The Space Environment
•
INTRODUCTION
•
THE SOLAR CYCLE
•
THE GRAVITATIONAL FIELD AND MICROGRAVITY*
•
THE UPPER ATMOSPHERE*
•
RADIATION AND ASSOCIATED DEGRADATION
•
HARDNESS AND SURVIVABILITY REQUIREMENTS
•
STRATEGIES FOR ACHIEVING SURVIVABILITY
Introduction

Near-Earth space and atmospheric environments strongly influence :

Performance

Life time of operational space system

They effect their size weight complexity and cost

They can also lead to loss of components

Once orbit is obtained, the spacecraft is subjected to a very tenuous
atmosphere [Tascione, 1994]. At lower orbits a spacecraft will be bombarded
by the atmosphere at orbital velocities on the order of -8 kmls.

Interactions between the satellite and the neutral atmosphere can erode
satellite surfaces, affect the thermal and electrical properties of the surface,
and possibly degrade spacecraft structures.
More glimpse
. More energetic space mdiation, such as electrons with energies from about 200 ke V to 15 MeV
can become embedded in dielectric components and produce electrostatic discharg~ in cable
insulation and circuit boards.
* Very energetic (MeV -Ge V) charged particles can be found in the trapped radiation
belts, solar flare protous, and galactic cosmic rays.
* The total dose effects of this highenergy radiation can degrade microelectronic devices,
solar arrays, and sensors.
• A single energetic particle can also cause single-event phenomena within
microelectronic devices which can temporarily disrupt or permanently damage
components.
• Lastly, orbiting spacecraft are periodically subjected to hypervelocity impacts by
1 J1IIl or larger sized pieces of dust and debris. If the impacting particles originate in
nature they are termed micrometeoroids
The Solar Cycle
• This subject is of particular interest because of the fact that the solar
activity is seen to vary with an ll-year cycle as shown in Fig. 8-1 [NOAA,
1991).
• The plot shows the FlO.7 index, which is the mean daily flux at 10.7 em
wavelength in units of 10-22 W/m2 • Hz.
• The peaks in the FI0.7 index are called solar maxima, while the valleys
are called solar minima. N
The Gravitational Field and Microgravity*
* Microgravity, also called weightlessness, free fall, or zero-g, is the nearly complete absence of
any of the effects of gravity.
* In the microgravity environment of-a satellite, objects don't fall. particles don't settle out of solution,
bubbles don't rise, and convection CUITents don't occur.
* Yet in low-Earth orbit, where all of these phenomena occur, the gravitational force is about 90% of
its value at the Earth's surface. Indeed, it is the gravitational field that holds the satellite in its orbit.
source
X direction
Y direction
X direction
(velocity)
(orbit normal)
(nadir)
Aerodynamic Drag
Gravity Gradient
Centrifugal (due to
spacecraft rotation in
inertial frame
Sinusoidal Vibration
along X axis of
frequency f and
amplitude A
Coriolis force from
material moving in
the spacecraft frame
Satellit
e
Mass
(kg)
Shape
Max
Min
Max
Max
Min
Type
XA
XA
XA
Drag
coef
Ballisti
c
Ballisti
c
Of
Coef
Coef
missio
n
The Upper Atmosphere*
The upper atmosphere affects spacecraft by generating aerodynamic drag lift and
heat, and through the chemically corrosive effects of highly reactive elements such as atomic
oxygen. The effects of aerodynamic lift and heating are important during launch and reentry
Radiation and Associated
Degradation

..
Trapped Radiation·
The Van Allen radiation belts are a permanent hazard to orbiting
spacecraft.
They consist of electrons and ions (mostly protons) having energies
greater than 30 ke V and are distributed nonuniformly within the
magnetosphere.
As illustrated in Fig. 8-7, the energetic electrons preferentially
populate a pair of toroidal regions centered on the magnetic shells L 1.3 (inner zone) and L - 5 (outer zone).
Hardness and Survivability
Requirements
1. Survivability is the ability of a space system to perform its intended function after being
exposed to a stressing natural environment or one created by an enemy or hostile agent.
2. Hardness is an attribute defining the environmental stress level which a space system can
survive.
3. As an example, a satellite or spacecraft which can withstand an X-ray fluence of 1.0 calIcm2
or absorption of 107 rads (Si) of total dose (a rad of absorbed dose is approximately 100 ergs/g)
has a hardness of that amount.
4. (Fluence is the time integral of flux. Flux is the flow of energy per unit time and per unit
crosssectional area.)
Radiation Effects on Optical link
components
NOTES:
Optical fibres
• Damage worse and annealing slower for lower temperatures. Losses generally lower for Increasing
wavelength (to 1.5 pm)
• Polymer clad silica cores have lowest losses but losses Incraase below - 20 ·C. Max dose usage 01
_107_1OS rads
Transmitters
• At higher temperatures, threshold current and peak wavelength Increase while output power decreases
for laser diodes
• LEOs have better temperature thermal stability, longer lifetimes, greater reliability and lower cost
Detectors
• APDs are predicted to be more sensitive than PIN diodes to total dose, neutrons and dose rate
• AlGaAs/GaAs pholodtodes shown to be more radiation resistant than hard PIN photodiodes.
Device type
Optical Fibres
Total dose
Neutron
Prompt Dose Rate
Natural Van Allen belts, manmade events
Man-made events primarily
Man made events with shortterm irradiation times
Depends on device and device
technology
Depends on device and device
technology . Circuit upset and
bumout possible.
>=100 k rad, polymer clad silica,
20 ·C, 0.85 cm2
0.02-0.5 dB/m loss (1-2 rate,
wavelength and orders less loss
at 1.5 pm).
Transmitters
detectors
Opto- modulators
Depends on device and device
technology