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