Spacecraft charging

Space Environment

An overview

Introduction
• Space is often incorrectly thought of as a vast, empty vacuum
that begins at the outer reaches of the Earth's atmosphere and
extends throughout the universe. In reality, space is a dynamic
place that is filled with energetic particles, radiation, and trillions
of objects both very large and very small. Compared to what we
experience on Earth, it is a place of extremes. Distances are vast.
Velocities can range from zero to the speed of light.
Temperatures on the sunny side of an object can be very
high, yet extremely low on the shady side, just a short distance
away. Charged particles continually bombard exposed surfaces.
Some have so much energy that they pass completely through
an object in space. Magnetic fields can be intense. The
environment in space is constantly changing. All of these factors
influence the design and operation of space systems.

Solar activity
• Solar activity is also characterized by cycles of
various lengths. The Sun has a rotation period of
28 days, which exposes Earth to the surface
features of the Sun, such as sunspots. The
number of sunspots is characterized by an
11-year cycle. Sunspots are normally associated
in a complex, but not completely
understood, way with solar flares, i.e., the more
the sunspots, the more the solar flares. A change
in polarity of the overall solar magnetic field is
characterized by a 22 year cycle

EM radiation of the sun
• The spectrum of solar electromagnetic radiation extends
from the radio frequency (RF) range to the x-ray
frequencies, expanding to slightly higher frequencies during
solar flares The radiation received from the Sun varies
according to the Sun's rotation period. The sun emits three
general classes of radiation in the RF range. First, there is a
constant background noise over the whole radio spectrum
from the "quiet sun". Second, there is a slowly varying
component related to sunspots. Third, there is sporadic
emission related to centers of activity, such as solar flares.
Solar activity during solar maximum can be
catastrophic, especially if solar flares are involved. The
greatest concerns are possible interference with
communications and the threat to humans.

Solar Wind
• Because of the high temperature of the Sun's corona, solar protons
and electrons acquire velocities in excess of the escape velocity
from the sun. The result is that there is a continuous outward flow
of charged particles in all directions from the sun. This flow of
particles is called the solar wind. By the time the solar wind reaches
Earth's orbit, it is traveling at 185 - 435 mi/sec (300 to 700 km/sec).
The density is 1 - 10 particles per cubic centimeter. The velocity and
density of the solar wind vary with sunspot activity. The solar wind
causes a radiation pressure on satellites in orbit around the Earth.
Radiation pressure is a significant source of
perturbations, especially for satellites with large area to mass
ratios. Typically such a large ratio is the result of large solar panels
Radiation pressure causes frictional drag. It is present only on the
daylight side of Earth. The satellite is essentially shielded from solar
wind when it is on the night side of Earth. This causes irregular
perturbations of a satellite's orbital elements and corresponding
ground traces.

Solar Flares
• High speed solar protons emitted by a solar flare are radiation
hazards to space flight. Flares themselves are among the most
spectacular disturbances seen on the Sun. A flare may spread in
area during its lifetime, which may last from several minutes to a
few hours. There is a relationship between the number of sunspots
and the frequency of flare formation, but the most intense flares do
not necessarily occur at solar cycle maximum. There are many
events that may occur on Earth following a solar flare. In addition to
increase in visible light, minutes later there is a Sudden Ionosphere
Disturbance (SID) in Earth's ionosphere. This, in turn, causes short
wave fade-out, resulting in the loss of long-range communications
for 15 minutes to 1 hour. During the first few minutes of a
flare, there may be a radio noise storm. The first few minutes of this
storm causes noise over a wide range of frequencies that can be
heard as static in radio transmissions.

Cosmic Rays
• Cosmic rays originate from two sources: the Sun (solar
cosmic rays), and other stars throughout the universe
(galactic cosmic rays). This radiation is primarily high
velocity protons and electrons. The galactic cosmic rays are
extremely energetic, but do not pose a serious threat, due
to the low flux, the rate at which they enter the
atmosphere. The solar cosmic rays are not a serious threat
to humans, except during periods of solar flare
activity, when the radiation can increase a thousand-fold
over short periods. Cosmic ray particles can also cause
direct damage to internal components through collision.
Cosmic rays have the most impact on polar and
geosynchronous orbits. This is due to the fact that they are
outside or near the edge of the protective shielding
provided by Earth's magnetic field.

The first physical regime

Interplanetary space, where the dominant
environment is the solar wind. Typical values
characteristic of this region are (for high speed solar
wind streams at 1A.U., ref. 37): ion number density ~ 3
cm-3, ion temperature ~ 0.01 keV, flow speed velocity ~
500 km/s, magnetic field ~ 4 nT
The second physical regime

is the bow shock/magnetosheath region where the plasma environment of interplanetary space
and the geophysical magnetic field become separated. The magnetopause is the boundary
where the solar wind exerts a force on the magnetosphere and the solar wind particles
(electrons/protons) are repelled. The magnetosheath is the region between the magnetopause
and the bow shock, where a shockwave forms from the encounter of the supersonic solar wind
and the magnetosphere.

The third physical regime

The magnetosphere region, including the inner magnetosphere or
plasmasphere, dipole field, radiation belts, auroral regions, plasma/current sheet, and
the magnetotail. The plasmasphere is a doughnut-shaped region located near the Earth
with radial distance less than a few RE (RE = 6378 km) in the equatorial plane, and it
contains relatively dense (ne >102 cm-3)

EM forces
• As a satellite orbits the Earth, it travels through the magnetic fields
which cause the satellite to act like a magnet. The electrical or
electronic components within or outside the satellite set up
magnetic fields, which react with the Earth's magnetic field.
Another reason a satellite may act as a magnet is due to a negative
electrical charge that is generated by the satellite passing through
the partly ionized medium which produces a negative charge on the
satellite's skin. The negative charge is higher on the day side of the
orbit than on the night side. The motion of a charged satellite made
of conductive materials through Earth's magnetic field also results
in the satellite acquiring an electrical potential gradient which is
proportional to the intensity of the magnetic field and the velocity
of the satellite as it passes through the field. These cause a
magnetic drag to act upon the satellite. The drag can cause torquing
of the satellite.

Space environment Effets on
Satellites
• Satellite Charging
Satellite charging is a variation in the electrostatic
potential of a satellite with respect to the
surrounding low density plasma around the
satellite or to another part of the satellite. The
extent of the charging depends on both design
and orbit. The two primary mechanisms
responsible for charging are plasma
bombardment and photoelectric effects.

• Plasma bombardment occurs due to varying plasma
density, resulting in the surface of the satellite becoming
electrostatically charged. This can occur in the proximity of the Van
Allen radiation belts and the magnetotail. Charging from plasma
bombardment usually results in a negative charge on the surface of
the satellite. The photoelectric effect results from solar radiation
which liberates electrons on a satellite's surface, resulting in a
positive charge on the satellite's sunlit side. A satellite will usually
have a negative potential on shaded areas (due to plasma charging)
and a positive potential on sunlit areas (due to the photoelectric
effect). If the surface of the satellite is conductive, a current will
develop to cancel these potentials. For a non-conducting
surface, the charge separation will be maintained until voltage
exceeds the resistive threshold of the material. This leads to a
sudden electrostatic discharge.

Satellite Discharging
• The satellites most vulnerable to
charging/discharging are those located at
geosynchronous altitude. Discharges as high as
20,000 volts (V) have been experienced. Satellites
in geosynchronous orbits typically move both in
and out of the upper regions of the Van Allen
Radiation Belts and the Earth's magnetotail. This
results in a low plasma density around the
satellite which does not allow the charge to bleed
off or neutralize before a discharge occurs.

Hardware Damage
Sudden electrostatic discharge (high current
or arc) can cause hardware damage, such as:
Blown fuses or exploded
transistors, capacitors and other electronic
components.
• Vaporized metal parts.
• Structural damage
• Breakdown of thermal coatings

Electric Problems
• These discharges can result in electrical or
electronic problems, such as: False commands
• On/Off circuit switching
• Memory changes
• Solar cell degradation
• Degradation of optical sensors

Particle Collision
• High energy solar flare particles and galactic
cosmic rays can cause direct damage to the
surface of a satellite. The damage can include
vaporization of surface materials and
structural damage. These particles can also
enter star or horizon sensors and mimic
reference points. This can lead to false
readings resulting in loss of attitude: antennas
and solar panels pointing in the wrong
direction, miscorrection of orbit.

Outgassing
• Although the environment in space is not benign, the density of
particles above 100 miles altitude is extremely low. There is almost
no atmospheric pressure, similar to a complete vacuum. As a
result, satellites and the materials they are made of experience
phenomena which are not encountered on Earth. In a
vacuum, some materials experience outgassing. Outgassing is a
phenomena where molecules of material evaporate into space.
Although many materials experience outgassing, composite
materials and those made with volatile solvents are particularly
susceptible. These include electronic microchips, plastics, glues.
Outgassing can result in changes to the physical properties of a
material, In addition, the evaporating molecules can form a thin
film over other components of the satellite, thereby affecting their
performance. Outgassing can be minimized through careful
selection of component materials but eventually some components
will exhibit different characteristics and properties.

Space Debris
• Space debris is defined as any non-operational man-made object of
any size in space. The size of space debris varies from complete
inoperative satellites and expended rocket bodies to small chips of
paint. Of the almost 10,000 man-made items in space currently
tracked and catalogued, only about 5% are operational space
systems. The rest is space debris. Space debris smaller than
approximately 2 cm (0.78 inches) cannot be detected and tracked
reliably, therefore it is reasonable to assume that there is
significantly more space debris than we know about. It has been
estimated that as many as 100 satellites have broken up while in
orbit, sometimes due to explosions of propulsion systems, and at
other times due to impact with other space debris. The result is a
an estimated 40,000 to 80,000 pieces of debris in orbit around the
Earth. There is even a wrench that became space debris when it
drifted away from an astronaut during a space walk. Most debris is
small but it can be traveling at relatively high speeds.

Meteoroids
• It is estimated that approximately 20,000 tons of
natural material is added to the Earth each year from
impacts of meteoroids and asteroid fragments with
the Earth's atmosphere. Most of these particles are
the size of dust particles, however, some are much
larger. When meteoroids enter the Earth's
atmosphere they usually burn up due to the friction
with the air molecules. Larger meteoroids generate
enough light to be seen as meteors streaking across
the night sky. Occasionally, larger objects don't
completely vaporize. When a piece strikes the
surface of the Earth it is called a meteorite.

Contd……
• These particles represent a constant natural danger to
satellites in orbit around the Earth. The Long Duration
Exposure Facility (LDEF) was a U.S. research satellite that
remained in orbit for six years before it was recovered by
the Shuttle and returned to the Earth. Examination of the
exposed surfaces indicate thousands of impacts by micro-
meteoroids. Microscopic examination has revealed
extensive damage to metal surfaces. Most meteoroids are
too small and traveling too fast to be detectable in time for
satellite controllers to direct a satellite to change it's orbit
to avoid collision. Shielding and other design considerations
are the most effective means to protect satellites from
catastrophic damage.

Deep Charging

Deep charging of a satellite occurs when
cosmic ray particles pass through a satellite
and ionize atoms within, through collisions.
Some of these particles are solar in origin, but
the majority are galactic and with no
preference to time or light conditions. They do
show some dependence on the solar cycle.

Outline
• Charging
• Charging currents
• Surface charging
• Absolute/differential charging
• Spacecraft charging environments

Spacecraft Environment
Many spacecraft have been lost due to lack of
full understanding environment
• Design to fly in space
• Flight environment
• Qualification test (design is correct)

Earth Environment
Atmosphere a primary source of problems
• Corrosive Atmosphere
Disintegration of atom due to chemical reaction
1. It destroys or irreversible damage to the
substance (strong acid or bases)
2. Stress concentration or embrittlement
(Loss of ductility, making a material brittle)

• Humidity control
To exclude oxygen and moisture using dry
nitrogen or helium purge
Low humidity-------- built up of static charge

40-50% range is a good compromise
Airborne particulate contamination (Sensors
whose location is air)

• Triboelectric effect (Contact electrification)
Ic’s or MOS are extremely sensitive to high
voltage and can easily damage by a discharge
Precautions: Clean room workers must be
grounded, i.e, conductive flooring, conductive
shoes
• Road vibration and shocks (a transient physical
excitation, sudden acceleration or deceleration
), could be higher than imposed by launch

Space and upper atmosphere
environment
• Hard Vaccum
• Low gravitational acceleration
• Ionizing radiations
• Several thermal gradients
• Micrometeoroids
• Orbital Debris
• Out gassing

Radiation Interactions

• Permanent radiation effects
– Change in material that persists after material
removed from radiation source
– Typically caused by atomic displacements in the
material
• Transient radiation effects
– Change in material does not persist after material
removed from radiation source
– Alters material properties during exposure

• Geostationary orbit (GEO), the orbital location
where a body holds a fixed position relative to
the rotating Earth, is located at 6.6 RE. This region
is a region of high variability. Geostationary orbit
tends to skim the inner boundary of the plasma
sheet, and the radiation belts and ring current are
located near GEO, as well. During solar quiet
times, the bodies orbiting at this location are
typically at dipolar-like field lines. During large
magnetic storms, the bodies orbiting at GEO are
within stretched field lines.

• Plasma Source
The Sun ejects charged particles and energetic
photons into space. These charged particles and
photons create an electrically active plasma
around the Earth. The collective behavior of
plasma generates an electric field that, in
turn, affects the charged particle motion. Source
of plasma produced within the space vehicle is
the ionized gas from the
thrustering, plume, venting and man-made
hardware design such as plasma contactor.

Plasma Effects on Spacecraft Electrical
Systems
• As a spacecraft travels through this ionized portion of
the atmosphere, it may be subjected to an unequal flux
of ions and electrons and may develop an induced
charge. Plasma flux to the spacecraft surface can
charge the surface and disrupt the operation of
electrically biased instruments. This is why the choice
of spacecraft ground is an important consideration in
determining where the spacecraft will float electrically
relative to the surrounding plasma. The graphic depicts
an electrical interaction between a spacecraft and the
surrounding plasma.

Plasma Effects on Spacecraft
• The plasma environment around the space vehicle will be altered
by the presence, operation, and motion of the vehicle. Potential
adverse impacts of interactions between the space vehicle and the
ionospheric plasma make it important to mitigate(moderate) the
plasma effects. Some of these effects are:
• plasma wave generation
• arcing and sputtering at significantly high negative potential relative
to the plasma
• spacecraft charging at high elliptical orbits
• corona/ESD phenomena
• current balance between the space vehicle and the ambient plasma
• geomagnetic field effect

LEO Plasma Environment
• Quasi-neutral plasma
• At 300 km, n ~ 105 cm-3
• Te,i ~ 1000 K (quasi-equilibrium)
• Je ~ 1 mA/m2
• Photoemission ~ 10 A/m2
• Secondary electron emission ~ 0.01 Je
• Sputtering yield is negligible
• LEO major source is incident ambient plasma
• Enhancement of plasma environment at high
inclinations (auroral zones)
– High density
– High energy (several keV)

GEO Plasma Environment
• Plasma is not quasi-neutral
• At GEO, n ~ 1 cm-3
• Energies
– Ions: 10 keV (H+)
– Electrons 2.4 keV
• Je ~ 10 nA/m2
• Photoemission ~ 10 A/m2
• Secondary electron emission and sputtering yield are
not negligible
• Enhanced by solar storms / events

Space plasma and spacecraft charging
• Plasma
• Source of charge particles: auroras, radiation
belts
• Charging currents
• Absolute and differential charging

Charging Processes
• (i) interaction of spacecraft with gaseous
plasma particles,
• (ii) interaction of spacecraft with energetic
• particles (electrons and ions)
• (iii) interaction of spacecraft with photons.

Relevance with the charging of a dust
Grain
• When dust grains are immersed in a gaseous
plasma, the plasma particles (electrons and ions) are
collected by the dust grains which act as probes. where
j represents the plasma species (electrons and ions)
and I_ j is the current associated with the species j .

• At equilibrium the net current flowing onto the dust
grain surface becomes zero, i.e.

• This means that the dust grain surface acquires some
potential φg which is −2.5kBT/e (where T = Te Ti) for a
hydrogen plasma and −3.6kBT/e for an oxygen plasma

Charging Phenomenon
• When energetic plasma particles (electrons or ions) are incident
onto a dust grain surface,
• they are either backscattered/reflected or they pass through the
dust grain/Spacecraft material.
• During their passage they may lose their energy partially or fully.
• A portion of the lost energy can go into exciting other electrons that
in turn may escape from the material. The emitted electrons are
known as secondary electrons.
• The release of these secondary electrons from the dust grain tends
to make the grain surface positive.
• The interaction of photons incident onto the dust grain surface
causes photoemission of electrons from the dust grain surface. The
dust grains, which emit photoelectrons, may become positively
charged. The emitted electrons collide with other dust grains and
are captured by some of these grains

• There are, of course, a number of other dust
grain charging mechanisms, namely
• Thermionic emission
• field emission
• radioactivity
• impact ionization
These are significant only in some different
special circumstances.

Field Emission
• Field emission (FE) (also known as field electron
emission and electron field emission) is emission
of electrons induced by an electrostatic field. The
most common context is FE from a solid surface
into vacuum. The terminology is historical
because related phenomena of surface
photoeffect, thermionic emission or Richardson-
Dushman effect and "cold electronic
emission", i.e. the emission of electrons in strong
static (or quasi-static) electric fields

• Field emission in pure metals occurs in high
electric fields: the gradients are typically
higher than 1 gigavolt per meter and strongly
dependent upon the work function. Electron
sources based on field emission have a
number of applications, but it is most
commonly an undesirable primary source of
vacuum breakdown and electrical discharge
phenomena, which engineers work to
prevent.

• Examples of applications for surface field
emission include construction of bright
electron sources for high-resolution electron
microscopes or to discharge spacecraft from
induced charges. Devices which eliminate
induced charges are termed charge-
neutralizers.

• As spacecraft moves through plasma, it
encounter with electrons
• Negative current tending to charge the spacecraft
• Coulomb force build up enhancing the attraction
of positive ions
• Floating potential (Ie=Ii)
• FP depend upon orbit parameters, spacecraft size
and geometry, solar cycle and terrestrial seasons
etc.

Current due to electron and ion
collection

Current due to Photoemission
• When a flux of photons with energy (hν) larger than
the photoelectric work function (Wf) of the dust grain
incidents on the dust grain surface, the latter emits
photoelectrons, where .h is Planck’s constant and ν is
the photon frequency.
• The photoemission of electrons depends on
• (i) the wavelength of the incident photons,
• (ii) the surface area of object
• (iii) the properties of the material
• This mechanism contributes to a positive charging
current and tries to make the dust grain positively
charged.

Various metals typically have photoelectric work function
Wf < 5 eV, such as Ag (Wf = 4.46 eV), Cu (Wf = 4.45
eV), Al (Wf = 4.2 eV), Ca (Wf = 3.2 eV) and Cs (Wf = 1.8
eV). There are also a number of low work
functionmaterials, e.g. carbides (binary compounds of
carbon and more electropositive metals) with work
functions Wf 2.18–3.50 eV, borides (binary compounds
of boron and more electropositive metals) with work
functions Wf = 2.45–2.92 eV, oxides of metals with
work functions ranging from Wf = 1 eV(Cs) to Wf = 4 eV
(zirconium)

Space charging environment
• High energy Electrons
• Auroral regions and the GEO environments include
high-energy electron populations which typically have
a Maxwellian distribution and a characteristic
temperature Te. When a significant plasma
environment and photoelectrons arising from solar
radiation are not present, the potential to which a
spacecraft will charge is directly proportional to the
electron temperature and varies between 1 to 20 kV.
Electrons with energies between 1-100 keV contribute
to surface charging, while trapped electrons with
energies above 100 keV penetrate the surface and
contribute to internal charging effects.

Solar Radiation
• Photons emitted from the Sun have an important effect in
surface charging. UV and EUV photon impacts on
spacecraft surfaces result in the emission of photoelectrons
(by the photoelectric effect). These photoelectrons
constitute a current out of the spacecraft
• surface, which can reduce the effect of negative surface
charging and hence it can be an important contributor to
the surface charging mechanism described in section 2. The
effect of solar photons is particularly important for GEO
orbits where the plasma density is low and the contribution
of photoelectrons is not negligible. The photoelectron
current depends on the surface material of the
spacecraft, the solar activity, solar incidence angle and
spacecraft potential (Lucas, 1973).

Magnetic Fields
• The Earth's magnetic field is approximately a magnetic dipole which is
displaced from the center of the Earth by ~ 436 km. The geomagnetic
axis is inclined at 11.5° with respect to the rotational axis of the Earth.
The Earth's magnetic field has great influence on plasma motions and
on trapped high-energy charged particles, which lead to spacecraft
charging and damages to electronics. The magnetic field determines
the regions of the space environment where spacecraft charging can
occur. It also plays a role in the surface charging mechanism since it
can affect the escape of electrons (such as photoelectrons) emitted
from the spacecraft surface. This idea is illustrated in Figure
(Laframboise, 1983). If the magnetic field is nearly perpendicular to the
spacecraft surface, electrons can escape along the field lines (Figure
3(a)). On the other hand, if the magnetic field is inclined with respect
to the surface, electrons may be redirected to the surface by the
Lorentz force law: (Figure 3(b)). This prevention of electron escape
from the surface increases the negative surface charge.

Surface Charging
• Mechanism
1. Low energy plasma and photoelectric current
2. The midnight to dawn sector is a favored region for surface
charging
• The spacecraft surface potential is a function of the net
current flow to/from the spacecraft surface.
• These currents are from solar photon-induced
photoelectrons
• leaving the surface, plasma electrons and ions impinging on
the surface, and charged particles emitted from the vehicle
(e.g., from active ion emission). A balance equation for
current density can be written as:

• Spacecraft charging is a function of the space environment
characteristics, including sunlight/eclipse, solar
activity, geomagnetic activity, electron flux magnitude and
spectrum. These effects can be (dis)advantageous,
• for instance, sunlight exposure provides photoemission, that can
act as a charge drain to neutralize the surface potential, or that can
act as a discharge trigger upon emergence from eclipse/shadow.
Table I is a reproduction of extreme satellite potentials in different
plasma which indicate for sun exposure that at lower altitudes (< 2
RE) the flux of plasma electrons to the satellite is greater than the
photoelectric flux, so the satellite becomes negatively charged;
whereas, for higher altitudes (> 3 RE) the photoelectric flux
dominates and the spacecraft becomes positively charged.

• At geosynchronous orbit (GEO), surfaces exposed to sunlight charge
to 2-3 volts (positive) due to the photoelectron current emitted
from the surface; during eclipses, a negative surface potential is
observed.[2] Photoemission from the extreme ultraviolet
wavelength range (< 2000 Å) is the most important since in that
region many materials have rather large photoelectric yields and
the solar spectrum also has significant energy there. In eclipse the
spacecraft roughly charges to a negative potential equivalent to the
electron temperature (i.e., kT). Shadowed dielectric or isolated
surfaces, the potential may charge to 1 to 10 kV (negative) from
local electrons. Surface discharges are primarily caused by low
energy electrons (up to a few tens of keV). In low Earth orbit
(LEO), the thermal electron currents are the largest and satellites
tend to be slightly negative

Differential Charging
• Typically, differential charging has occurred after
geomagnetic substorms, which result in the injection of keV
electrons into the magnetosphere.
• While in eclipse, the spacecraft may negatively charge to
tens of kilovolts.
• A potential sufficient for discharge is easily created when
the satellite emerges into sunlight, which results in positive
surface charge due to photoelectron emission.
• Differential charging can also be caused by satellite self-
shadowing.
• The basic solution to differential charging problems is to
provide a common ground for the spacecraft surface
(including internal structures).

• Spacecraft in geosynchronous orbit are more
likely to undergo differential charging.
However, use of a high-voltage power system
in a low earth orbit satellite can increase the
adverse environmental effects.

Conditions required for spacecraft charging

Spacecraft charging

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