the natural space environment: effects on spacecraft

The Natural Space Environment:Effects on Spacecraft

NASA Reference Publication 1350

November 1994

Bonnie F. James, CoordinatorO.W. Norton, Compiler, andMargaret B. Alexander, Editor

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Thermal Environment

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Solar Environment

Ionizing Radiation

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Gravitational Field

Solar Environment

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Solar Environment

Ionizing Radiation

Geomagnetic

Field

Gravitational Field

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Downloaded from http://www.everyspec.com

The Natural Space Environment:Effects on Spacecraft

National Aeronautics and Space AdministrationMarshall Space Flight Center • MSFC, Alabama 35812

NASA Reference Publication 1350

November 1994

Bonnie F. James, CoordinatorO.W. Norton, Compiler, andMargaret B. Alexander, EditorMarshall Space Flight Center • MSFC, Alabama

Prepared byElectromagnetics and Environments BranchSystems Definition DivisionSystems Analysis and Integration LaboratoryScience and Engineering Directorate


ii

PREFACE

The effects of the natural space environment on spacecraft design, development, andoperations are the topic of a series of NASA Reference Publications currently being developed by theElectromagnetics and Environments Branch, Systems Analysis and Integration Laboratory,Marshall Space Flight Center.

This primer provides an overview of the natural space environment and its effects onspacecraft design, development, and operations, and also highlights some of the new developmentsin science and technology for natural space environment. It is hoped that a better understanding ofthe space environment and its effects on spacecraft will enable program management to moreeffectively minimize program risks and costs, optimize design quality, and successfully achievemission objectives.


iii

TABLE OF CONTENTS

Page

INTRODUCTION 1

NEUTRAL THERMOSPHERE 5

THERMAL ENVIRONMENT 8

PLASMA 11

METEOROID/ORBITAL DEBRIS 15

SOLAR ENVIRONMENT 18

IONIZING RADIATION 20

GEOMAGNETIC FIELD 22

GRAVITATIONAL FIELD 24

CONCLUSION 25


iv

LIST OF ACRONYMS

AO atomic oxygen

CRRES combined release and radiation effects satellite

DMSP defense meteorological satellite program

EMI electromagnetic interference

ERBE Earth radiation budget experiment

EUV extreme ultraviolet

GCM general circulation model

GCR galactic cosmic rays

GN&C guidance, navigation, and control

IGRF international geomagnetic reference field

IRI international reference ionosphere

LEO low-Earth orbit

MET Marshall engineering thermosphere

M/OD meteoroid/orbital debris

MSFC Marshall Space Flight Center

MSIS mass spectrometer incoherent scatter

NASA National Aeronautics and Space Administration

NOAA National Oceanic and Atmospheric Administration

NORAD North American Air Defense Command

OLR outgoing long-wave radiation

POLAR potential of large spacecraft in auroral regions (computer model)

SAMPEX solar, anomalous, and magnetospheric particle explorer

S/C spacecraft

SAA South Atlantic anomaly

UV ultraviolet


REFERENCE PUBLICATION

THE NATURAL SPACE ENVIRONMENT: EFFECTS ON SPACECRAFT

INTRODUCTION

The natural space environment refers to the environment as it occurs independent of thepresence of a spacecraft; thus, it includes both naturally occurring phenomena such as atomic oxygen(AO) and atmospheric density, ionizing radiation, plasma, etc., and a few man-made factors such asorbital debris. Figures 1 through 3 list the breakout of the natural space environments and theirmajor areas of interaction with spacecraft systems.

This primer provides an overview of these natural space environments and their effect onspacecraft design, development, and operations, and it also highlights some of the new devel-opments in science and technology for each space environment.

Understanding these natural space environments and their effect on spacecraft enables pro-gram management to more effectively optimize the following aspects of a spacecraft mission:

• Risk—Increasingly, experience on past missions is enabling NASA to provide statisticaldescriptions of important environmental factors, thus enabling the manager to make informeddecisions on design options.

• Cost—Selection of design concepts and missions profiles, especially orbit inclination and altitudewhich minimizes adverse environmental impacts, is the first important step toward a simple,effective, high-quality spacecraft design and low operational costs.

• Quality—New environment simulators and models provide effective tools for optimizing sub-system designs and mission operations.

• Weight—Consideration of environmental effects early in the mission design cycle helps to mini-mize weight impacts at later stages. For example, early consideration of directionality effects inthe orbital debris and ionizing radiation environments could lead to reduced shielding weights.

• Verification—A unified, complete environments description coupled with a clear mission profileprovides a sound basis for analysis and test requirements in the verification process and elimi-nates contradictory, unnecessary, and/or incomplete performance assessments.

• Science and Technology—The natural space environment is not static. Not only is our under-standing improving, but new things occur in nature which have not been observed before (forexample, a new transient radiation belt was recently encountered). Perhaps more importantly,engineering technology is constantly changing and with this the susceptibility of spacecraft toenvironmental factors. Early consideration of these factors is key to converging quickly on aquality system design and to successfully achieving mission objectives.


2

Nat

ura

l Sp

ace

En

viro

nm

ents

NE

UT

RA

L

TH

ER

MO

SP

HE

RE

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ER

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T

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ITA

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HE

RE

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&C

sys

tem

des

ign,

Mat

eria

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egra

datio

n/

surf

ace

eros

ion

(ato

mic

oxy

gen

fluen

ces)

, D

rag/

deca

y, S

/C li

fetim

e, C

ollis

ion

avoi

danc

e,

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sor

poin

ting,

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erim

ent d

esig

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rbita

l po

sitio

nal e

rror

s, T

rack

ing

loss

Pas

sive

and

act

ive

ther

mal

con

trol

sys

tem

de

sign

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iato

r si

zing

/mat

eria

l sel

ectio

n,

Pow

er a

lloca

tion,

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ar a

rray

des

ign

EM

I, S

/C p

ower

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tem

s de

sign

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ater

ial d

eter

min

atio

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/C h

eatin

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S/C

cha

rgin

g/ar

cing

Col

lisio

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nce,

Cre

w s

urvi

vabi

lity,

S

econ

ary

ejec

ta e

ffect

s, S

truc

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l des

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ield

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eria

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olar

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el d

eter

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ar p

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ntry

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odel

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erat

ions

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lect

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ingl

e ev

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ting

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atio

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ital m

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entr

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ater

ials

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Tet

her

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rimen

t des

ign

Figu

re 1

. A

bre

akou

t of

the

natu

ral S

pace

env

iron

men

ts a

nd ty

pica

l pro

gram

mat

ic c

once

rns.

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ITIO

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GR

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MA

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UE

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OD

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MS

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& E

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riatio

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posi

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(Ato

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Oxy

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ds

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ar r

adia

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(alb

edo

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var

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adia

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sfer

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osph

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oral

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sma,

Mag

neto

sphe

ric

plas

ma

M/O

D fl

ux, S

ize

dist

ribut

ion,

M

ass

dist

ribut

ion,

Vel

ocity

di

strib

utio

n, D

irect

iona

lity

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hysi

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ynam

ics,

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eom

etric

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olar

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netic

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olar

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olar

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Tra

pped

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ton/

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tron

rad

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Gal

actic

cos

mic

ray

s (G

CR

’s),

S

olar

par

ticle

eve

nts

Nat

ural

mag

netic

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d

Nat

ural

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ld

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osph

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den

sity

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riatio

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at

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ind

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Gen

eral

Circ

ulat

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Mod

els

(GC

M’s

)

Inte

rnat

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l Ref

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ce

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sphe

re M

odel

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AS

CA

P/L

EO

N

AS

CA

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LAR

Flu

x m

odel

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EL

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rato

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odel

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pr

edic

tion

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tistic

al

mod

els,

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ar d

atab

ase

CR

EM

E, A

E-8

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, A

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olpr

o,

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IELD

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E

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0, U

AR

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atab

ase,

“s

cien

ce”

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AM


3

Sp

ace

En

viro

nm

ent

Eff

ects

Figu

re 2

. Sp

ace

envi

ronm

ent e

ffec

ts o

n sp

acec

raft

sub

syst

ems.

SP

AC

EC

RA

FT

S

UB

SY

ST

EM

S

Avi

on

ics

Ele

ctri

cal P

ow

er

GN

&C

/Po

inti

ng

Mat

eria

ls

Op

tics

Pro

pu

lsio

n

Str

uct

ure

s

Tel

emet

ry,

Tra

ckin

g, a

nd

C

om

mu

nic

atio

ns

Th

erm

al C

on

tro

l

Mis

sio

n

Op

erat

ion

s

Neu

tral

Th

erm

osp

her

eT

her

mal

En

viro

nm

ent

Pla

sma

Met

eoro

ids/

Orb

ital

Deb

ris

SP

AC

E E

NV

IRO

NM

EN

TS

MS

FC

EM

& E

nviro

nmen

ts B

ranc

h/E

L54

Deg

rada

tion

of S

olar

Arr

ay

Per

form

ance

Ove

rall

GN

&C

/Poi

ntin

g S

yste

m

Des

ign

Mat

eria

l Sel

ectio

n, M

ater

ial

Deg

rada

tion

S/C

Glo

w, I

nter

fere

nce

with

S

enso

rs

Dra

g M

akeu

p/F

uel R

equi

rem

ent

Pos

sibl

e T

rack

ing

Err

ors,

Pos

sibl

e T

rack

ing

Loss

Ree

ntry

Loa

ds/H

eatin

g, S

urfa

ce

Deg

rada

tion

due

to A

tom

ic

Oxy

gen

Reb

oost

Tim

elin

es, S

/C L

ifetim

e A

sses

smen

t

The

rmal

Des

ign

Sol

ar A

rray

Des

igns

, Pow

er

Allo

catio

ns, P

ower

Sys

tem

P

erfo

rman

ce

Mat

eria

l S

elec

tion

Influ

ence

s O

ptic

al D

esig

n

Influ

ence

s P

lace

men

t of

The

rmal

ly S

ensi

tive

Sur

face

s,

Fat

igue

, The

rmal

ly In

duce

d V

ibra

tions

Pas

sive

and

Act

ive

The

rmal

C

ontr

ol S

yste

m D

esig

n, R

adia

tor

Siz

ing,

Fre

ezin

g P

oint

s

Influ

ence

s M

issi

on P

lann

ing/

S

eque

ncin

g

Ups

ets

due

to E

MI f

rom

A

rcin

g, S

/C C

harg

ing

Shi

ft in

Flo

atin

g P

oten

tial,

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rent

Los

ses,

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eattr

actio

n of

Con

tam

inan

ts

Tor

ques

due

to In

duce

d P

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tial

Arc

ing,

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tterin

g,

Con

tam

inat

ion

Effe

cts

on S

urfa

ce P

rope

rtie

s R

eattr

actio

n of

Con

tam

inan

ts,

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nge

in S

urfa

ce O

ptic

al

Pro

pert

ies

Shi

ft in

Flo

atin

g P

oten

tial D

ue

to T

hrus

ter

Firi

ngs

mak

ing

Con

tact

with

the

Pla

sma

Mas

s Lo

ss F

rom

Arc

ing

and

Spu

tterin

g, S

truc

tura

l Siz

e In

fluen

ces

S/C

Cha

rgin

g E

ffect

s

EM

I Due

to A

rcin

g

Rea

ttrac

tion

of C

onta

min

ants

, C

hang

e in

abs

orpt

ance

/ em

ittan

ce p

rope

rtie

s

Ser

vici

ng (

EV

A)

Tim

elin

es

EM

I Due

to Im

pact

s

Dam

age

to S

olar

Cel

ls

Col

lisio

n A

void

ance

Deg

rada

tion

of S

urfa

ce

Opt

ical

Pro

pert

ies

Deg

rada

tion

of S

urfa

ce

Opt

ical

Pro

pert

ies

Col

lisio

n A

void

ance

, Add

ition

al

Shi

eldi

ng In

crea

ses

Fue

l Req

uire

men

t, R

uptu

re o

f Pre

ssur

ized

Tan

ks

Str

uctu

ral D

amag

e, S

hiel

ding

Des

igns

, O

vera

ll S

/C W

eigh

t, C

rew

Sur

viva

bilit

y

EM

I Due

to Im

pact

s

Cha

nge

in T

herm

al/O

ptic

al P

rope

rtie

s

Cre

w S

urvi

vabi

lity


4

Sp

ace

En

viro

nm

ent

Eff

ects

Figu

re 3

. Sp

ace

envi

ronm

ent e

ffec

ts o

n sp

acec

raft

sub

syst

ems.

SP

AC

EC

RA

FT

S

UB

SY

ST

EM

S

Avi

on

ics

Ele

ctri

cal P

ow

er

GN

&C

/Po

inti

ng

Mat

eria

ls

Op

tics

Pro

pu

lsio

n

Str

uct

ure

s

Tel

emet

ry,

Tra

ckin

g a

nd

C

om

mu

nic

atio

n

Th

erm

al C

on

tro

l

Mis

sio

n

Op

erat

ion

s

SP

AC

E E

NV

IRO

NM

EN

TS

So

lar

En

viro

nm

ent

Ion

izin

g R

adia

tio

nM

agn

etic

Fie

ldG

ravi

tati

on

al F

ield

Mes

osp

her

e

MS

FC

EM

& E

nviro

nmen

ts B

ranc

h/E

L54

The

rmal

Des

ign

Sol

ar A

rray

Des

igns

, Pow

er

Allo

catio

ns

Influ

ence

s D

ensi

ty a

nd D

rag,

D

rives

Neu

tral

s, In

duce

s G

ravi

ty

Gra

dien

t Tor

ques

Sol

ar U

V E

xpos

ure

Nee

ded

for

Mat

eria

l Sel

ectio

n

Nec

essa

ry D

ata

for

Opt

ical

D

esig

ns

Influ

ence

s D

ensi

ty a

nd D

rag

Influ

ence

s P

lace

men

t of T

herm

al

Sen

sitiv

e S

truc

ture

s

Tra

ckin

g A

ccur

acy,

Influ

ence

s D

ensi

ty a

nd D

rag

Influ

ence

s R

eent

ry T

herm

al

Load

s/H

eatin

g

Mis

sion

Tim

elin

es, M

issi

on

Pla

nnin

g

Deg

rada

tion:

SE

U’s

, Bit

Err

ors,

Bit

Sw

itchi

ng

Dec

reas

e in

Sol

ar

Cel

l Out

put

Deg

rada

tion

of M

ater

ials

Dar

keni

ng o

f Win

dow

s an

d F

iber

Opt

ics

Cre

w R

epla

cem

ent

Tim

elin

es

Indu

ced

Pot

entia

l E

ffect

s

Indu

ced

Pot

entia

l E

ffect

s

Siz

ing

of M

agne

tic

Tor

quer

s

Indu

ces

Cur

rent

s in

Lar

ge S

truc

ture

s

Loca

ting

Sou

th

Atla

ntic

Ano

mal

y

Sta

bilit

y an

d C

ontr

ol,

Gra

vita

tiona

l T

orqu

es

Influ

ence

s F

uel

Con

sum

ptio

n R

ates

Pro

pella

nt B

udge

t

May

Indu

ce

Tra

ckin

g E

rror

s

Effe

ct o

n G

N&

C

for

Re-

entr

y

Deg

rada

tion

of M

ater

ials

Due

to

Atm

osph

eric

Inte

ract

ions

Tet

her

Str

uctu

ral D

esig

n


5

NEUTRAL THERMOSPHERE

Environment Definition

The region of the Earth’s atmosphere containing neutral atmospheric constituents and locatedbetween about 90 and 600 km is known as the neutral thermosphere, while that region above 600 kmor so is known as the exosphere (fig. 4). The thermosphere is composed primarily of neutral gasparticles which tend to stratify based on their molecular weight. AO is the dominant constituent inthe lower thermosphere with helium and hydrogen dominating the higher regions. As figure 4 shows,the temperature in the lower thermosphere increases rapidly with increasing altitude from a minimumat 90 km. Eventually, it becomes altitude independent and approaches an asymptotic temperatureknown as the exospheric temperature. Thermospheric temperature, as well as density and compo-sition, are very sensitive to the solar cycle because of heating by absorption of the solar extremeultraviolet (EUV) radiation. This process has been effectively modeled using a proxy parameter, the10.7-cm solar radio flux (F10.7).

Spacecraft Effects

Density of the neutral gas is the primary atmospheric property that affects a spacecraft’sorbital altitude, lifetime, and motion. Even though space is thought of as a vacuum, there is enoughmatter to impart a substantial drag force on orbiting spacecraft. Unless this drag force is compen-sated for by the vehicle’s propulsion system, the altitude will decay until reentry occurs. Densityeffects also directly contribute to the torques experienced by the spacecraft due to the aerodynamicinteraction between the spacecraft and the atmosphere, and thus, must be considered in the designof the spacecraft attitude control systems.

Many materials used on spacecraft surfaces are susceptible to attack by AO, a major con-stituent of the low-Earth orbit (LEO) thermosphere region (fig. 5). Due to photodissociation, oxygenexists predominantly in the atomic form. The density of AO varies with altitude and solar activityand is the predominant neutral species at altitudes of about 200 to 400 km during low solar activity.Simultaneous exposure to the solar ultraviolet radiation, micrometeoroid impact damage, sputtering,or contamination effects can aggravate the AO effects, leading to serious deterioration of mechanical,optical, and thermal properties of some material surfaces. A related phenomenon which may be ofconcern for optically sensitive experiments is spacecraft glow. Optical emissions are generated frommetastable molecules which have been excited by impact on the surface of the spacecraft. Investi-gations show that the surface acts as a catalyst, thus the intensity is dependent on the type of sur-face material.

New Developments

Two important extensions to the Marshall engineering thermosphere (MET) model weredeveloped in 1992–93 (fig. 6). One provides a statistical definition of density variations in the day-to-week time scales for all LEO applications; the other is a simulation model for very short timescale variations which impact guidance, navigation, and control (GN&C) design, microgravity, andtorque equilibrium attitude calculations, applicable to low inclination orbits.


6

5,000

Miles

2,000

1,000

500

200

100

50

20

50,00010

30,000

20,000

10,000

5,000

–100 °C –50 °C 0 °C

Temperature

50 °C 100 °C

Feet Kilo

met

ers

10,000

5,000

2,000

1,000

500

200

100

50

20

10

5

2

1

Mount Everest

Mount Blanc

Ben Nevis

STRATOSPHERE

MESOSPHERE

IONOSPHERE

EXOSPHERE

MAGNETOSPHERE

Spray Region

Pres

sure

Mol

ecul

ar M

ean

Free

Pat

h

Aurora

AirglowNoctilucent

Cloud

F2

F1

E

D

Warm Region

Sound Waves Reflected Here

Maximum Height For

BalloonsMother-of-Pearl

Clouds

Tropopause

Cirrus Cloud

Altocumulus Cloud

Cumulus Cloud

Temperature Curve

Stratus Cloud

Ozone Region

10–107

mb100

7

km

10–87

mb

10–6mb

10–4mb

10–2mb

10–4cm

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cm

1 mb

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1 cm

THERMOSPHERE

Sunlit Aurora

TROPOSPHERE

Figure 4. The layers of the Earth’s atmosphere.


7

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01

Using MSFC/MET Number Density (cm )

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OMIN

OMAX

AMIN

AMAX

HMAX

N2 MAX

HMIN

HeMAX

HeMIN

O2MIN

O2MAX

N2MIN

–3

Figure 5. Variations of neutral species concentration as a function of altitude. Neutral atmosphericconstituent densities vary with solar conditions also. Typically, AO is the constituent of concern at

orbital altitudes. (MAX refers to solar maximum and MIN to solar minimum.)

5.0

4.0

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20 May 8847400 47500 47600 47700 47800

1 Oct. 89Modified Julian Day

Actual Decay Rate of LDEF

Density Calculated From MET Model

Den

sity

(E–1

1 kg

/m3 )

Dec

ay R

ate

(km

/day

)

Average Orbital Density 350 km alt Resolution—3 hours

465–390 km alt Beta 23 lb/ft2

Resolution —~2 days

Figure 6. Comparison of calculated and measured thermosphere drag. MSFC can now providestatistical descriptions of these rapid density fluctuations to support control system design.


8

THERMAL ENVIRONMENT


Spacecraft may receive radiant thermal energy from three sources: (1) incoming solar radi-ation (solar constant), (2) reflected solar energy (albedo), and (3) outgoing long-wave radiation(OLR) emitted by the Earth and atmosphere. If one considers the Earth and its atmosphere as awhole and averages over long time periods, the incoming solar energy and outgoing radiant energyare essentially in balance; the Earth/atmosphere is nearly in radiative equilibrium with the Sun.However, it is not in balance everywhere on the globe and there are important variations with localtime, geography, and atmospheric conditions. A space vehicle’s motion with respect to the Earthresults in its viewing only a “swath” across the full global thermal profile, so it sees these vari-ations as a function of time in accordance with the thermal time constants of its hardware systems(figs. 7 and 8).

Spacecraft Effects

Correct definition of the orbital thermal environment is an integral part of an effective space-craft thermal design. This thermal environment varies over orbits and over mission lifetime, whiletypical temperature control requirements for spacecraft components cover a predetermined range oftemperatures. Changes in temperature need to be minimized because they may lead to systemfatigue. An issue frequently encountered is the ability to provide adequate capability to cool sen-sitive electronic systems. Temperature fluctuations may fatigue delicate wires and solder joints,promoting system failures. Abrupt changes in the thermal environment may cause excessive freeze–thaw cycling of thermal control fluids. Too extreme an environment may require oversizing of radi-ators or possibly cause permanent radiator freezing. The thermal environment is also an importantfactor in considering lifetimes of cryogenic liquids or fuels.

New Developments

Advances in technology have led to stronger material bonding which has provided the spacecommunity with stronger flight materials at much reduced weight. These lightweight materials, how-ever, are more susceptible to changes in the thermal environment of space. This has led to a need fora much more accurate description of thermal environment variations. Previously, the design engineerhad only long-term, global mean thermal parameters to aid in the design process. Marshall SpaceFlight Center (MSFC) has completed analysis of data from the Earth radiation budget experiment(ERBE) which provides a significant advancement in the description of this near-Earth thermalenvironment. The new results provide thermal environment parameters which vary with orbitinclination and can be matched to a system’s thermal response time. The new thermal environmentdescription has already been incorporated in the space station program and other NASA programs.


9

781,366

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r Con

stan

t (W

/m2 )

Zuri

ch S

unsp

ot N

umbe

r84 85

Solar Max Acrim

Nimbus 7 ERB

86 87 88 89 90 91

Figure 7. A sample of the solar constant measurements from an instrument on the solar maximumsatellite and an instrument flying on a Nimbus satellite. (The dashed line represents the

trend in sunspot number.)


10

Square Triangle Diamond Cross Plus

Extremes 1–99 3–97 5–95 50

Percentile Range Percentile Range Percentile Range Percentile Value

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e Av

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lbed

o

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Square Triangle Diamond Cross Plus

Extremes 1–99 3–97 5–95 50

Percentile Range Percentile Range Percentile Range Percentile Value

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Time Interval (hours)

Tim

e Av

erag

e O

LR—

W/m

2

1.2 1.4 1.6 1.8

Figure 8. New engineering thermal model results for polar orbits.


11

PLASMA


The major constituents of the Earth’s atmosphere remain virtually unchanged up to an alti-tude of 90 km, but above this level the relative amounts and types of gases are no longer constantwith altitude. Within this upper zone of thin air, shortwave solar radiation causes various photo-chemical effects on the gases. A photochemical effect is one in which the structure of a molecule ischanged when it absorbs radiant energy. One of the most common of these effects is the splitting ofdiatomic oxygen into atoms. Another common effect is that atoms will have electrons ejected fromtheir outer shells. These atoms are said to be ionized. A small part of the air in the upper atmo-sphere consists of these positively charged ions and free electrons which cause significant physicaleffects. The electron densities are approximately equal to the ion densities everywhere in the region.An ionized gas composed of equal numbers of positively and negatively charged particles is termed aplasma. Therefore, it is because of these characteristics that this electrically charged portion of theatmosphere is known as the ionosphere, and the gas within this layer is referred to as the iono-spheric plasma. The electron and ion densities vary dramatically with altitude, latitude, magneticfield strength, and solar activity (figs. 9 and 10).

Spacecraft Effects

As a spacecraft flies through this ionized portion of the atmosphere, it may be subjected to anunequal flux of ions and electrons and may develop an induced charge. Plasma flux to the spacecraftsurface can charge the surface and disrupt the operation of electrically biased instruments (fig. 11).In LEO, vehicles travel through dense but low energy plasma. These spacecraft are negativelycharged because their orbital velocity is greater than the ion thermal velocity but slower than theelectron thermal velocity. Thus, electrons can impact all surfaces, while ions can impact only ramsurfaces. LEO spacecrafts have been known to charge to thousands of volts, however charging atgeosynchronous orbits is typically a greater concern. Biased surfaces, such as solar arrays, canaffect the floating potential. The magnitude of charge depends on the type of grounding configurationused. Spacecraft charging may cause: biasing of spacecraft instrument readings, arcing which maycause upsets to sensitive electronics, increased current collection, reattraction of contaminants, andion sputtering which may cause accelerated erosion of materials. High magnitude charging will causearcing and other electrical disturbances on spacecraft. Figure 12 shows some typical charging eventsthat have occurred on a polar orbiting spacecraft. A listing of some of the disturbances that havebeen noted by spacecraft charging is provided in figure 11.

New Developments

Spacecraft charging due to plasma interactions has been studied for some time for spacecraftat geosynchronous altitudes. Recently, however, there has been a new emphasis on studyingcharging effects on LEO spacecraft, especially those in polar orbits. A new computer model calledPOLAR has been developed to analyze spacecraft charging in low-Earth polar orbit. POLAR pro-vides detailed information on charging potentials anywhere on the spacecraft structure.


12

1,000

800

600

400

200

01.00E+02 1.00E+03 1.00E+04

Log Electron Density (cm^–3)

Altit

ude

(km

)

1.00E+05 1.00E+06 1.00E+07

DayNight

Midlatitude Electron Density—Solar Maximum

Figure 9. LEO plasma density at solar maximum.

Altit

ude

(km

)

1,000

800

600

400

200

01.00E+02 1.00E+03 1.00E+04 1.00E+05 1.00E+06 1.00E+07

Midlatitude Electron Density—Solar Minimum

Log Electron Density (cm^–3)

Day

Night

Figure 10. LEO plasma density at solar minimum


13

Effects attributed to spacecraft charging can be of serious engineering concern:• Increased surface contamination• Compromised scientific missions• Physical surface damage• Operational anomalies ranging from “minor irritations to the fatally catastrophic”

Arc discharging is seen as the primary mechanism by which spacecraft charging disturbs mission activities• Occurs when generated electric fields exceed a breakdown threshold• Results in a rapid release of large amounts of charge• Coupling of generated structural transients into spacecraft electronics

Types of discharges are:• “flashover” discharge from one surface to another• “punch-through” discharge through surface material from underlying structure• “discharge to space” discharge from spacecraft to ambient plasma

Spacecraft Known Anomaly Spacecraft Known Anomaly_________________________________ ________________________________ Voyager I Power-on resets Intelsat 3 and 4 Spin upSCATHA 34 pulses detected Skynet 2B Telemetry problemsDSP False flags from star sensor ANIK Power downsDSCS II Spin up, amplifier gain change, CTS Short circuit

power switching events Meteosat Status changesGPS Clock shift, false commands GOES 4 and 5 Upsets and failure

Figure 11. Spacecraft charging.


14

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49820 13:50:20

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–3)

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llite

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entia

l, –

Øsc

(vol

ts)

Inte

gral

Ele

ctro

n Fl

ux (E

lect

ron)

DMSP F7

E > 30 eV E > 14 keV

Figure 12. An example of the type of charging events encountered by the DMSP satellite F7 onNovember 26, 1983. DMSP 7 is a defense meteorological satellite flying in LEO/polar.


15

METEOROID/ORBITAL DEBRIS


The meteoroid population consists primarily of the remnants of comets. As a cometapproaches perihelion, the gravitational force and solar wind pressure on it are increased, resulting ina trail of particles in nearly the same orbit as the comet. When the Earth intersects a comet’s orbit,there is a meteor shower, and this occurs several times per year. The Earth also encounters manysporadic particles on a daily basis. These particles originate in the asteroid belt, and are themselvesthe smallest asteroids. Radiation pressure from the Sun causes a drag force on the smallest particlesin the asteroid belt. In time, these particles lose their orbital energy and spiral into the Sun.

Since the beginning of human activity in space, there has been a growing amount of matter leftin orbit (figs. 13 through 16). In addition to operational payloads, there exist spent rocket stages,fragments of rockets and satellites, and other hardware and ejecta, many of which will remain in orbitfor many hundreds of years. Currently, the U.S. Air Force Space Command tracks over 7,000 largeobjects (>10 cm) in LEO, and the number of smaller objects is known to be in the tens of thousands.Since the orbital debris population continually grows, it will be an increasing concern for future spaceoperations.

Spacecraft Effects

Meteoroids and orbital debris pose a serious damage and decompression threat to spacevehicles. In the orbital velocity regime, collisions are referred to as hypervelocity impacts. Such animpact, for example, by a 90-gram particle, will impart over 1 MJ of energy to the vehicle. Thus,practically any spacecraft will suffer catastrophic damage or decompression if it receives a hyper-velocity impact from an object larger than a few grams. Collisions with smaller objects cause serioussurface erosion with subsequent effects on the surface thermal, electrical, and optical properties. Netrisk to a mission depends on the orbit duration, vehicle size and design, launch date (solar cyclephase), orbit altitude, and inclination. Protective shielding is often necessary to minimize the threatfrom the meteoroid/orbital debris environment. If a system cannot be shielded, operational con-straints or procedures may be imposed to reduce the threat of damage. The debris threat is highlydirectional, so risk can also be mitigated by careful arrangement of critical components.

New Developments

The orbital debris environment continues to increase because of continued activity in spaceand continued on-orbit fragmentation events. New data from radar measurements confirm the currentNASA models, in general, and will lead to improved descriptions of the altitude, inclination, andvelocity distributions of the debris environment.


16

Fragmentation Debris (43%)

Active Pay- loads (5%)Launch

Debris (16%)

Rocket Body (15%)

Inactive Payloads

(21%)

5-21106

Figure 13. Sources of the catalogued debris population.

Figure 14. The state of the Earth debris environment is illustrated in this snapshot of all cataloguedobjects in July 1987.


17

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ber o

f Tra

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ects

Inclination (degree)

Figure 15. Inclination distribution of the USSPACECOM tracked objects,August 1988—all altitudes.

2,400

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00 200 400 600 800 1,000 1,200 1,400 1,600

Altitude (km)

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f Sat

ellit

es

Space Debris as a Function of Altitude

5-21092

Figure 16. Based on data from the January 1987 NORAD satellite catalog,the altitude regime between 800 and 1,000 km is the most populous.


18

SOLAR ENVIRONMENT


The Sun emits huge amounts of mass and energy; enough energy in 1 second to powerseveral million cars for over a billion years. This tremendous emission of energy has important con-sequences to spacecraft design, development, and operations. Over short periods of time and in cer-tain locations, solar intensity can fluctuate rapidly. It is thought that a major factor causing thesefluctuations is the distortion of the Sun’s large magnetic field due to its differential rotation. Two ofthe most common indicators of locally enhanced magnetic fields are sunspots and flares. Sunspotsare probably the most commonly known solar activity feature. The average sunspot number is knownto vary with a period of about 11 years (fig. 17). Each cycle is defined as beginning with solar mini-mum (the time of lowest sunspot number) and lasting until the following solar minimum. Forexample, cycle 22, which began in late 1986, reached solar maximum in 1991. A solar flare is a highlyconcentrated explosive release of energy within the solar atmosphere. The radiation from a solarflare extends from radio to x-ray frequencies. Solar flares are differentiated according to their totalenergy released. Ultimately, the total energy emitted is the deciding factor in the severity of a flare’seffects on the space environment.

Spacecraft Effects

The solar environment has a critical impact on most elements within the natural spaceenvironment. Variations in the solar environment impact thermospheric density levels, the overallthermal environment a spacecraft will experience, plasma density levels, meteoroids/orbital debrislevels, the severity of the ionizing radiation environment, and characteristics of the Earth’s magneticfield. The solar cycle also plays an important role in mission planning and mission operations activi-ties. For instance, when solar activity is high, ultraviolet and extreme ultraviolet radiation from theSun heats and expands the Earth’s upper atmosphere, increasing atmospheric drag and the orbitaldecay rate of spacecraft. Solar flares are a major contributor to the overall radiation environment andcan add to the dose of accumulated radiation levels and to single event phenomena which affectelectronic systems.

New Developments

The variability of the solar cycle has been effectively modeled using a proxy parameter, the10.7-cm solar radio flux (F10.7). The Environments Team at the Marshall Space Flight Centercurrently predicts and publishes a monthly solar activity memorandum which gives long-rangeestimates of both the F10.7 index and the geomagnetic activity index, Ap. Also, solar science ismaking progress toward predicting the sites for flare activity on the Sun in much the same way asmeteorologists have developed criteria for probable tornadic activity within the Earth’s atmosphere.As more is learned about the Sun’s behavior, more realistic modeling of the Sun is possible and moreaccurate predictions of the Sun’s future behavior can be made.


19

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pot N

umbe

r

Yearly Mean Sunspot Numbers 1700–1992

Figure 17. Solar cycle history of interest is that the dawn of the space age in the 1950’s occurredduring the “largest” cycle on record, cycle number 19. Space travel has been faced with dramatic

solar cycle characteristics since that time.


20

IONIZING RADIATION


The particles associated with ionizing radiation are categorized into three main groupsrelating to the source of the radiation: trapped radiation belt particles, cosmic rays, and solar flareparticles. Results from recent satellite studies suggest that the source of the trapped radiation belts(or Van Allen belts) particles seems to be from a variety of physical mechanisms: from the accel-eration of lower-energy particles by magnetic storm activity, from the trapping of decay products ofenergetic neutrons produced in the upper atmosphere by collisions of cosmic rays with atmosphericparticles, and from solar flares. Solar proton events are associated with solar flares. Cosmic raysoriginate from outside the solar system from other solar flares, nova/supernova explosions, orquasars.

The Earth’s magnetic field concentrates large fluxes of high-energy, ionizing particlesincluding electrons, protons, and some heavier ions. The Earth’s magnetic field provides the mecha-nism which traps these charged particles within specific regions, called the Van Allen belts. Thebelts are characterized by a region of trapped protons and both an inner and an outer electron belt.The radiation belt particles spiral back and forth along the magnetic field lines (fig. 18). Because theEarth’s approximate dipolar field is displaced from the Earth’s center, the ionizing radiation beltsreach their lowest altitude off the eastern coast of South America. This means as particles travel intothis region they will reach lower altitudes, and particle densities will be anomalously high over thisregion. This area is termed the South Atlantic Anomaly (SAA). For the purpose of this document,the term “cosmic rays” applies to electrons, protons, and the nuclei of all elements from other thansolar origins. Satellites at low inclination and low altitude experience a significant amount of naturalshielding from cosmic rays due to the Earth’s magnetic field. A small percentage of solar flares areaccompanied by the ejection of significant numbers of protons. Solar proton events occur sporadically,but are most likely near solar maximum. Events may last hours or up to more than a week, but typi-cally the effects last 2 to 3 days. Solar protons add to the total dose and may also cause single-event effects in some cases (figs. 19a and 19b).

Spacecraft Effects

The high-energy particles comprising the radiation environment can travel through spacecraftmaterial and deposit kinetic energy. This process causes atomic displacement or leaves a stream ofcharged atoms in the incident particle’s wake. Spacecraft damage includes decreased power pro-duction by solar arrays, failure of sensitive electronics, increased background noise in sensors, andradiation exposure of the spacecraft crew. Modern electronics are becoming increasingly sensitive toionizing radiation.

New Developments

A new transient radiation belt containing large numbers of highly energetic electrons wasrecently encountered by the combined release and radiation effects satellite (CRRES). The solaranomalous and magnetospheric particle explorer (SAMPEX) satellite recently discovered a belt oftrapped anomalous ray particles that will effect LEO missions. Also, it is now realized that manyheavy ion cosmic rays are only partially ionized so that geomagnetic shielding of these particles isnot as effective as once thought.


21

Flux Tube

Trajectory of Trapped Particle

Mirror Point

Magnetic Conjugate Point

North

Drift of Protons

Magnetic Field Line

Figure 18. Trapped particles spiral back and forth along magnetic field lines.

0Shielding (g/cmˆ2)

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iatio

n D

ose

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)

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Figure 19a. Average radiation dose from a large solar proton event.

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Figure 19b. Variation of solar flare proton events as a function of solar activity.


22

GEOMAGNETIC FIELD


The Earth’s magnetic field exerts a strong influence on space environmental phenomena suchas plasma motions, electric currents, and trapped high-energy charged particles. This influence hasimportant consequences to spacecraft design and performance. The Earth’s natural magnetic fieldcomes from two sources: (1) currents inside the Earth which produce 99 percent of the field at thesurface, and (2) currents in the magnetosphere. The magnetosphere is the outer region in theEarth’s atmosphere where the Earth’s magnetic field is stronger than the interplanetary field. Thedipole is about 436-km distance from the center of the planet. The geomagnetic axis is inclined at an11.5° angle to the Earth’s rotational axis. The International Geomagnetic Reference Field (IGRF)predicts the Earth’s equatorial magnetic field to decrease by 0.02 percent each year. The IGRFprediction of the Earth’s magnetic field is shown in figures 20 and 21.

Spacecraft Effects

The geomagnetic field influences the motions of particles within the Earth’s orbital envi-ronment and also deflects incoming high-energy particles such as those associated with cosmic rays.These high-energy particles may charge spacecraft surfaces causing failure of or interference withspacecraft subsystems. Due to the dipole field geometry, the magnetic field strength is lowest overthe southern Atlantic ocean, which leads to a higher concentration of trapped radiation in this region(figs. 20 and 21). It is in the vicinity of the SAA that a spacecraft may encounter electronics“upsets” and instrument interference. An accurate depiction of the geomagnetic field is also neededto properly size magnetic torquers, which are used in GN&C systems.

Geomagnetic storms may also affect orbiting spacecraft. Disturbances in the geomagneticfield which last one or more days are called geomagnetic storms. When a geomagnetic storm occurs,large numbers of charged particles are dumped from the magnetosphere into the atmosphere. Theseparticles ionize and heat the atmosphere through collisions. The heating is first observed minutes tohours after the magnetic disturbance begins. The effects of geomagnetic heating extend from at least300 km to well over 1,000 km and may persist for 8 to 12 hours after the magnetic disturbance ends.

New Developments

Technology advancements have provided electronics which are far more sophisticated, but atthe same time are far more vulnerable to radiation damage. Therefore, correct determination of theSAA has become essential to successful spacecraft design and to successful mission operations.The IGRF is updated with new coefficients approximately every 5 years. The most recent revisionwas in 1991, but the model allows projection of future trends.


23

180° W

90° N

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90° S150° W 120° W 90° W 60° W 30° W 0° 30° E 60° E 90° E 120° E 150° E 180° E

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Figure 20. Geomagnetic field at sea level.

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Figure 21. Geomagnetic field at 650 km-altitude.


24

GRAVITATIONAL FIELD


Since the advent of Earth satellites, there has been a considerable advancement in the accu-rate determination of the Earth’s gravitational field. The current knowledge regarding the Earth’sgravitational field has advanced far beyond the normal operational requirements of most space mis-sions. Adequate accuracy for determining most spacecraft design values of gravitational interactionsis obtained with the central inverse square field:

F = –µEm

r 2 r .

The above central force model accurately represents the gravitational field to approximately0.1 percent. If this accuracy is insufficient for particular program needs, a more detailed model of thegravitational field can be used that accounts for the nonuniform mass distribution within the Earth.This model gives the gravitational potential, V, to an accuracy of approximately a few parts in a mil-lion. The gravitational acceleration is expressed in terms of the negative gradient of the potential.The potential is then expressed using harmonic expansion:

F = mg = m(–∇V ) .

Spacecraft Effects

Accurate predictions of the Earth’s gravitational field are a critical part of mission planningand design for any spacecraft. The Earth’s gravitational field will affect spacecraft orbits and trajec-tories. Gravity models are used to estimate the gravitational field strength for use in designing theGN&C/pointing subsystem, and designing the telemetry, tracking, and communications. Theavailable gravitational models have sufficient accuracy for estimating the gravitational field strengthfor spacecraft planning and design.

New Developments

Gravitational models of the Earth are continually being updated and upgraded. Currently,gravitational models are available for varying degrees of accuracy. For operational applications andother situations where computer resources are a premium, “best fit” approximations to the highestaccuracy model may be utilized which minimize the computational requirements yet retain therequired accuracy for the specific application.


25

CONCLUSION

For optimum efficiency and effectiveness, it is important that the need for definition of theflight environment be recognized very early in the design and development cycle of a spacecraft.From past experience, the earlier the environments specialists become involved in the design pro-cess, the less potential for negative environmental impacts on the program through redesign, opera-tional work-arounds, etc. The key steps in describing the natural space environment for a particularprogram include the following:

• Definition of the flight environment is the first critical step. Since the environment dependsstrongly on orbit and phase of the solar cycle, environment effects should be reviewed prior tofinal orbit selection.

• Not all space environments will have a critical impact on a particular mission. It is the role of theenvironments specialists to find all the environmental limiting factors for each program and tooffer design or operational solutions to the program where possible. This typically requires aclose working relationship between the environments specialists, members of the design teams,and program management. Typically, once the limiting factors are established, trade studies areusually required to establish the appropriate environmental definition.

• After definition of the space environment is established including results from trade studies, thenext important step is to establish a coordinated set of natural space environment requirementsfor use in design and development. These requirements are derived directly from the definitionphase and again, typically are derived after much interaction among the design developmentengineering staff, program management, and environments specialists.

• Typically, the space environment definition and requirements are documented in a separate pro-gram document or are incorporated into design and performance specifications.

• The environments specialist then helps insure that the environment specifications are understoodand correctly interpreted throughout the design, development, and operational phases of the pro-gram.

This primer has provided an overview of the natural space environments and their effect onspacecraft design, development, and operations. It is hoped that a better understanding of the spaceenvironment and its effect on spacecraft will enable program management to more effectivelyminimize program risks and costs, optimize design quality, and successfully achieve missionobjectives. If you have further questions or comments, contact the MSFC Systems Analysis andIntegration Laboratory’s Electromagnetics and Environments Branch, Steven D. Pearson at 205–544–2350.


the natural space environment: effects on spacecraft

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