The Space Environment I: Characteristics of the Vacuum,
Neutral, and MMOD Environments
Dr. Andrew KetsdeverMAE 5595Lesson 4
Solar Structure
Solar Properties• The Sun is the dominant energy source driving
the structure of the atmosphere– Photon radiation– Particle emission
• Sun can be modeled as a black body with T = 5800 K– Good approximation for visible and IR emission
• Wavelength of peak spectral intensity (Wien’s Law) ~ 0.5 µm• Spectral intensity as a function of wavelength given by
Planck function– Does not model high energy photon emission well
• Under-estimates UV and higher energy photon emission• Structure of UV emission is important for upper atmospheric
processes
Solar Properties
Solar Properties
“Continuum” Radiation
Solar Properties
Discrete Radiation
Solar Structure
• Photosphere– Visible disk of the sun– Temperature ranges from 6000 K to 4300 K near the
boundary with the chromosphere– Mostly visible wavelength emission
• Chromosphere– Temperatures rise rapidly up to 25,000 K
• Corona– Low density (not visible without eclipse)– High temperature ~ 2x106 K– Reaches as far as 10x the photosphere diameter
Solar Emission• EM Radiation
– 3.9 x 1033 ergs/sec– 1360 W/m2 at Earth’s upper
atmosphere• 52% IR• 41% Visible• <7% UV• 0.1% EUV• 0.1% Radio
– EUV is extremely important• Ability to dissociate and ionize upper atmospheric
species• Similar processes can be active on spacecraft
materials
Solar Emission• Particles
– Solar Wind• 96% Protons• 4% Alpha Particles• Mean velocity ~ 450 km/s (max ~
800 km/s)• Particles follow solar magnetic
field lines• Number density ~10 cm-3 at 1 AU• Temperatures from 104 – 105 K
– Solar Flares / Coronal Mass Ejections
• Events that can increase flux of particles
• Can also increase energy of particles
Solar Emission
Solar Cycle
Current Solar Cycle
F10.7
• F10.7 is the flux of the 10.7 cm (radio) frequency emitted from the sun– Of interest since the intensity of the 10.7 cm emission from the sun
follows the solar EUV emission– 10.7 cm wavelengths can be measured from ground based facilities
Earth’s Neutral Environment
Earth’s Neutral Environment• Earth’s Neutral Atmosphere is divided into several
regimes– Temperature– Chemical composition
• Vertical distribution of pressure
−−=Hzzzpzp o
o exp)()(
mgkTH =
Earth’s Neutral Environment
Earth’s Neutral Environment
UV Atmospheric Windows
IR Atmospheric Windows
Vacuum Environment
Characteristics of the Vacuum Environment
• Solar UV Radiation (Relatively low energy)– UV radiation from the sun is not absorbed
above the Stratosphere– Highly energetic photons interact with
spacecraft surfaces (degradation)• Vacuum
– Space environment is extremely rarefied above 100 km
– Reduction in pressure and/or an increase in temperature causes outgassing
Solar UV Environment
• UV / Surface Interaction– Extinction
• Loss of the photon in the interaction• Energy absorbed by the material
– Dissociation– Ionization– Photo-electron production
– Scattering• Reflection• Transmission
Effects of the Space Environment
Year on-orbit
Material Outgassing• Molecular release from a material into the
gaseous phase– Reduced pressure– Elevated temperature
• Highly volatile, loosely bound molecules• Solar UV can enhance surface outgassing
– Direct energy coupling– Surface material heating
• Contamination potential for critical spacecraft components
Contamination
• Sources– Molecular Outgassing– Particulate– Spacecraft Thrusters– Water Dumps– Gas Vents– Cabin Leakage
Contamination
Molecular Particulate
Neutral Environment
Spacecraft Drag• Existence of molecules and atoms in low-Earth
orbit causes spacecraft drag– Takes energy out of the orbit– Causes orbit to decay (decrease in size)
fD
fDD
ACmB
vBmAvCF
=
== 22
21
21 ρρ
EXAMPLE MISSION• Real nano-satellite (m < 20kg) mission
– Original launch date: Mid-2003 on Shuttle• Shuttle grounded• New launch on Delta IV Heavy Lift Demo
December 2004– REQUIREMENT: For all the science to be
performed on-board, want the satellite lifetime in orbit > 85 days.
– QUESTION: What is the minimum altitude for the nanosat operations?
Spacecraft Altitude• Determines orbital
velocity
• Determines atmospheric density
.)( altRV
Ecircular +
=µ
0.10
10
103
105
107
109
1011
200 400 600 800 1000
Num
ber D
ensi
ty (c
m-3
)
Altitude (km)
N2
He
O
H
O2Ar
F10.7 = 139.5 sfu
Atmospheric Density Variations
• Can depend on– Solar output
• Highly variable• 11 year solar cycle
– Location in orbit (lat/long)– Diurnal variations– Seasonal variations– Whether the Dodgers will
make the playoffs
What do we need to know?• Launch date (why?)• Mission requirements• Spacecraft configuration…
66.2Maximum Ballistic Coefficient (kg/m2)37.0Minimum Ballistic Coefficient (kg/m2)2.7Maximum Drag Coefficient
0.150Maximum Cross Sectional Area (m2)18Maximum Total Mass (kg)2.0Minimum Drag Coefficient
0.136Minimum Cross Sectional Area (m2)15Minimum Total Mass (kg)
LAUNCH DATE DETERMINES:Atmospheric Density Due to Solar
Variations
50
100
150
200
250
Actual, Penticton, B.C., CanadaPredictedHighLow
F10.
7 (x
10-2
2 W /
m2 H
z)
Date
1991
2001
2003
Potential Launch
1993
1995
1997
1999
2005
Atmospheric Density• Based on predicted solar activity, a model is used
to determine the range of atmospheric density
10-19
10-17
10-15
10-13
10-11
200 400 600 800 1000
F10.7 = 70 sfuF10.7 = 100 sfuF10.7 = 150 sfuF10.7 = 200 sfuF10.7 = 250 sfu
Mas
s Den
sity
(g/c
m3 )
Altitude (km)
Predicted Solar Output• Not highly accurate, like predicting Tropospheric
weather• A range is necessary (max, min, mean)
10-16
10-15
10-14
10-13
10-12
10-11
10-10
0 100 200 300 400 500 600
PredictedHighLow
Mas
s Den
sity
(g/c
m3 )
Altitude (km)
Orbital Variations
• The atmospheric density can also vary with latitude and longitude or orbital true anomaly
• Diurnal• Etc.
3.5 108
4.0 108
4.5 108
5.0 108
5.5 108
6.0 108
0 20 40 60 80
Tota
l Num
ber D
ensi
ty (c
m-3
)
Orbital Time (min)
F10.7 = 139.5 sfu
Orbital Lifetime• A good approximation of ρ leads to
– Good estimate of orbital drag force– Good estimate of satellite lifetime (integrated effect)
• A bit complicated. Drag reduces orbital altitude, which increases drag which reduces orbital altitude which….
• A lot of effort being placed on predicting Space Weather– Affects all spacecraft
• LEO – density– Changes in drag force, atomic oxygen concentration
• POLAR – high energy particles (aurora)– Spacecraft charging
• GEO – high energy solar particles– Spacecraft charging
Satellite Orbital Lifetime
0.0
100.0
200.0
300.0
400.0
500.0
600.0
360 380 400 420 440
Deorbit to 65 kmDeorbit to 250 km
Life
time
(Day
s)
Initial Altitude (km)
F10.7 = 139.5 sfu
Satellite Orbital Lifetime
57.2584450
42.9312425
28.6174400
14.3100375
058350
∆v to raise orbit from 350 km (m/sec)
Lifetime (days) for B = 66.2 kg/m2
Final Altitude (km)
Questions• The mission was delayed from early 2003 to
winter of 2004. Was this a benefit to the overall mission from a drag standpoint?
• Are there scenarios where delays could cause a mission to be completely scrubbed?
• If the drag at a given altitude is too high, what can be done about it?
• What are some ways that the lifetime could be increased even at the original 350 km altitude?
• Is atmospheric drag a problem for MEO or GEO spacecraft?
• If we want to use thrusters to maintain our orbit (i.e. counteract drag), is the ∆V positive or negative w.r.t. the satellite velocity vector?
Effects of Atomic Oxygen• Atomic oxygen is formed
from the photo-dissociation of molecular oxygen by solar EUV
• Atomic oxygen in extremely reactive
• Atomic oxygen interacts with spacecraft in LEO with relative energies of approximately 5 eV (Vs/c ~ 8 km/s, VO ~ Thermal)
• Material Degradation• Shuttle Glow
Material Degradation
Spacecraft Glow• Source of optical emission
that originates from the spacecraft itself– Driven by molecular
adsorption and atomic oxygen
• Observed on many LEO satellites
• Source of contamination for optical observations
Shuttle Glow
Shuttle Glow
Shuttle Glow
Micrometeoroid and Orbital Debris Environment
Micrometeoroids
Orbital Debris
MMOD
Earth Orbiting Satellites
MMOD
MMOD
MMOD
MMOD
MMOD
MMOD
MMOD