery,”and
“▶Water Quality Monitoring”). Currently, the Water Recovery System (WRS) on
theISS(seeFig.7)isable torecycle upto93%oftheliquids itprocesses(NASA
2012). The efficiency of the WRS is sufficient to support the ISS crew of six, but
onlywhencomplementedwithperiodicresupplyshipmentsofwaterfromtheEarth.
Indeepspaceorondistantplanetary habitats,therewillbemuchlesscapabilityto
resupplycrewwithconsumablessuchaswaterfromEarth.Waterrecyclingsystems
inthosedistanthabitatswouldhavetoapproach100%efficiency.
The influent wastewater through the WRS may contain solid particulates and
gases, producing multiphase flows in tubes (see chapters “▶Manned Spaceflight
WasteManagement,”“▶EssentialsofLifeSupportSystems:WasteManagement,”
and “▶Waste Management”). On the ground, engineers would expect that heavy
solidscollectatthebottomofthetube,whilethelightergasesflowatthetopofthe
tube. This aids in separating the phases. However, as the velocity of the flow
increases, the flow patterns change, as indicated in Fig. 8 for the flow of a liquid-
gas system. In near weightlessness, the number of flow patterns is simplified since
thedensitydifferencebetweentheliquidandgaswillnotcausethe“lighter”gasto
stay above the liquid. As indicated in Fig. 9, the three main flow regimes in near
weightlessnessarebubbly,slug,andannular(McQuillenetal.2003).Afundamental
understanding of how fluids flow in pipes of different sizes, shapes, and junctions
willbeimportanttothedesignofanylifesupportsystem.
EffectsofReducedGravity 9
H
b c
θ θ
Liquid
Vapor Vapor
R
c L
2
θ
c h c
Single-Interface
Configuration Double- Interface
Configuration
Liquid
R
R d
Vapor Vapor b
Liquid
Vapor
Liquid Vapor
θ θ
Bridge Configuration Bubble Droplet
Configuration Configuration
θ
Rsd Rsb
Liquid
θ
Vapor
Vapor
Liquid
Sessile Droplet Sessile Bubble
Configuration Configuration
Fig. 5 In free fall, a single-component (water), vapor-liquid system confined to an isothermal,
closedaxisymmetriccylindricalvesselmayreachanyoneofanumberofpossiblestableequilib-
riumconfigurationsshown(Sasgesetal.1996)
10 A.H.Persad
Fig.6 Thepredicted
configurationofwaterina
glasscylinderinnear
weightlessnessisshownto
agreecloselywith
observationsmadeonthe
SpaceShuttlemissionSTS-87
(Wardetal.2000)
Fig. 7 The Water Recovery System is used onboard the International Space Station to convert
wastewaterintopotablewater,therebyconservingresources(NASA2012)
Boiling
In the WRS, water is recycled through a distillation process (NASA 2012). The
reader may already be familiar with water distillation on the ground: a pool of
wastewater is first boiled and turned into steam; the steam risesand leaves heavier
impuritiesbehindthepool;thesteamiscooledandrecondensedbackintoastorage
containerascleanwater.ThisprocessisanalogoustotheEarth’shydrologicalcycle,
Fig.10.However,inthenear-weightless environment,theprocessofdistillation is
complicatedbyseveralfactors.
