Spacecraft Power Systems: Chapter 2. Near - Earth Space Enviroment

Chapter 2Near-Earth Space Environment

2.1 Introduction

The near-Earth space environment, i.e., thermosphere and ionosphere,presents a substantial risk to space systems. The spacecraft power systemmust withstand the space environment and meet full performancespecifications over the entire mission life, starting from the launch phaseto disposal at the end of life. General design criteria required for thispurpose are studied and documented by space research organizations suchas NASA.1 As the solar array is directly exposed to the space environment,it is particularly vulnerable to damage. In fact, the rate of damage on a solararray determines the spacecraft life in many cases. The environmentalfactors also affect the overall design of all other components of the powersystem.

2.2 Launch and Transfer Orbit Environment

High levels of acceleration, shock, and vibration are present during lift-offand during pyro-driven deployments at various stages. The resulting stresslevels, varying with the launch vehicle, have effects on the power systemdesign, particularly on the solar panels. For example, the solar panels arerequired to withstand the launch acceleration of about 3g on the spaceshuttle Orbiter, and about 10g on the launch vehicle Saturn, where g is theacceleration due to gravity on the Earth’s surface. The pyro shock levelsduring deployments can be extremely high for a few milliseconds. Theshock spectrum is generally characterized by high-frequency components.The peak shock level in Ariane during payload separation, for example,could be about 2000g at frequencies above 1.5 kHz.

In transfer orbit, the solar panel is still folded, but must withstand theaccelerating force at perigee and the braking force at apogee. Thermally, theouter solar panels must withstand the Earth’s heat radiation and albedo, inaddition to the solar radiation, without exceeding the specified temperaturelimit.

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2.3 On-orbit Environment

The power system design is influenced primarily by the following on-orbitenvironmental factors.

2.3.1 Lack of Gravity and Atmosphere

The lack of gravity and atmosphere in space has significant effects on thepower system design. Zero gravity and vacuum jointly deprive allspacecraft equipment from the natural convection cooling which is normalon Earth. The internal thermal design in space, therefore, depends primarilyon conduction cooling and, to some extent, on radiation cooling. The heatrejection to the outer space, however, must depend only on radiationcooling.

The vacuum causes sublimation and outgassing, more in some materialsthan others. The subsequent condensation of the sublimed vapor on coldsurfaces can cause short circuits in electrical parts. Since zinc has a highsublimation rate, and some polymers have high outgassing rates, their usein space is restricted. The solar array current collecting slip rings andbrushes can experience cold welding under high contact pressure in avacuum. The use of lubricants having low sublimation rates, or keepingthem in sealed pressurized enclosures, is therefore important.

2.3.2 Magnetic Field

The interaction of the Earth’s magnetic field, B, and the magnetic dipolemoment, M, created by an electrical current loop produces torque, T, on thespacecraft. The cross-vector product of M and B gives this torque (in Nm),i.e.,

T ¼ M � B ð2:1Þ

where M is expressed in A�m2 and B in tesla. The magnetic moment isproduced by current loops in the spacecraft power circuits. The moment (inA �m2) is defined as the vector product of the current and the loop area, i.e.,M ¼ I � A. Its direction is perpendicular to the loop area as shown in Figure2.1.

In the geostationary orbit, the normal component of B is constant around0.104 mT, and the radial component varies between �0.042 mT as the satelliterevolves around the Earth. In orbits at other altitudes, the magnetic fieldvaries inversely with the orbit radius cubed. The normal component of Bproduces a torque in the equatorial plane, averaging to zero over the orbitperiod. Both components of the torque affect the satellite’s attitude. Thepower system contribution to the magnetic torque comes from the solarpanel, battery, and wires connecting various system components.

Near-Earth Space Environment 27


The magnetic moment of the spacecraft is minimized by design andcompensation in the following ways:

� Lay out the current carrying conductors such that they form thesmallest possible loops.

� Make two adjacent loops in opposite directions, so that they compen-sate each other.

� Twist wires wherever possible to neutralize the moments of adjacenttwists.

After using one or more of the above design means, the net residualmagnetic moment is determined by tests after the satellite is assembled. Amoment that would limit the resulting torque below 100 mNm in theoperational orbit is generally considered acceptable.

2.3.3 Meteoroids and Debris

Impact of solid objects can cause damage to the solar array. Small particlesnot large enough to cause immediate damage can cause gradual degrada-tion in output power over time. The particle mass and the hit rate (flux)depends on the orbit. Several NASA studies conducted for predicting themeteoroid flux in the Earth’s orbit have given consistent results. They showthat the average number varies inversely with the meteoroid mass as givenby the following simple expression:

meteoroid flux ¼1

mð2:2Þ

where the flux is measured in number of particles of mass greater than mgrams per square kilometer per year. The most common meteoroids aresmall, having masses between 0.1 and 100 mg, generally called micro-meteoroids. The probability of having n impacts with particles of massbetween m1 and m2 on a given surface area during y years is estimatedusing Poisson’s probability distribution function. In the absence of accurateinformation, their mass density is assumed to be 0.5 g/cm3 and a mean

28 Spacecraft Power Systems


FIGURE 2.1 Definition of magnetic moment.

impact speed of 20 km/s. The actual fluence varies with the orbit. Forexample, in the geostationary orbit of TDRS, the estimated meteoroidfluence for a 15-year mission is shown in Figure 2.2. The yearly fluence ratein other orbits is shown in Figure 2.3.

In addition to the natural micrometeoroids, the man-made debris canvary from 1 mm to 10 mm in diameter. They have an average density equalto that of aluminum, the most widely used material for satellite and launchvehicle components. Their relative velocity with respect to the spacecraftvaries from zero to twice the orbital velocity, with an average around10 km/s. The impact energy of large meteoroids can instantly damage thesolar array protective glass and cells. Micrometeoroids, on the other hand,erode the glass surface gradually with subsequent degradation in the poweroutput over time.

2.3.4 Atomic Oxygen

Atomic oxygen is present in low Earth orbit. It severely erodes somematerials, such as silver, which is extensively used in the solar array



FIGURE 2.2 TDRS meteoroid environment fluence over 15 years.

FIGURE 2.3 Debris flux rate estimate in various orbits.

construction. Similar erosion is also seen in some electrical insulation suchas KaptonTM and silicon rubber. The erosion comes not only from thechemical activity, but also in large part by the atoms travelling atseveral km/s velocity relative to the spacecraft. In addition to the erosion(surface recession), atomic oxygen can also form stable oxides on the metalsurface. For these reasons, exposed bare silver in solar arrays and silicon orKapton insulation on electrical wires are undesirable. This topic is dealtwith in Chapter 5.

2.3.5 Charged Particles

The sun continuously radiates energy in several forms — visible light,invisible infrared, ultraviolet, X-rays, gamma rays, radio waves, electrons,protons, and plasma (electrically charged hot gas). The abundance ofcharged particles that come with the sun’s radiation makes space a hostileenvironment. The energy in these particles impinging on a surface causesdamage, which accumulates over a period of time. The following terms arewidely used in discussing the radiation of charged particles in the spaceenvironment.

One million electron volts (1 MeV) is the unit of the equivalent energylevel of various charged particles. It is defined as the energy releasedby the total charge of one million electrons falling through one voltin the electric potential field. With the electron charge of0:1592� 10�18 C, 1 MeV equals 0:1592� 10�12 J.

Flux density expresses the number of charged particles present per unitvolume.

Flux is the term that expresses the rate of flux hitting a surface. It is givenas the number of particles impinging upon a unit surface area perunit time, i.e., in number/m2 s or in MeV/m2 s to account for theirtotal energy level. The flux varies as the spacecraft moves around theorbit. It is greater on the sun-side of the Earth than on the farther side.Often the orbit average flux is quoted.

Integrated flux or fluence expresses the accumulated number of particlesimpinging upon a unit surface area over some time duration in orbit.Its unit is number/m2 year or often MeV/m2 over the mission life toaccount for the total energy level the surface must withstand.

Dose expresses the amount of energy absorbed per unit mass of thesubject matter. It is measured in rad or rad (Si). Since silicon makesup the bulk of micro-electronic devices, it is taken as the referencematerial for comparing the radiation energy. One rad is equal to 100ergs of absorbed energy in 1 g of target material. One rad (Si) is the



unit of energy that will generate 1.7 � 1013 electron–hole pairs in 1 gof silicon.

The common sources of charged particles in the space environment are asfollows:

Solar radiation, consisting of mostly electrons and protons coming fromthe sun.

Solar wind brings bursts of charged particles in addition to the normalradiation from the sun. It is primarily made of protons and someelectrons radiating from the sun. Their flux and energy level aroundthe Earth’s orbit are low during normal solar wind. The mean protonflux is about 200 million protons/cm2 with a mean energy level ofseveral keV. However, the energy levels can reach 100 MeV duringsolar flares, and even 1 GeV during intense flares.

Cosmic radiation comes mostly from the outer space and some probablyfrom the sun. It consists of about 85% proton, 12% alpha particlesand 3% electrons. The proton energy level is high in GeV, but the fluxrate is low: in the order of a few particles/cm2s.

2.4 Van Allen Belts

The Earth’s magnetic field covers the magnetosphere, a doughnut shapedregion of space influenced by the Earth’s magnetic field. It acts on theelectrons and protons coming from the space. The Van Allen radiation beltsare parts of the magnetosphere that contain a large number of particles. Themagnetosphere normally shields the Earth from these particles. However,when a disturbance on the sun radiates an abundance of particles, somereach the Earth’s atmosphere near the magnetic poles and cause a glowknown as an aurora, popularly known as the northern lights (or thesouthern lights, in the southern hemisphere).

The Earth’s magnetic field varying with the radius cubed has a radialgradient, and it converges near the magnetic poles a shown in Figure 2.4.The solar wind brings charged particles to the Earth’s magnetic field. Theseparticles are deflected by the Lorentz force, F, given by the following cross-vector product:

F ¼ qV � B ð2:3Þ

where q is the particle charge, V is the particle velocity, and B is the Earth’smagnetic field intensity. The particle moving radially towards the Earth atvelocity V generates a circumferential force that will impart velocity V�. Theresultant velocity is then VR, as shown in the plan view. The radial gradient



in the field imparts a spiral motion to the particle. The spiraling particlecomes to rest once all the energy is absorbed, and then springs back andforth as explained below.

A particle moving in a converging magnetic field generates a force thatpushes the particle into a weak magnetic field (Figure 2.5), causing it to driftat velocity Vz. The Vz decreases as the particle moves towards the equatorialplane of weak magnetic field; and V� increases to keep the kinetic energyconstant. The particle eventually comes to a stop in the z-direction and isreflected back into the weaker field. Since the field is again converging onthe other side, the particle is reflected back again. Thus, the converging fieldconfines the charge particle between two magnetic mirrors. This way, most



FIGURE 2.4 Lorentz force on a charged particle moving in Earth’s magnetic field.

FIGURE 2.5 Charged particle moving in a convergent magnetic field.

particles remain trapped in a certain belt all the time. However, particleshaving sufficiently high axial kinetic energy do not get reflected, and escapethe confinement belt.

Most particles become trapped in two doughnut-shaped radiation beltsknown as the Van Allen belts, which are a part of the magnetosphere. Thebelts are stronger on the sun-side than on the side away from the sun, asshown in Figure 2.6. The trapping is concentrated in two belts shown inFigure 2.7. The electrons get trapped in a belt spanning 2 to 5 Earth radiiwith a heavy concentration between 3 and 4 Earth radii. The high-energyprotons penetrate closer to the Earth and get trapped in another beltspanning around 1 to 2 Earth radii with a heavy concentration around 1.5Earth radius. Since the charged particle flux varies over the satellite orbit,the integrated radiation flux (fluence) is used in the power system design.



FIGURE 2.6 Van Allen radiation belts.

FIGURE 2.7 Extent of Van Allen radiation belts around Earth per NASA’s AP-8 EnvironmentModel.

The solar array, in particular, requires protection against radiationdamage. The geosynchronous satellites orbiting at 6.6 times the Earthradius are way outside the inner proton belt and at the outer edge of theouter electron belt. Therefore, they are exposed to only a small part of theouter electron belt. However, they are more vulnerable to high-energyproton generated by solar flares.

2.5 Solar Wind and Solar Flare

A continuous stream of protons, electrons, and ionized gas leaving the sunand sweeping the solar system is called the solar wind. It is very thin,containing about 5 charged particles per cubic centimeter of space. Thesolar wind streams constantly, but not evenly. These particles travel pastthe Earth at speeds of about 480 km/s. In comparison, the Earth travels inits orbit at 30 km/s. When these particles flow past the Earth, some gettrapped in the Earth’s magnetosphere.

A solar flare is a gush of ionized gas with charged particles spewed outby the corona, the sun’s glowing outer shell. It is associated with darksunspots bounded by intense magnetic fields that come and go in 11-yearcycles. The flare can cause auroras and disturb radio signals on Earth. Theionized gas is extremely hot (>1 million �C), such that atoms of hydrogenand helium are homogenized into dilute plasma, composed mainly ofnegatively charged electrons and positively charged protons. Although thisconstitutes a million tons of matter moving at a million miles per hour, itsdensity is so low that it is still considered a vacuum. However, its effectscan be devastating as depicted in Figure 2.8.

Solar flares occur in 11-year cycles. The last major period was Novemberand December 2000. Several flares occur during each solar cycle in which 7years are active and 4 inactive. The intensity levels of the flares vary over awide range. Therefore, their energy levels and the corresponding prob-abilities that must be considered in designing the power system aremutually agreed upon by the satellite customer and the manufacture. Therelatively strong 1989–1990 flares were extensively analyzed by spacecraftengineers because of the quantity and quality of data obtained and theirimpact on many spacecraft in orbit at the time. The newly developed modelsuggests that the proton fluences below 10 MeV were significantlyunderestimated in the past. This causes more degradation of the photo-voltaic (PV) cells in the Isc, although not much in Voc. From this study, it isestimated that a solar array design based on the old King model2 of 1974would have about 20% probability of missing the power output require-ments by a few percent compared to the new Feynman model3 of 1989–1990flares.



Table 2.1 gives the total annual fluence in a typical geosynchronous orbit.The integrated flux of solar flare protons per 11-year cycle based on NASA/JPL model with 80% confidence level is given in Table 2.2.

Spacecraft going beyond the solar system, such as Pioneer and Voyager,experience the so-called termination shock wave at the edge of the solarsystem. The termination shock wave is a zone around the solar systemwhere the solar wind crashes into the tenuous gas and dust that fillsinterstellar space.



FIGURE 2.8 Solar flare and effects on Earth.

Table 2.1 Total annual fluence in a typical geosynchronous orbit(number/cm2)a

Particle type Normal solar flares Intense solar flares

Trapped electrons � 1014 � 1014

Trapped protons Negligible NegligibleHigh energy solar flare protons � 107 � 1010

aThe energy level of the electrons varies from a few keV to several MeV,and that of protons in several hundreds MeV range.

2.6 Geomagnetic Storm

There is a well-recognized link between solar activity, disturbances in thegeomagnetic field, and disruptions to man-made systems such as satellites,communications networks, defense systems, and even the sturdy powergrids on Earth. Analysis suggests that these vulnerabilities to magneticdisturbances will continue and perhaps even increase as these systemscontinue to evolve and grow.

What could happen to the spacecraft power system under the geomag-netic storm can perhaps be appreciated by examining what has happened tothe power grids on Earth in the past. Under such storms, the magnetic fieldof the Earth is disturbed, causing the induced currents in overheadtransmission lines. The high-voltage lines, due to their long length andlarge loop area with the Earth’s surface, can induce a high current. This lowfrequency d.c.-like geomagnetically induced current then enters the highvoltage transformers, driving them in deep magnetic saturation, causing thevoltage collapse and severe overheating. The great storm of March 13, 1989affected large areas of North America with a power blackout for 6 millionHydro Quebec customers in Canada for a long time. Voltage regulation wasaffected, undesired relay operations occurred on system equipment, andnew areas of vulnerability were exposed due to unanticipated commonmode currents saturating the transformer cores, drawing high currents andtripping off the transmission lines. Less severe storms in September 1989,March 1991, and October 1991 reinforced the fact that geomagneticdisturbances can hamper utility operation. The October 2003 storm knockedout satellites (e.g., Japan’s Earth observation satellite ADEOS-11), hamperedradio communication and caused an electrical blackout in Sweden. Toprotect airline passengers against the charged particles, the U.S. Federal



Table 2.2 Integratedflux of solar flareprotons per 11-yearsolar cyclea

Energy

(>MeV)

Solar protons

(number/cm2)

1 5.3� 1011

5 2.0� 1011

10 1.0� 1011

30 2.3� 1010

50 9.7� 109

70 6.6� 109

100 2.5� 109

aBased on the NASA/JPL model with 80%confidence level.

Aviation Administration warned pilots to stay below 25,000 feet whenflying near the poles in order to avoid excess radiation. The storm arrivedwithin a few hours of NOAA’s prediction time.

The space research models of plasma population and circulation,monitoring the interplanetary real-time solar wind and the geomagneticallyinduced currents, and tracking the auroral zone current can provide timelyinformation for the next solar maxima, which begins in 2010–2011.

Major effects of solar flares and geomagnetic storms on the spacecraftpower system are:

� Rapid degradation of the solar array power generation. This is a life-shortening, not a life-threatening, effect.

� Common-mode EMI noise entering the electrical power systemcomponents through exposed cables. This can upset the power systempossibly causing a permanent damage. In the 1990s, the power systemsof a few communications satellites (e.g., Anik-E1 of Canada and Intelsat-K) were damaged, and a few were suspected to have failed underintense solar flares. An in-depth investigation indicated that a sparkcaused by geomagnetic storm shorted out connections between thesatellite solar panels and several dozens of relays.

2.7 Nuclear Threat

Some defense satellites are required to withstand a man-made threat ofnuclear detonation of a specified intensity. The energetic particles releasedduring a nuclear event are fission electrons, neutrons, gamma rays andX-rays. Their energy levels depend solely on the destructive power ofthe nuclear device deployed. The threat level specified for the design isdetermined from the probability and the consequence considerations.The level of nuclear threat, however, is always a classified number underthe U.S. Department of Defense (DoD) secret classification system.

2.8 Total Radiation Fluence

The energy levels of various sources of the natural radiation in the Earth’sorbit are as follows:

� Trapped electrons 0.1 to 7 MeV

� Trapped protons 0.1 to 100 MeV

� Solar flare protons 1 to 200 MeV

� Solar flare alphas 1 to 300 MeV

� Galactic cosmic rays >1 GeV



The total fluence is calculated for a given mission using the environmentalmodel specified by the customer. As an example, the orbit integrated flux oftrapped proton and electron environment in a 852-km, 99�-inclined circularorbit is shown in Figures 2.9 and 2.10, where the y-axis is the fluence withenergy levels on the x-axis.4

The total fluence in equivalent MeV/cm2 day is the area under such anumber (n) versus energy level (E) curves for the electrons, protons, solarflares, and any other source of radiation, natural or man-made. That is,



FIGURE 2.9 Trapped proton environment in 852-km, 99�-inclined circular orbit.(Source: NASA/GSFC.)

FIGURE 2.10 Trapped electron environment in 852-km, 99�-inclined circular orbit.(Source: NASA/GSFC.)

fluence ¼

Z1o

ndE electrons þ

Z1o

protonsþ

Z1o

flaresþ

Z1o

nuclear

þ

Z1o

any other

ð2:4Þ

The integrated fluence determined by Equation 2.4 is then used todetermine the damage on the solar array and other components duringthe mission life.

Several recently completed cross-disciplinary scientific and engineeringresearch projects are significantly improving space environment character-ization. Operational users of space systems are the direct beneficiaries ofthese research advances.

References

1. NASA, ‘‘Space Vehicle Design Criteria (Environment),’’ Technical Report No.SP-8005, 1980.

2. King, J.H., ‘‘Solar proton fluence for 1977–1983 space missions,’’ Journal ofSpacecraft and Rockets, 11, 401–407, 1974.

3. Feynman, J., ‘‘New interplanetary proton fluence model,’’ Journal ofSpacecraft, 27(4), 403–408, 1990.

4. Stassinopoulos, J.M., Barth, J.M., and Smith, R.L., ‘‘METSAT ChargedParticle Environment Study, Revised Edition, Method 2,’’ NASA Report No.GSFC X-600-87-11, 1987.



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Spacecraft Power Systems: Chapter 2. Near - Earth Space Enviroment