THE ENVIRONMENT
OF SPACE
Col. John Keesee
1
Image courtesy of NASA.
OUTLINE
? Overview of effects
? Solar Cycle
? Gravity
? Neutral Atmosphere
? Ionosphere
? GeoMagnetic Field
?Plasma
? Radiation
2
OVERVIEW OF THE EFFECTS OF
THE SPACE ENVIRONMENT
? Outgassing in near vacuum
? Atmospheric drag
? Chemical reactions
? Plasma-induced charging
? Radiation damage of microcircuits, solar
arrays, and sensors
? Single event upsets in digital devices
? Hyper-velocity impacts
3
4
? Solar Cycle affects all space environments.
? Solar intensity is highly variable
? Variability caused by distortions in magnetic field
caused by differential rotation
? Indicators are sunspots and flares
Solar Cycle
LONG TERM
SOLAR CYCLE INDICES
5
? Sunspot number R
10 (solar min) d R d 150 (solar max)
? Solar flux F
10.7
Radio emission line of Fe (2800 MHz)
Related to variation in EUV
Measures effect of sun on our atmosphere
Measured in solar flux units (10
-22
w/m
2
)
50 (solar min) d F
10.7
d 240 (solar max)
SHORT TERM
SOLAR CYCLE INDEX
? Geomagnetic Index A
p
– Daily average of maximum variation in the earth’s
surface magnetic field at mid lattitude (units of
2 u 10
-9
T)
A
p
= 0 quiet
A
p
= 15 to 30 active
A
p
> 50 major solar storm
6
GRAVITY
force
At surface of earth
G
m
1
m
2
r
2
r
G 6.672u 10
11
m
3
kg
1
2
2
E
e
2
E
e
g
sec
m
9.8
R
Gm
g
R
Gm
mf |
7
MICROGRAVITY
8
? Satellites in orbit are in free fall - accelerating
radially toward earth at the rate of free fall.
? Deviations from zero-g
– Atmospheric drag
– Gravity gradient
– Spacecraft rotation
(rotation about Y axis)
– Coriolis forces
22
5.0 ZU a
m
AC
x
D
?
?
?
?
?
1
·
¨
¨
¨
¨
¨
?
§
222
2zwzywxxwx
22
ZZ zzxx
ZZ xzzoyzx
2
ATMOSPHERIC MODEL
NEUTRAL ATMOSPHERE
9
? Turbo sphere (0 ~ 120Km) is well mixed (78% N
2
, 21%
O
2
)
– Troposphere (0 ~ 10Km) warmed by earth as heated
by sun
– Stratosphere (10 ~ 50 Km) heated from above by
absorption of UV by 0
3
– Mesosphere (50 ~ 90Km) heated by radiation from
stratosphere, cooled by radiation into space
– Thermosphere (90 ~ 600Km) very sensitive to solar
cycle, heated by absorption of EUV.
? Neutral atmosphere varies with season and time of day
10
Layers of the
Earth’s
Atmosphere
TEMPERATURE
MAGNETOSPHERE
Pressure
Molecular
m
ean
free
p
ath
EXOSPHERE
Sunlit
Spray region
Warm region
Maximum
height for
balloons
THERMOSPHERE
Aurora
Aurora
Airglow
MESOSPHERE
IONOSPHERE
Noctilucent
cloud
D
E
F
1
F
2
Ozone region
Sound waves
reflected here
Mother-of-pearl
clouds
Cirrus
clouds
Altocumulus
clouds
cumulus
clouds
Stratus
clouds
Tropopause
STRATOSPHERE
Mount
Blanc
Ben
Nevis
Temperature
curve
-100
0
C -50
0
C 0
0
C 50
0
C 100
0
C
1,000
mb
10
-8
mb
10
-6
cm
10
-4
cm
1cm
1km
100
km
100
mb
1
mb
10
-2
mb
10
-4
mb
10
-6
mb
10
-10
mb
Miles
10,000
5,000
5,000
2,000
2,000
1,000
1,000
500
500
200
200
100
100
50
50
20
20
10
50,000
30,000
20,000
10,000
5 ,000
10
5
2
1
Kilometers
Feet
Mount
Everest
TROPOSPHERE
DENSITY ALTITUDE MODEL
Assume perfect gas and constant temperature
n is number density (number/m
3
) dpA - n m g A d h = o
k is Boltzmann’s constant
M is average molecular mass
H ~ 8.4km h ~ 120km n = n
o
exp (-h/H)
H { kT/mg (scale height)
dh
nkTd
dh
dp
Tknp
?
?
1
·
¨
¨
?
§
dh
nkTd
nMg
dh
dp
?
?
1
·
¨
¨
?
§
dh
KT
Mg
n
dn
p
A
dh
p+dp
11
Atmospheric Gases
? At higher altitudes O
2
breaks down into O by UV
? Primarily O from 80 - 90 km to 500 km
? Hydrogen and Helium beyond 500 km
? Kinetic energy of O atom at 7.8 km/s ~ 5eV (enough to
break molecular bonds ~1 - 2eV)
? O is highly reactive and destructive to spacecraft
? Temperature at LEO increases with altitude
? Atmosphere expands when heated by high UV (solar max)
? LEO densities ~ 10
8
particles/cm
3
12
ATMOSPHERIC MODEL
13
Most common Mass Spectrometer and Incoherent
Scatter model - 1986 (MSIS - 1986)
– Based on measured data
– Requires A
p
, F
10.7
, month as input
– Gives average values of n, n
o
, T, atomic mass as
function of altitude
– Instantaneous values can vary by factor of 10
http://nssdc.gsfc.nasa.gov/space/model/atmos/msis.html
AERODYNAMIC DRAG
Drag
Ballistic coefficient
U=density of the atmosphere=m
o
n
o
=16x1.67x10
-27
x10
13
=2.67x10
-13
kg/m
3
V=7.8km/s
C
D
- Drag coefficient
A - Cross sectional area
D
1
2
Uv xv (
v
v
)CDA
D m
dv
dt
'v
1
2
Uv
2
CDA
m
a
?
?
?
o
?
?
?
't
E
m
CDA
a
?
?
?
o
?
?
?
14
DRAG COEFFICIENTS
Derived from Newtonian Aerodynamics. Depends
on what air molecule does at impact
– Reflected C
D
= 4
– Absorbed C
D
= 2
Since F = d(mv)/dt
D = - F = - d(mv)/dt o
m = U Av
i
dt
C
D
= - 2 (v
f
-v
i
)/v
i
= 2 if v
f
= o in rarefied atmosphere
= 4 if v
f
= - v
i
AdtV
VVm
AV
D
C
i
if
D
2
2
2
1
2
1
U
U
?
?
1
·
¨
¨
?
§
A
15
TYPICAL DRAG PARAMETERS
16
E (kg/m
2
) C
D
LANDSAT 25 - 123 3.4 - 4
ERS - 1 12 - 135 4
Hubble 29 - 192 3.3 - 4 90,000Kg
Echo 1 0.515 2
Typically C
D
~ 2.2 - 4 for spacecraft. (see SMAD Table 8.3)
'V over one year (E = 100 kg/m
2
)
h (km) 'V /year (m/s)
100 10
7
200 2 - 5 u 10
3
solar (min - max)
300 40 - 600
400 3 - 200
17
SATELLITE LIFETIMES
Large variation depending on initial altitude and
solar min/max condition (see SMAD Fig. 8 - 4)
At LEO, design must compensate for effects of
drag.
MAGNETIC FIELD EFFECTS
18
? Deflects charged particles/solar wind.
– South Atlantic Anomaly
? Creates the structure of the ionosphere/plasmasphere
– Magnetosphere
– Van Allen radiation belts
? Direct effects on Spacecraft systems
– Avionics - induced potential effects
– Power - induced potential effects
– GN&C - magnetic torquer performance, sizing
– Structures - induced currents
– TT&C - location of SAA
GEOMAGNETIC FIELD
19
? Earth’s Magnetic field comes from three sources
– internal field (99%)
? currents inside the Earth
? residual magnetism of elements contained in crust
– External field 1%
? Currents in the magnetosphere
?B
i
internal field varies slowly
on the order of 100 years
(0.05%/year.)
? Poles of magnetic field lie in
Siberia and South Australia.
20
GEOMAGNETIC FIELD
MAGNETOSPHERE
21
Magnetosphere (continued)
22
? Earth’s field extends 10 Earth Radii (R
H
) toward the sun
- terminates at magneto pause
? Earth’s field slows and deflects solar wind
– Compressed, heated, turbulent
– Bow shock at about 14 R
H
? Polar field lines are swept back in night-side tail
– Does not close
– Neutral sheet
? Surface of discontinuity in magnetic field implies
current flow in the surface
– Sunward magnetopause - eastward current flow across sub-
solar point.
– Neutral sheet current flow is westward across the tail
EXTERNAL MAGNETIC FIELD
23
?B
e
generated by ring currents and solar wind. Large
variation with time
– Milliseconds to 11-year cycle scales.
? Variations caused by
– Magnetosphere fluctuations (geomagnetic storms)
– Solar activity
? Geomagnetic storms dump large numbers of charged
particles from magnetosphere into atmosphere
– Ionizes and heats the atmosphere
– Altitudes from 300 km to over 1000 km
– Persist 8-12 hours after storm subsides
GEOMAGNETIC
COORDINATE SYSTEMS
24
Geomagnetic B - L
B
L=8
Greenwich
meridian
Geog
r
aphic
nor
th
pole
Geographic
north pole
Geographic Geomagnetic
Solar-ecliptic Solar-magnetospheric
Solar-magnetic
Colatitude
z
z
se
x se x sm
x sm
y
sm
z
sm
z
sm
y
sm
T
o
sun
T
os
u
n
y
se
y
r
r
m
z
m
r se
x mx
y
m
Magnetic
colatitude
East longitude
T
o
nor
th
ecliptic
pole
Sun direction
Several coordinate systems used in geomegnetism.
Direction of
geographic
north pole
Dipole axis
direction
Dipole
axis
Dipole
axis
Magnetic longitude
φ
φ
se
θ
φ
m
φ
se
θ
m
B=.01
.02
.05
0.1
30
20 15
10
9
8
L=2 3 4 5 6 7
0
0
-90 -60 -30
1
2
3
4
0.2
.005
.002
.001
Field
Strength
(Oersteds)
North Latitude (Degrees)
Geocentric
Distance
(Re)
GEOMAGNETIC FIELD
Magnitude Formula/Models
Tilted dipole (11q from geographic north)
at LEO
where
M = 0.311 u 10
-4
=7.9u 10
15
T - m
3
International Geomagnetic Reference Field
1987 (IGRF1987)
B
i
r,T
m
,I
m
§
?
¨
·
1
?
M
r
3
3cos
2
T
m
§
?
¨
·
1
?1
§
?
¨
·
1
?
1/2
B
r
M
r
3
2cosT
m
B
Tm
M
r
3
sinT
m
B
Im
0
TR
e
3
25
FIELD VALUES
? Minimum (near equator) = 0.25 u 10
-4
T
? Maximum (near polar caps) = 0.50 u 10
-4
T
? Two peaks near north pole
? Two minimum near equator
? Largest minima is known as South Atlantic
Anomaly
– Much higher radiation exposure at LEO
? Geomagnetic storms impose variations of
0.01 u 10
-4
T
26
TOTAL FIELD INTENSITY
27
28
SOUTH ATLANTIC ANOMALY
Reduced protection in SAA allows greater effect of high
energy particles - electronic upsets, instrument interference.
PLASMA EFFECTS OVERVIEW
29
? Plasma is a gas made up of ions and free electrons in
roughly equal numbers.
?Causes
Elecromagnetic Interference
Spacecraft charging & arcing
Material effects
? Effects
Avionics - Upsets from EMI
Power - floating potential, contaminated solar arrays, current
losses
GN & C - torques from induced potential
Materials - sputtering, contamination effects on surface
materials
PLASMA EFFECTS (cont.)
? Effects continued
Optics systems - contamination changes properties of
surface materials.
Propulsion - Thruster firings change/shift the floating
potential by contacting the plasma.
30
PLASMA GENERALIZATION
31
? Plasma is caused by UV, EUV, X-ray photoelectric
effect on atmospheric molecules.
– Breaks diatomic molecule bonds.
– Ejects electrons from outer shells.
? As UV, EUV, X-ray penetrate the atmosphere, ion
density increases with atmospheric density until most
UV, EUV have been absorbed (>60 Km altitude).
Varies dramatically with altitude, latitude, magnetic
field strength, time of day and solar activity.
? Electrically charged region of atmosphere is called the
ionosphere.
? Gas in ionosphere is called ionospheric plasma.
LEO PLASMA ENVIRONMENT
32
? Balance between increasing density and increasing absorption
leads to formation of ionization layers.
F - layer 150 km - 1000 km
E - layer 100 km - 150 km
D - layer 60 km - 100 km
? Transition region from ion-free atmosphere to fully ionized
region called the plasmasphere.
? Plasmasphere ion densities peak at 10
10
/m
3
to 10
11
/m
3
at
1000 km
– Drops to 10
9
/m
3
at its boundary
? Outer boundary called plasmapause
– Density drops to 10
5
/m
3
to 10
6
/m
3
– Height is ~ 4 RH between 0000 and 1800 hours
– Expands to ~ 7 RH during the local dusk (dusk bulge)
ELECTRON DENSITY
33
Solar Max
Solar Min
Daytime Electrons
Nightime Electrons
10
1
100
1000
Altitude
(km)
10
3
10
5
10
7
Density (cm
-3
)
K
p
is Magnetic Activity Index
PLASMAPAUSE HEIGHT VS
LOCAL TIME
34
K
p
<1
K
p
=2
K
p
=4-5
K
p
=3
10
-2
N(H
+
)
(cm
-2
)
OGO S, Nightside, 1968
1234567
10
0
10
2
10
4
L
ION CONCENTRATIONS
– Similar to neutral atmosphere
D - layer NO
+
/O
+
E - layer O
+
F - layer O
+/
H
+
-Daytime F layer density peaks at 10
12
/m
3
(300 km)
-Nighttime F-layer density drops to 10
11
/m
3
(500 km)
– Composition transitions from O
+
to H
+
35
ION CONCENTRATIONS (cont.)
36
10
1
He++
O
2
+
Altitude
(km)
Ion Concentrations (ions/cm
3
)
O
+
H
+
NO
+
150
200
250
300
350
400
450
500
550
600
650
10
3
10
5
PLASMA TEMPERATURES
Increases from ~100K at 50 - 60 km to
2000 - 3000K above 500 km
Electron temperature T
e
= 4000K - 6000K
Ion temperature T
i
= 2000K - 3000K
Density much higher at solar maximum due to
higher UV/EUV fluxes.
37
LEO PLASMA
ENVIRONMENT MODELS
International Reference Ionosphere (IRI)
-Outputs - electron density n
e
- ion composition n
i
- Temperature T
e
, T
i
-Inputs (latitude, longitude, altitude, solar activity (R),
time).
Available at :
http://nssdc.gsfc.nasa.gov/space/model/ionos/iri.html
“Ionospheric models” Carlson, Schunk, Heelis, Basu
38
RADIO FREQUENCY
TRANSMISSIVITY
– Plasma transitions from a perfect conductor to
perfect dielectric as a function of frequency.
– Plasma frequency
– Dielectric constant
–ForZ >> Z
pe
the plasma appears like free space
–ForZ~ Z
pe
electromagnetic waves cannot
propagate
? Transmissions from below are reflected
? Transmissions from within are absorbed
–ForZ!~ Z
pe
random variations in n
e
can cause
random delays and phase shifts
Z
pe
n
e
e
2
H
o
m
§
?
¨
¨
·
1
?
?
1
2
H H
o
1
Z
pe
Z
§
?
¨
¨
·
1
?
?
2§
?
¨
¨
¨
·
1
?
?
?
39
SPACECRAFT CHARGING
? At LEO spacecraft become negatively charged
– Plasma is dense but low energy
– Orbital velocity is higher than ion thermal velocity
– Lower than electron thermal velocity
– Electrons impact all surfaces
– Ions impact ram surfaces only
? Geo spacecraft charge during magnetospheric substorms
between longitudes corresponding to midnight and dawn
? Biased surfaces (solar arrays) influence the floating
potential
40
CHARGING EFFECTS
? Instrument reading bias
? Arcing-induced EMI, electronics upsets
? Increased current collection
? Re-attraction of contaminants
? Ion sputtering, accelerated erosion of materials
Spacecraft must be designed to keep differential
charging below the breakdown voltages or must
tolerate the effects of discharges.
41
42
RADIATION
? Most radiation effects occur by energy depostion
– Function of both energy, type of particle and material into
which energy is deposited.
? Definitions
1 rad (Si) = 100 ergs/gm into Silicon
1 Cray (Si) = 1 J/kg into Si
1 rad (Si) = 10
-4
Cray
Adapted from SMAD.
43
RADIATION DAMAGE THRESHOLDS
In many materials the total dose of radiation is the most
critical issue. In other circumstances the time over which
the dose is received is equally important.
Material Damage Threshold (rad)
Biological Matter 10
1
-10
2
Electrical Matter 10
2
-10
4
Lubricants, hydraulic fluid 10
5
-10
7
Ceramics, glasses 10
6
-10
8
Polymeric materials 10
7
-10
9
Structural metals 10
9
-10
11
SPACECRAFT EFFECTS
? High energy particles travel through spacecraft
material and deposit kinetic energy
– Displaces atoms.
– Leaves a stream of charged atoms in their wake.
? Reduces power output of solar arrays
? Causes sensitive electronics to fail
? Increases sensor background noise
? Radiation exposure to crews
44
HIGH ENERGY RADIATION
? Definition
For Electrons E > 100 keV
For protons and heavy ions E > 1 MeV
? Sources
– Van Allen Belt (electrons and protons) (trapped
radiation)
– Galactic cosmic rays interplanetary protons and ionized
heavy nuclei
– Protons associated with solar proton events
45
VAN ALLEN BELTS
? Torodial belts around the earth made up of electrons
and ions (primarily protons) with energies > 30 keV.
? Two big zones
– Inner belt ~ 1000 Km 6000 km altitude
? Protons E > 10’s of MeV
? Electrons E ~ 1 - 10 MeV
– Outer belt 10,000 - 60,000 km
? Electrons E ~ 0.04 - 4.5 MeV
46
VAN ALLEN BELTS (cont.)
? Sources
– acceleration of lower-energy particles by magnetic
storm activity
– trapping of decay products produced by cosmic ray
collisions with the atmosphere
– solar flares
47
CONCENTRATION MECHANISM
? Earth’s magnetic field concentrates on large fluxes of
electrons, protons and some heavy ions.
? Radiation belt particles spiral back and forth along
magnetic field lines.
– Ionizing radiation belts reach lowest altitude of the eastern
coast of the eastern coast of South America (SAA).
48
(Image removed due to copyright considerations.)
ELECTRON AND PROTON
FLUXES
49
AP8Min Proton Fluxes (cm
-2
s
-1
)
2x10
6
2x10
6
3x10
6
10
6
-2.5
z(
R
e
)
x(R
e
)
2.5
2.5 5.0 7.5 10.0
0
0
10
5
10
4
10
3
10
2
10
5
10
4
10
3
10
2
AE8Max Electron Fluxes (cm
-2
s
-1
)
5 YEAR DOSE
50
10
2
10
2
10
3
10
4
5
YEAR
DOSE,
Rad
(Si)
ALTITUDE, nml
DMSP
Altitude
(96
0
)
GPS
Altitude
(55
0
-63
0
)
Synchronous altitude
(DSP, DSCS, Fltsatcom - 0
o
)
5Times
synchronous alt
Free space
(Flare only)
0
0
63.4
0
90
0
Inclination
Natural environment
No operational
satellites
10
5
10
6
10
3
10
4
10
5
10
6
TRAPPED RADIATION BELTS
51
10
4
10
8
10
7
10
6
10
9
5x10
8
N
Electrons >40 Kev
1
2
3
10
2
10
1
10
3
Distance from center
of earth (Earth Radii)
Protons >100 Mev
VAN ALLEN BELT
RADIATION STABILITY
? Inner belt
– Fairly stable with changes in solar cycle
– May change by a factor of three as a result of geomagnetic
storms loading in high energy electrons.
? Outer belt
– Electron concentrations may change by a factor of 1000 during
geomagnetic storms.
? Standard Models (AP8 protons) and (AE8 electrons)
– Require B, L and whether solar min/solar max
– Provide omni-directional fluxes of protons 50 keV < E < 500
MeV and electrons 50 keV < 7 Mev
52
53
SOLAR CELL DEGRADATION
0.4
NORMALIZED
EFFICIENCIES
0
10
13
10
14
1 MeV ELECTRON FLUENCE (cm
-2
)
Degradation caused by the radiation of InP, GaAs,
conventional (8mil) Si, and thin (3 mil) Si solar cells.
10
15
10
16
0.6
0.8
1.0
InP
GaAs/Ge
S1 (2.6-3.1 mils thick)
S1 (8 mils thick)
GALACTIC COSMIC RAYS
? Primarily interplanetary protons and ionized heavy
nuclei
– 1 MeV < E < 1 GeV per nucleon
Cause Single Event Upsets (SEU)
? Sources are outside the solar system
– other solar flares
– nova and supernova explosions
– quasars
54
PARTICLE RANGE
55
Ranges of Protons and Electrons in Aluminum
PARTICLE ENERGY (MeV)
RANGE
(cm)
Electrons
Protons
0.1
0.01
0.1
1
10
1 10 100
MAGNETIC SHIELDING
56
Magnetic Equator
12
Any
Ion
β
1147 MeV/n
2900 MeV/n
907 MeV
384 MeV
173 MeV
87 MeV
48 MeV
313 MeV/n
109 MeV/n
46 MeV/n
23 MeV/n
12 MeV/n
34 567
SOLAR PROTON EFFECTS
? Solar flares often eject high energy hydrogen
and other nuclei
– 1 MeV < E < 10 GeV/nucleon
– At low energies the number can be much greater
than galactic comic radiation level
? Solar events are sporadic but correlate
somewhat with the solar cycle
? These events make a Mass Mission hazardous
57
PARTICLE ENERGY
58
Energy (MeV)
10
1x10
-6
Galactic Cosmic
Rays
Particles/
m
2
sec
(MeV)
ster
Worst Case
Solar Flare Event
1x10
-4
1x10
-2
1x10
2
1
1x10
4
1x10
6
1x10
8
100 1000 10000 100000
SOLAR PROTON DOSE
59
FEYNMAN MODEL
Based on data from 1963 to 1991
60
1 Year
2 Year
FLUENCE (cm
-2
)
10
9
0.001
0.01
0.1
1
10
10
10
11
PROBABILITY
3 Year
5 Year
7Year
ELECTROMAGNETIC RADIATION
?Radio
– 1 - 10 MHz galactic electromagnetic radiation
– terminal noise
– not significant for single event environment
? Visible/IR
– solar flux
– heating
? UV/EUV/X-ray
– EUV @ 100 to 1000 ? is significant for surface chemistry
61
References
? Wertz, James R. and Wiley JH. Larson, Space Mission
Analysis and Design, Third edition, Microcosm Press, El
Segundo CA 1999
? Pisacane, Vincenti and Robert C. Moore, Fundamentals of
Space Systems, Oxford University Press, NY, 1994.
? http://nssdc.gsfc.nasa.gov/space/model/models_home.html
? http://nssdc.gsfc.nasa.gov/space/model/magnetos/igrf.html
62