JOURNAL OF SPACECRAFT AND ROCKETS Vol. 51, No. 6, November–December 2014 Feasibility of Detecting Spacecraft Charging and Arcing by Remote Sensing Dale C. Ferguson,∗ Jeremy Murray-Krezan,† and David A. Barton‡ Space Vehicles Directorate, U.S. Air Force Research Laboratory, Kirtland Air Force Base, New Mexico 87117 J. R. Dennison§ Utah State University, Logan, Utah 84322 and Stephen A. Gregory¶ Boeing LTS Inc., Kirtland Air Force Base, New Mexico 87117 DOI: 10.2514/1.A32958 More than 50 years after the dawn of the space age, most spacecraft still do not have sensors onboard capable of detecting whether they are at potentials likely to put them at risk of severe charging and the concomitant arcing, or, indeed, even capable of detecting when or if they undergo arcing. As a result, anomaly resolution has often been hit or miss, and false diagnoses are probably common. In this paper, a few remote sensing techniques that could be applied for remotely detecting spacecraft charging and/or arcing, and their feasibility, are examined: surface glows from high-energy electron impact, x-rays from bremsstrahlung, and radio and optical emission from arcs and after arcing. Nomenclature LThermal = spectral radiance that results from thermal −1 −2 μ −1 A = projected area of sample, relative to observer, m2 emittance of sample, W · sr · m · m 2 R = stand-off range between observer and satellite, m ABus = projected area of satellite bus, relative to observer, m A = projected area of solar panel, relative to observer, m2 T = sample temperature, K SolarPanel σ = rms width of specular reflection from sample, deg; ATotal = total projected area of satellite (bus and solar panel) 2 rms standard deviation of photon noise, counts as seen relative to observer, m θ a = sample effective albedo SPA = solar phase angle, deg aBus = average, diffuse bus albedo aSolarPanel = average, diffuse solar panel albedo B = magnetic field strength, T I. Introduction e = sample emissivity PACE situational awareness (SSA) is the capability to determine LC = spectral radiance expected from charging effects S what is happening and why on satellites in space. It is important propagated to the observer location, W · sr−1 · for satellite operators to have good SSA so that they can respond to m−2 · μm−1 anomalies and plan for events (like meteor showers) when avoidance LCharging = spectral radiance expected from charging effects, is necessary. Easily understood examples of SSA are when ground W · sr−1 · m−2 · μm−1 station operators plan for losing the signals from their satellites when LEarthShine = spectral radiance due to Earthshine reflected from they are too close to the sun in the sky (during eclipse seasons) or sample, W · sr−1 · m−2 · μm−1 when space weather conditions are likely to produce spacecraft LSkyGlow = radiance estimate resulting from expected sky charging arcing anomalies on satellites. The Air Force must maintain background, both terrestrial and zodiacal, W · SSA to determine whether satellite anomalies are due to operations in sr−1 · m−2 the natural environment or to hostile acts. In any case, SSA is of great LSun = spectral radiance due to sunlight reflected from importance. More than 50 years after the dawn of the space age, most sample, W · sr−1 · m−2 · μm−1 spacecraft still do not have sensors onboard capable of detecting Downloaded by UTAH STATE UNIVERSITY on January 26, 2015 | http://arc.aiaa.org DOI: 10.2514/1.A32958 −1 −2 LTotal = total spectral radiance expected, W · sr · m · whether they are at potentials likely to put them at risk of severe μm−1 charging and the concomitant arcing, or, indeed, even capable of detecting when or if they undergo arcing. As a result, anomaly resolution has often been hit or miss, and false diagnoses are probably common. Until spacecraft are routinely launched with charging and Presented as Paper 2013-2828 at the 5th AIAA Atmospheric and Space arcing monitors, the best that can be achieved is detection through – Environments Conference, San Diego, CA, 24 27 June 2013; received 12 remote sensing, from the ground or by satellites. In this paper, a few February 2014; revision received 18 March 2014; accepted for publication 20 June 2014; published online 7 October 2014. This material is declared a work remote sensing techniques are examined that could be applied for of the U.S. Government and is not subject to copyright protection in the United detecting spacecraft charging and/or arcing. States. Copies of this paper may be made for personal or internal use, on But first, one must define a few terms. A satellite is said to be in condition that the copier pay the $10.00 per-copy fee to the Copyright eclipse when it passes into Earth’s shadow. Satellites in geo- Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923; include synchronous Earth orbit (GEO, on the equator at about 36,000 km the code 1533-6794/14 and $10.00 in correspondence with the CCC. altitude) can only be in eclipse during two eclipse seasons every year, *Lead, Spacecraft Charging Science and Technology, RVBXR, 3550 each lasting about 2 months at the spring and autumnal equinoxes, and Aberdeen Dr. SE; [email protected]. Associate Fellow AIAA. † for a maximum of about 1 hour each day during these seasons. GEO Program Manager, Advanced Research Collaboration and Development satellites are subject to spacecraft charging, due to fluxes of high- Environment (ARCADE), RVSV, 3550 Aberdeen Dr. SE. ‡Director, Calibration and Test Chamber for Electron Radiation energy electrons onto and beneath their surfaces, usually coincident (CaTChER), RVBXR, 3550 Aberdeen Dr. SE. with geomagnetic storms. Geomagnetic storms are rapid changes in §Professor, Materials Physics Group, Physics Department. Senior Member Earth’s magnetic field due to impingement of plasmas from the sun on AIAA. the magnetosphere. During these storms, entire satellites can charge ¶P. O. Box 5670, 3550 Aberdeen Dr. SE. tens of thousands of volts negative of their surrounding space plasma, 1907 1908 FERGUSON ET AL. and spacecraft surfaces can charge thousands of volts with respect to The terms in Eq. (1) are the reflected sunlight LSun, the reflected each other. The ensuing electric fields can cause local discharges Earthshine incident on the satellite LEarthShine, the thermal emittance (commonly called arcs), which through their high currents and radiated LThermal, and the skyglow LSkyGlow. To obtain an estimate of each of signals can cause disruptions in command and control signals, latchups the contributing spectral radiance terms, several assumptions are of electronic components, short circuits, and even surface property made that are reasonable for a large GEO-synchronous com- changes. When behavior on a spacecraft suddenly deviates from munication satellite, such as one of the DirecTV satellites, observed nominal, the event is called an anomaly. Anomalies range in severity from a ground-based sensor. The expected spectral radiance due to from simple bit flips in nonsensitive circuits to losses of entire reflected sunlight LSun is calculated assuming that the observed command and/or communications circuits or permanent destruction of satellite has an albedo described by the equation solar array strings or power supplies. Especially sensitive to spacecraft charging-related anomalies are the solar arrays, because they typically θ − θ ∕ σ2 2 have grounded conductors exposed to the space plasma, surfaces a SPA expf SPA 2 g already at high potentials with respect to each other, large areas of connected capacitance that can contribute to arc currents, surfaces A A a SolarPanel a Bus cos2 θ (2) always in sunlight, and surfaces always in shade. SolarPanel A Bus A SPA It is even possible for small transient arcs on solar arrays to turn into Total Total sustained arcs powered by the solar arrays themselves. Most GEO satellite anomalies occur during eclipse seasons, during eclipse and for For a large communications satellite, the solar panel and bus sizes (A) 2 a few hours afterward. The so-called deep-dielectric discharges are due and albedos (a) can be approximated as ASolarPanel 60 m , 2 to very high-energy (penetrating) electrons of 2 MeVand higher. These aSolarPanel 0.04, ABus 10 m , and aBus 0.6. The notional θ electrons can build up for hours or days inside spacecraft electronics observed solar phase angle in this scenario is SPA 60°. For these until the electric field builds up to discharge levels. Surface discharges conditions, the expected reflected sunlight from the satellite is LSun − − − − − are due to electrons of 5–50 keV that differentially charge spacecraft 140 W·m 2 ·μm 1 (3.5 × 1016 photons · s 1 · cm 2 · μm 1)atan surfaces. Again, when local electric fields build up to discharge levels, optical wavelength near 0.5 μm. Using a similar set of assumptions, an anomaly can occur. GEO charging conditions can last for an hour up the spectral radiance due to Earthshine can be estimated as −2 −1 15 −1 −2 −1 to several hours, and then typically abate for a while. LEarthShine 4W·m ·μm (1 × 10 photons · s · cm · μm ) To be able to remotely sense spacecraft charging and its discharges, at an optical wavelength near 0.5 μm. The expected spectral radiance one must be able to detect the high-energy electrons (or ions) as they due to thermal emission of the notional, large communication hit the spacecraft surfaces, to detect the radiated emissions from the satellite, assuming an emissivity even as unrealistically high as ≈ −37 passage of the electrons through the material, or to detect the radiated e 1, gives an upper bound on the value of LThermal 10 W · emissions from the arcs themselves. In this paper several of these m−2 · μm−1 (2.5 × 10−23 photons · s−1 · cm−2 · μm−1) at an optical options are investigated, to see if remote sensing is feasible.