What is Spacecraft Charging?
The following is presented for those who would like an introduction to the field of spacecraft charging.
1 Spacecraft Charging
The charging referred to here is defined as the electrostatic potential of the spacecraft frame relative to the surrounding, undisturbed space plasma. At times this is referred to as ‘floating potential’, ‘spacecraft potential’ ‘spacecraft charge’, ‘surface potential’, ‘chassis potential’, ‘frame potential’, ‘absolute potential’ etc. This is not to be confused with spacecraft ‘internal’, ‘deep dielectric’, or ‘differential’ charge, which are not discussed here.
The electrostatic charging of objects in outer space was hypothesized by scientists as early as the 1930s. As early as 1955 charged particle energy analyzers detected spacecraft charging. The ATS-5 spacecraft (launched in the late 1960s) was shown to have charged to kilovolt levels. The large magnitude charging noted for the ATS-5 spacecraft “provided a major impetus to the discipline of spacecraft charging”. In the early 1970s unexplained performance anomalies were noted for spacecraft in GEO. In 1972 it was determined that a strong solar flare might be responsible for anomalous satellite behavior. Analysis done between 1973 and 1974 showed a strong correlation between times of geosynchronous satellite anomalies and space weather.
Even low level charging (on the order of a few to a few tens of volts) will complicate space plasma measurements and can attract contaminants to spacecraft surfaces. Higher level charging can damage spacecraft surfaces through ion sputtering. Even minor physical damage to spacecraft surfaces (such as through the attraction of contaminants or through ion sputtering) can seriously degrade sensitive satellite components such as thermal control or optical surfaces. Higher level charging might cause discharges into space (a “blow-off” discharge). When the spacecraft frame charges relative to the space plasma it is believed that differential charging takes place. The charge differential between adjacent regions can lead to electrical breakdown (arc discharge) to redistribute charge. Potential differences as low as 100 V are believed capable of inducing arcing. Arc discharge can induce undesired signals into spacecraft electronics or even induce currents in internal satellite wiring. At it s worst arc discharges may cause short-circuits, open circuits or other catastrophic damage. Spacecraft charging has been implicated in spacecraft anomalies, some of which result in the partial, or even total, loss of the use of the spacecraft. It has been claimed that satellite charging was responsible for 161 out of 198 environment-related anomalies included in one study. It has also been stated that the largest cause of mission failures (lost or terminated missions) related to the space environment is surface electrostatic discharge (spacecraft charging). In GEO, 200 annoying to serious and 10 critical operational anomalies due to electrostatic surface discharge are expected over the lifetime of the spacecraft.
The vacuum of space allows a spacecraft to charge up to extreme values, as high as -20K volts in geosynchronous earth orbit (GEO). Significant effort has been focused on the study of charging in GEO. Recent effort has been focused on charging in low earth orbit (LEO). High level charging (to 100 volts or more negative) occurred on average almost one hundred times per year for the polar orbiting Defense Meteorological Satellite Program (DMSP) spacecraft. Prior to an electrical malfunction the DMSP F13 spacecraft’s floating potential rose to 459 volts. Spacecraft in LEO equipped with high voltage power systems may charge to levels that cause destructive arcing. It has been shown that a potential difference in the kV range may exist between docking spacecraft under certain conditions. Spacecraft charging in the solar wind, in orbit around Jupiter and Saturn, and to the farthest extremes reached by spacecraft have been studied. The use of new materials for spacecraft present new spacecraft charging challenges.
Positive spacecraft floating potential may reach tens of volts but negative floating potential may reach tens of kilovolt. The asymmetry in equilibrium charge is due to the higher mobility of electrons in the spacecraft materials and in the space plasma. It is expected that satellite floating potential in GEO will range from +30 to -20,00V, with the highest negative potential expected in the midnight (‘00’) to 9:00 local time (09) sector of orbit (midnight to dawn). A spacecraft in GEO will generally charge to just a few volts but it is considered normal for some satellites in GEO to routinely and predictably charge to a few hundred volts negative at certain times. The exact charging characteristics differ from spacecraft to spacecraft and from orbit to orbit. For the GEO satellites ATS-5 and -6 satellites, the probability of charging to greater than -10kV during one pass from 00 to 06 LT was between 6% and 12% at times of high geomagnetic activity. Hazardous discharges due to spacecraft charging in GEO are most likely during geomagnetic storms, in the midnight to dawn sector of orbit, when the satellite emerges from or goes into the Earth’s shadow, or when spacecraft thrusters fire.
Spacecraft charging models, such as Nascap-2k and spacecraft design guidelines exist so that spacecraft can be built to minimize charging due to design. For those who wish to learn more about the spacecraft charging phenomena, many scholarly introductions to the field exist.
2 The Need for Spacecraft Charge Measurement
Relatively few spacecraft have been equipped with spacecraft floating potential monitors. Spacecraft charging is generally determined the cause of anomalies through deduction rather than direct measurement.
It may be possible to include a floating potential monitor on a spacecraft to warn of times when anomalies are likely. Arcing between the parts of a spacecraft electrically connected to the frame and those that are not electrically connected to the frame is believed by some to be more likely when spacecraft floating potential is high. Sensitive systems could be placed in safe mode when floating potential is high in an effort to reduce the damage that might be done by arcing. The importance of monitoring floating potential to warn of arcing is unknown. It is believed that a growing proportion of spacecraft anomalies are not correlated to spacecraft floating potential. The makers of the Compact Environmental Anomaly Sensor (CEASE), which does not monitor spacecraft floating potential, contend that operational anomalies are at least as likely to be caused by deep dielectric charging as surface charging. The flight of floating potential monitors and internal charging monitors on the same spacecraft might help demonstrate their relative merits as predictors of spacecraft anomalies.
There are other uses for floating potential monitors. Knowledge of spacecraft frame potential is especially important to the interpretation of the data from on-board instruments designed to measure the properties of charged particles and electric fields in the spacecraft environment. Knowledge of spacecraft floating potential is also useful in evaluating active charge control. Numerous charge control devices have been used or proposed. One such charge control device flies on the International Space Station (ISS) because it has been hypothesized that spacecraft charge could cause astronaut electrocution under certain conditions.
Charge monitors are needed to improve our understanding of the charging phenomenon. Spacecraft charging cannot yet be predicted in detail for a given vehicle in spite of significant modeling efforts. Until spacecraft charging modeling code results are verified for a wide variety of environments and satellite types, their validity will remain in question. There is no correlation between the potentials on adjacent vehicles during charging events in GEO, even as close as 4000 km and surface potential measurements are required on the spacecraft of interest and cannot be inferred from nearby vehicles.
3 Measuring Spacecraft Charge
Spacecraft charge is measured by four main techniques. The instruments for the four main techniques are: Langmuir probes, retarding potential analyzers (RPAs), floating probes, and charged particle energy analyzers. There is no‘standard method used to measure spacecraft charge. Spacecraft potential measurements may vary from instrument to instrument on a single spacecraft.
The table below gives method used, the major limitations, and the altitude (LEO or GEO) and degree of charge where useful, for each of the four classes of instruments. For those who wish to learn more, a reference is included in the “Major Limitations” column for each instrument.
Two of the techniques listed in the table work in both LEO and GEO. One of those two requires the use of a boom. Only charged particle energy analyzers are able to measure floating potential in both LEO and GEO without the use of a boom. Since charged particle energy analyzers require little more than a clear field of view to the surrounding space plasma to measure floating potential they can be made as compact devices that are mounted directly to the spacecraft frame.
The differential energy distributions gathered by charged particle spectrometers can be analyzed in a number of different ways to determine spacecraft floating potential. Since the SCM-1 and PASS use the ‘electron spectroscopic’ and the ‘low energy ion cutoff’ methods of charged particle energy analysis to measure floating potential, some discussion is in order. Both methods require the identification of features in the spectrum. It is assumed that an apparent shift in the energy location of the features is due to a non-zero spacecraft frame potential relative to the space plasma. The electron spectroscopic and low energy ion cutoff methods promise simplicity in data analysis: the spacecraft floating potential is the apparent shift in the energy location of feature(s) measured in eV, expressed in volts. For a negative floating potential, features in the electron spectrum are shifted to lower energies and in ion spectra features are shifted to higher energies.
4 How Charge Can be Determined with the SCM and PASS
The spectral features in electron and ion spectra can be thought of as the ‘signal’ used to pinpoint floating potential with the SCM-1 and PASS. Since neither the SCM nor the PASS have been tried in space, it is important to discuss the signal they will require to determine charge. Discussion will be limited to evidence that supports the electron spectroscopic and low energy ion cutoff methods.
Evidence that supports the electron spectroscopic method (used by the SCM and PASS) will be discussed first. Almost half of the solar energy deposited in the atmosphere above 120 km is given to electrons of energies of less than 100 eV which are produced by the photoionization of N2 and O. The electron spectroscopic method utilizes the sharp electron-spectral peaks that can be seen in the atmospheric photoelectron spectrum due to intense HeII line solar ionizing radiation. Photoionization by other solar lines and scattering between electrons produces electrons with other energies as well so that the photoelectron spectrum typically appears as an exponentially sloped ‘background’ from 0 to 60 eV with peaks due to the HeII line superimposed. Photoelectrons with energies higher than 60 eV are generally not produced. Thus, two features in the photoelectron spectrum can be used to determine spacecraft floating potential. The first feature is the characteristic set of peaks at 20-30 eV and the second is the steep ‘cut off’ in photoelectrons at 60 eV. The electrons used to determine spacecraft floating potential through electron spectroscopy have been created in the ‘photoelectron production region’ (from about 150 to 300 km altitude). Atmospheric photoelectrons are found at a great distance from the production region because they travel along field lines. The electron may move directly up or down the field line (with a pitch angle of 180˚ or 0˚), or it may move along the field line in a helical path with an intermediate pitch angle. The energy of the electron is distributed between ‘along field line’ motion and circular motion. Photoelectrons that have traveled along geomagnetic field lines from their origin to the other end of the field line (conjugate photoelectrons) have been found in the electron energy spectra gathered by satellite. In fact, electrons that originate from the daylit side of the Earth are even detected on the night side of the Earth. At altitudes above 250 km the photoelectrons detected are no longer locally produced and the photoelectron lines in the spectrum may be broadened and shifted (by less than 0.5 eV) because of scattering by the ambient thermal plasma. In general, the spectra collected after the photoelectrons have traveled great distances have the same features expected in a spectrum that is collected at the top of the production region. If electrons have passed through a region of high plasma density, the spectrum may exhibit a broadening and slight shift in the energy, but at least some evidence of the peaks at 20-30 eV is visible in the vast majority spectra published regardless of altitude at which they were collected. The effect of transport on the photoelectrons, however, is variable and, due to a lack of data, not well characterized. Theoretical modeling has been done to investigate photoelectron transport and some work has been done to
Spectra from altitudes above 1000 km have not yet been collected with instruments with the energy resolution needed to reveal the peaks at 20-30 eV. Atmospheric photoelectrons at GEO altitudes are evident in cruder spectra, such as that shown in the following image.
The image is an electron spectrograph from the Magnetospheric Plasma Analyzer (MPA). The MPA instruments are deployed on spacecraft with international designators of 1989-046, 1990-095, and 1991-080. The narrow band of intensity at look angles of approximately 360˚ and 180˚ (spacecraft North and South) from LT 6 to LT 16 are thought to be atmospheric photoelectrons. The flux of the electrons, their predominance at low energies (0-15 eV), and their directionality is consistent with what would be expected for atmospheric photoelectrons transported along geomagnetic field lines. More about the electron spectroscopic method can be found in Instrument for Measuring Spacecraft Potential.
The electron spectroscopic method is limited to positive charge or minor negative charge. The steep reduction in flux at 60 eV marks the end of the atmospheric photoelectron ‘signal’. If the spacecraft is floating at more than 60 volts negative then atmospheric photoelectrons will not reach the spacecraft. There is also a large flux of spacecraft photoelectrons of energies of 10 eV or less that may interfere with the electron spectroscopic method at negative floating potentials. If that interference is taken into consideration a maximum negative spacecraft charge of -15 volts for identification of features at 20-30 eV and -50 volts for identification of the steep reduction in atmospheric photoelectrons may be reasonable. Some method other than the electron-spectroscopic method (the only method used by the SCM) must be used to determine high negative floating potential. That method, the low energy ion cutoff method, will now be discussed.
Low energy ion cutoff method will allow the PASS to determine large negative floating potentials. The source of low energy ions in the magnetosphere is not well understood and it is beyond the scope of this paper to describe it. The low energy ion cutoff method works as follows. It is assumed that there are ambient ions around the spacecraft that would have very low kinetic energy if the spacecraft were not present. For a negatively charged spacecraft the energy spectrum of such ions will exhibit a ‘low energy cutoff’, the minimum energy for which spectral ‘counts’ will appear. Since the ion energy analyzer reference potential is that of the spacecraft frame, then the apparent minimum in ion energy (in eV) is taken to be the spacecraft’s floating potential in volts (negative) relative to space plasma. Examples of low energy ion spectra, and their analysis for floating potential, are shown in a number of publications. Low level as well as high level charging can be detected with this method.
Many ion spectrograms from the Magnetospheric Plasma Analyzer (MPA) show the low ion energy cutoff clearly and are currently used to determine spacecraft charge in GEO. The following is an ion spectrogram is from the MPA instrument.
The spectrogram is not an individual ion energy spectrum, but a series of spectra plotted in time with the ion density plotted so that color represents density (flux). The lowest energy ions detected gives the negative floating potential. In the MPA ion spectrograph one can see the Los Alamos satellite’s floating potential rise and fall as the spacecraft goes through a 24-hour (local time) orbit at GEO. The plots are for one full 24-hour geosynchronous orbit for the date March 21, 2007. The lowest part of the band of color (or greyscale if in black and white) indicates the spacecraft’s floating potential. The exact method used to determine spacecraft floating potential with MPA data is somewhat more complex becaus at times when the low energy ion cutoff is not clear, an analysis based on electron spectra is used.
Experience has shown that unless the charged particle analyzer is pointed in the approximately in the direction of the geomagnetic field and is configured to collect spectra in the energy range where a charging peak will appear a charging peak will not be observed in the data. The electron spectroscopic and the ion energy cutoff methods both benefit from the largest signal when the instrument field of view is aligned with geomagnetic field. There is some tolerance for pointing direction, at least for the electron spectroscopic method. The best photoelectron spectra were for one mission when the sensor’s look direction was always less than 55˚ from the local magnetic field direction. Other electron spectrometer placement recommendations are discussed in Plasma Analyzer for Measuring Spacecraft Floating Potential in LEO and GEO.
The SCM and PASS may be the first charged particle energy analyzers designed exclusively to measure spacecraft floating potential. Since particle detectors have not been placed on satellites with the detection of vehicle charging as their prime objective such instruments as flown in the past have not been efficient charge detectors. Spaceborne charged particle energy analyzers have had insufficient geometric factor and/or energy resolution to determine charge quickly and accurately. Even instruments with low geometric factor and/or poor energy resolution have been relatively bulky and expensive compared to other sensors that fly on spacecraft. The SCM and PASS have been developed as high performance, low mass, inexpensive alternatives to the charged particle energy analyzers that have been used to determine spacecraft floating potential in the past.
5 References and Further Reading
The spacecraft charging introduction given above is available complete with citations upon request.
References cited in Table 1 are:
Anderson, P. C.; Hanson, W. B.; Coley, W. R.; Hoegy, W. R., “Spacecraft potential effects on the Dynamics Explorer 2 satellite”, Journal of Geophysical Research (ISSN 0148-0227), vol. 99, no. A3, p. 3985-3997, 1994.
Brace, Larry H., Langmuir probe measurements in the ionosphere, Measurement technologies in space plasmas: particles, Robert F. Pfaff, Joseph E. Berovsky, and David T. Young, editors, Geophysical Monograph 102, AGU press, 1998.
Maynard, Nelson C., Electric field measurements in moderate to high density space plasmas with passive double probes, Measurement technologies in space plasmas: fields, Robert F. Pfaff, Joseph E. Berovsky, and David T. Young, editors, Geophysical Monograph 103, AGU press, 1998.
Moore, T. E., M. O. Chandler, C. J. Pollack, D. L. Reasoner, R. L. Arnoldy, B. Austin, P. M. Kintner, and J. Bonnell, Plasma heating and flow in an auroral arc, J. Geophys. Res., 101, A3, 5279-5297, 1996.