asen-5335 aerospace environments -- radiation effects on space systems 1 spacecraft charging...

ASEN-5335 Aerospace Environments -- Radiation Effects on Space Systems 1

SPACECRAFT CHARGING

Spacecraft charging is a variation in the electrostatic potential of a spacecraft surface.

Two categories of charging are of relevance:

1. Surface charging (also includes differential charging)

2. Internal dielectric (bulk, buried, deep or thick) charging

The relevant plasma energies are from eV to keV levels, as compared to MeV particles typical of ionizing radiation.

Differential charging correlates best with intensity of electrons with E ≤ 50 keV.

Electrons with energy > 50 keV can penetrate spacecraft surface metallization to cause internal discharge.


Surface Charging

Surface charging is created from low-energy plasma and photoelectric currents.

All currents (positive and negative) to and from the surface must balance; in order to obtain this balance, the surface potential (voltage) must vary. Some parts of the spacecraft will out of necessity generate higher potentials than others.


The balance equation for current density:

Jelec + Jion + Jpe + Jsec + Jback + Jart = 0Currents due toexternal plasmaelectrons andIons.

Netphotoelectroncurrent

Backscatteredelectron current fromelectronsreflected back from the surface with some energy loss.

Net current due to secondary electrons (few eV)generated by energetic primaries (electrons andions) at the satellite surface.

A possibleartificialcurrent(ion orelectronbeam).


A spacecraft placed in the plasma will assume a floating potential different from the plasma itself.

In shadow, a spacecraft will tendto charge negatively from theambient plasma electrons.

The plasma is neutral, with equalnumbers of electrons and ions.

However, the lighter electronsmove at higher velocities, andhence the negative electroncurrent to the spacecraft is greater than the positive ioncurrent.

Surface in Shadow


A spacecraft placed in the plasma will assume a floating potential different from the plasma itself.

Equilibrium is achieved when the flow of escaping

photoelectrons (photoelectron current) is equal to the difference between the

incoming flows of plasma ions and electrons (net other

current).

In sunlight at < 2 RE the flux of plasma electrons to the satellite is greater than the photoelectric

flux, so the satellite becomes negatively charged.

In sunlight at > 3 RE thephotoelectric flux dominates

and the satellite becomespositively charged.

Surface in Sunlight


Subject to the constraint of current balance as well as the kinetics of plasma-surface interactions, an equation for the electric potential over the spacecraft surface is solved numerically. A popular model is the NASCAP code (NASA Charging Analyzer Program). The following figure compares a NASCAP simulation with actual data from the SCATHA (Satellite Charging at High Altitudes) Satellite (P78-2):

NASA NASCAP representation of the FUSE satellite

The SCATHA Satellite was specifically designed to study spacecraft charging (launch, January, 1979).

Charging Simulation Using NASCAP


• Photoelectron flux (photoemission) tends to maintain a positively charged satellite. (Actually, satellites are generally negatively charged.) • When photoelectron flux is removed, satellite potential is driven further negative.

This has a finite time constant because eclipsing does not take place instantaneously, and it varies depending on the wavelength of light.

1979

Going into darkness (photoelectron flux removed); flow of electrons is now to satellite surface rather than away from it.


Spacecraft charging is vehicle as well as orbit dependent. For instance, a spherical satellite with a homogeneous conducting surface would be able to distribute charge evenly and effectively, so that the vehicle's potential would be uniform.

Thus vehicle design is an important factor in avoiding spacecraft charging problems.

Absolute vs. Differential Charging

Absolute charging occurs when the satellite potential relative to the ambient plasma is changed uniformly. Absolute charging is not generally detrimental.

Differential charging between different points on the spacecraftsurface is a serious problem.

Number of arcs per hour as a function of daily average ap for a geosynchronous satellite.


Differential charging refers to the variation of charge or potential between different points on the spacecraft surface.

This generally leads to discharges or arcing and EMI generation (and resultant transient pulses) which can lead to a several types of operational anomalies:

Differential Charging

• spurious switching activity (i.e., turning off a recorder or activating a radio or control system)

• breakdown of vehicle thermal coatings

• amplifier and/or solar cell degradation

• degradation of optical sensors by arcing and attraction of chemicals


Internal (or “Deep”) Dielectric Charging

Internal dielectric charging is caused by high-energy ( >100’s keV) electrons penetrating dielectric materials (i.e., printed circuit boards).

If sufficient charge builds up, an arc discharge ensues that appears as a pulse (~ tens of nanoseconds).

Deep charging in a cable and inside a “black” box.

• By internal charging we mean electrons have penetrated satellite material and deposit their charge within subsystem elements.

• This charge can end up on isolated conductors, such as ungrounded radiation shields, or buried in dielectrics.

• The charge can build up to “breakdown” levels leading to arc discharges into sensitive circuits.


Occurrence is highest in radiation belts 1-3 days after magnetic storms.

The use of leaky dielectrics, proper grounding and shielding can reduce the possibility of internal charging.

For instance, Kapton and Teflon are dielectric materials often used as thermal blankets on satellites; however, they poorly distribute electric charge.

Internal discharge is especially damaging because it often occurs within sensitive electronic circuitry.

Internal charging can affect cable wrap, wire insulation, circuit boards, electrical connectors, etc.

Internal Charging

Tree-like pattern in a ceramic material after

electric discharge induced in the laboratory


WHAT ARE THE TRIGGERS FOR DISCHARGE AND ARCING?

Any sudden changes in the electrical environment of the spacecraft:

• orbital maneuvers

• onset of downlink telemetry

• any other electrical activity on spacecraft

• movement into/out of eclipse or sunlight

• encountering an intense current or boundary of the magnetosphere

The occurrence of highly energetic (relativistic) electrons with energies greater than 2 Mev represents adverse space weather conditions hazardous for geosynchronous satellites. When this happens, there is a high likelihood of internal charging of satellite components by energetic electrons, with possible electric discharges that could result in malfunction or even complete failure of the satellite. Such an event was the likely cause of a number of satellite operational anomalies in January 1994, as shown above.

Satellite Anomalies Connected with Occurrence of Highly Energetic Electrons


OTHER EFFECTS OF SPACECRAFT CHARGING:

• The contaminants can be attracted to negatively-charged satellite surfaces where they modify the optical and thermal properties of the surfaces.

• Estimates indicate that about 50Å of material can be deposited on charged optical surfaces in ~ 100 days.

UV

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• Contaminants (i.e., due to thrusters or outgassing) from a satellite are ionized by solar UV, creating a positively-charged large molecule (“contaminant ion”).

• Charging obscures interpretation of ambient plasma measurements (for instance, a positively charged vehicle can re-attract secondary e-, backscattered e-, photo - e-, etc.).


Micrometeorites Generate EMI and Induce ESD

• Micrometeorite impacts destroy part of the surface material and create a cloud of charged particles (plasma) and molecules.

• The emitted plasma generates a electromagnetic “noise” pulse.

• The plasma from the impact site can discharge surface charge, resulting in a more intense “noise” pulse than that from the micrometeorite alone.

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Micrometeorite

ExpandingPlasma Cloud


SCATHA Satellite Mission (Spacecraft Charging at

High Altitude) 1979–1991

Joint Air Force/NASA Program to measure the plasma environment and

its effects on surface materials, internal and external charging. A major focus

was to understand the relation between system noise due to electrostatic

discharge (ESD) and properties of the space environment.

USAF & Martin Marietta

The kinds of effects identified were:

• Surface & internal charging with associated ESD.

• Attraction of outgassed contaminants by charged surfaces.

• Radiation induced conductivity of dielectric materials.

• Surface charging of materials in space which were not observed to charge in the laboratory.

• Dielectrics becoming weak conductors.

• Deterioration of thermal control materials, paints and coatings.


Correlationwith >300 keVelectronssuggests internalcharging.

IDM = internal discharge monitor

MEP = Micro-ElectronicsPackage

RADIATION

BIOLOGICAL EFFECTS & RADIATION HEALTH HAZARDS

The degree of damage caused by radiation on human tissue is generally related to the degree of ionization produced, i.e., the degree to which electrons are stripped from neutral atoms, leaving positive ions. An important effect is that the ability of a cell to reproduce properly is changed as the incident radiation interacts with a cell's DNA and RNA.

• Primary biological risk from space radiation exposure is cancer.

• When radiation is absorbed in biological material, the energy is deposited along the tracks of radiation.

• Neutrons and heavy ions produce much denser pattern of ionization more biological effects per unit of absorbed radiation dose.

• Secondary concerns such as cataracts are beginning to receive more attention.

He2+

e-

In biological applications, the terms REM and RBE are used.

As radiation dose is dependent on the type of materials, in electronics dose is often specified in Rad (Silicon).

SI unit = Sievert (Sv) 1 Sv = 100 REM

REM (Roentgen Equivalent in Man) is the product of the dose in Rad and the RBE factor.

REM and RBE

RBE (Relative Biological Effectiveness) is the number of rads of X-ray or gamma radiation that produces the same biological damage as 1 Rad of the radiation being used.

There are several physical processes by which ionization occurs; generally, though, the effects of ionizing radiation are proportional to the energy absorbed by the surrounding material.

As we said before, the rad is the unit of absorbed energy = .01 J/Kg of absorbing material.

However, 1 rad received from x-rays produces far less bodily damage than 1 rad received from high energy protons, even though both deposit equal amounts of energy.

Radiation RBE

5 MeV gamma rays 0.51 MeV gamma rays 0.7200 keV gamma rays 1.0Electrons 1.0Protons 2.0Neutrons 2-10Alpha particles 10-20

RBE OF VARIOUS RADIATION SOURCESThe RBE (relative biological effectiveness) was therefore defined to express the effects of radiation on biological tissue. The RBE is defined in comparison to a beam of 200 keV x-rays.

Recall that a rem relates biological damage to type of radiation:

rem = rad X RBE

So, a 1 rad dose of 200 keV x-rays gives a biological equivalent dose of 1 rem, but a 1 rad dose from protons gives a biological equivalent dose of 2 rem.

SI units: 1 Sievert (Sv) = 100 REM

Shuttle Flight~65-195 mr

What is a "normal" dose per year ?radioactive

rock : 20 mr (east) 90 mr (rockies)

cosmic rays : 40 mr 160 mr (rockies)

food/water : 20-50 mr 20-50 mr

each NY/Paris Trip : 4 mr 4 mr

___________ ____________totals ~100 mr ~300 mr

Single Dose Radiation Effects

During violent solar events, the Sun can accelerate electrons and protons to almost the speed of light which gives them huge amounts of energy. Protons and electrons at these high energies can be very dangerous to living cells.

Solar Proton EventsIntegrated Proton Fluence

1 Sievert (Sv)=100 REM

Acute Radiation Syndrome

Symptoms observed within a few months following radiation exposure are collectively called "acute radiation syndrome."

Among syndrome symptoms are vomiting, diarrhea, reduction in the number of blood cells, bleeding, epilation (hair loss), temporary sterility in males, and lens opacity (clouding ) as well as others.

Relation between the proportion of people with severe epilation (loss of more than 2/3 of hair) and estimated radiation dose.

Probably overestimateddoses, since not much survival for Gy > 5.0.With the exception of vomiting, these

symptoms are closely related to cell division because repeatedly dividing cells, e.g., bone marrow and intestinal lining, are more sensitive to radiation than nondividing cells, e.g., muscle and nerve.

Radiation Dangers to Astronauts

Between Apollo 16 and 17,one of the largest solar protonevents ever recorded arrivedat Earth. The radiation levelsan astronaut inside a satellitewould experience during thisevent were simulated. Eveninside a spacecraft, astronautswould have absorbed lethaldoses of radiation within 10 hrsafter the start of the event(4000 mSv).

• Before flight, blood sample is divided into 4 parts and exposed to 4 different dose levels of gamma radiation.

• The blood is processed and photographs are made of the chromosomes from these cells. Counts of identifiable damage are recorded.

Procedure for Determining Absorbed Radiation Dose in Astronauts

• On return, another blood sample is taken and chromosome damage counts are made once again.

• The Damage/Dose curve is then used to determine the equivalent dose of radiation received in space.

• These data are used to make a simple (roughly linear) graph relating measured chromosomal damage to dose (the Damage/Dose relationship).

• Legal and moral reasons require that NASA limit astronaut radiation exposures.

• U.S. Occupational Safety and Health Administration officially classifies astronauts as “radiation workers”.

• Adherence to ALARA (As Low As Reasonably Achievable) is recognized throughout NASA’s manned space flight requirements documents.

–Radiation protection philosophy--any radiation exposure results in some risk

• ISS astronaut exposures will be much higher than typical ground-based radiation worker

–Astronaut legal dose limits (In BFO: 50 REM/yr and 30 REM/mo) are 10 times that allowed ground based radiation workers

• Space radiation more damaging than radiation typically encountered by ground-based workers

PARTICLE ENERGIES OF CONCERN

• ISS originally planned at 28.5º latitude; now at 51.6 º.

Requires post-flight analysis on ground

• EVAs - additional radiation exposure concern

– Lower shielding

– Eye dose

– Skin dose

• 51.6 degrees, new concern for electron events

• Of all the risks encountered by astronauts during space flight, cancer induction from radiation exposure is one of the few that persists after landing.

• There is a relatively large probability that EVAs from the ISS will coincide with a radiation enhancement in the belts.

Extravehicular Activity (EVA)

NRC Report (2000)

“ When the intensity of relativistic electrons is greatest, a single ill-timed EVA could deliver a radiation dose big enough to push an astronaut over the short-term limit for skin and eyes. “

Recommendation 3c: A project should be initiated to develop a protocol for identifying the conditions that produce highly relativistic electron events based on the demonstrated good correlation between changes in solar wind conditions and the onset of such events.

1 sievert (Sv)=100 REM

From Francis Cucinotta, NASA JSCSpace Radiation Health Project(private communication)

Parameters That Affect Astronaut Exposure

1. Spacecraft structure2. Altitude3. Inclination4. EVA start time5. EVA duration6. Status of outer zone electron belts7. Status of interplanetary proton flux (SPE)8. Solar cycle position9. Geomagnetic field conditions

Red--Controlled by space weather activity

Italics--Opportunity for ALARA ---

‘as low as reasonably achievable’

Interplanetary missions, with lifetimes in years, may see even larger radiation doses, as the earth's magnetic field would not be present to shield the spacecraft from GCR's and SPE's.

Additional spacecraft shielding might therefore be required.

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The dose rate at an altitude of 39,000 ft (12 km) in mid-temperate latitudes (temperate zones are 23.5° to 66.5° North and South) is typically up to about 6 microSieverts (μSv) per hour, but near the equator only about 3μSv/hr. (The Sievert [1 Sv=1 Joule/kg] is a measure of potential harm from ionizing radiation.) Typically, a London to Los Angeles flight in a commercial aircraft accumulates ~65μSv (6μSv/hr); however, the solar cycle can give ~ 20% variations in dose from solar minimum to maximum.

The principal space weather hazard to humans is exposure to cosmic radiation, which is causedprimarily by GCRs. As discussed previously in connection with trans-polar flights and effects on avionics, these very energetic GCRs start interacting with the atmosphere at around 130,000 ft causing secondary particles to shower down into the denser atmosphere below. This “particle shower,” and the corresponding level of radiation dose, reach a maximum intensity ataround 66,000ft (~20 km) and then slowly decrease with decreasing altitude down to sea level. The dose rates also increase with increasing latitude until reaching about 50 degrees, where upon it becomes almost constant.

Future commercial air transport concepts and increased traffic over the poles raises concerns in connection with the natural radiation environment

asen-5335 aerospace environments -- radiation effects on space systems 1 spacecraft charging...

Documents

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aerospace environments

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