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Spacecraft charging


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  • 1. Space Environment An overview
  • 2. Introduction• Space is often incorrectly thought of as a vast, empty vacuum that begins at the outer reaches of the Earths 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.
  • 3. 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
  • 4. 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 Suns 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.
  • 5. Solar Wind• Because of the high temperature of the Suns 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 Earths 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 satellites orbital elements and corresponding ground traces.
  • 6. 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 Earths 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.
  • 7. 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 Earths magnetic field.
  • 8. 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 nTThe second physical regimeis the bow shock/magnetosheath region where the plasma environment of interplanetary spaceand the geophysical magnetic field become separated. The magnetopause is the boundarywhere 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 magnetopauseand the bow shock, where a shockwave forms from the encounter of the supersonic solar windand the magnetosphere.
  • 9. The third physical regimeThe magnetosphere region, including the inner magnetosphere orplasmasphere, dipole field, radiation belts, auroral regions, plasma/current sheet, andthe magnetotail. The plasmasphere is a doughnut-shaped region located near the Earthwith radial distance less than a few RE (RE = 6378 km) in the equatorial plane, and itcontains relatively dense (ne >102 cm-3)
  • 10. 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 Earths 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 satellites 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 Earths 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.
  • 11. 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.
  • 12. • 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 satellites surface, resulting in a positive charge on the satellites 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.
  • 13. 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 Earths 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.
  • 14. 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
  • 15. 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
  • 16. 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.
  • 17. 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.
  • 18. 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.
  • 19. 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 Earths atmosphere. Most of these particles are the size of dust particles, however, some are much larger. When meteoroids enter the Earths 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 dont completely vaporize. When a piece strikes the surface of the Earth it is called a meteorite.
  • 20. 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 its orbit to avoid collision. Shielding and other design considerations are the most effective means to protect satellites from catastrophic damage.
  • 21. Deep ChargingDeep charging of a satellite occurs whencosmic ray particles pass through a satelliteand ionize atoms within, through collisions.Some of these particles are solar in origin, butthe majority are galactic and with nopreference to time or light conditions. They doshow some dependence on the solar cycle.
  • 22. Radiation Interactions
  • 23. Spacecraft charging
  • 24. Outline• Charging• Charging currents• Surface charging• Absolute/differential charging• Spacecraft charging environments
  • 25. Spacecraft EnvironmentMany spacecraft have been lost due to lack of full understanding environment• Design to fly in space• Flight environment• Qualification test (design is correct)
  • 26. Earth EnvironmentAtmosphere a primary source of problems• Corrosive AtmosphereDisintegration of atom due to chemical reaction1. It destroys or irreversible damage to the substance (strong acid or bases)2. Stress concentration or embrittlement(Loss of ductility, making a material brittle)
  • 27. • Humidity controlTo exclude oxygen and moisture using dry nitrogen or helium purgeLow humidity-------- built up of static charge40-50% range is a good compromiseAirborne particulate contamination (Sensors whose location is air)
  • 28. • 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
  • 29. Space and upper atmosphere environment• Hard Vaccum• Low gravitational acceleration• Ionizing radiations• Several thermal gradients• Micrometeoroids• Orbital Debris• Out gassing
  • 30. 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
  • 31. • 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.
  • 32. • 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.
  • 33. 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.
  • 34. 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
  • 35. Plasma Interactions
  • 36. 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)
  • 37. 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
  • 38. Spacecraft Charging
  • 39. Space plasma and spacecraft charging• Plasma• Source of charge particles: auroras, radiation belts• Charging currents• Absolute and differential charging
  • 40. 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.
  • 41. 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
  • 42. 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
  • 43. • There are, of course, a number of other dust grain charging mechanisms, namely• Thermionic emission• field emission• radioactivity• impact ionizationThese are significant only in some different special circumstances.
  • 44. 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
  • 45. • 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.
  • 46. • 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.
  • 47. • 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.
  • 48. Current due to electron and ion collection
  • 49. 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.
  • 50. 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)
  • 51. 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.
  • 52. 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).
  • 53. Magnetic Fields• The Earths 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 Earths 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.
  • 54. Surface Charging• Mechanism1. Low energy plasma and photoelectric current2. 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:
  • 55. • 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.
  • 56. • 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
  • 57. 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).
  • 58. • 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.
  • 59. Conditions required for spacecraft charging