On the Feasibility of Detecting Spacecraft Charging and Arcing by Remote Sensing

摘要

It is a sad fact that 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. Until spacecraft are routinely launched with charging and arcing monitors, the best that can be achieved is detection through remote sensing, from the ground or by satellites. In this paper we examine a few remote sensing techniques that could be applied for detecting spacecraft charging and/or arcing.The first technique considered depends on the fact that when bombarded by high energy electrons, many types of dielectrics emit a glow that could be observed remotely, and would change with the degree of spacecraft charging. Only kilovolt electron strikes are effective at producing the glow. Thus, under geomagnetically calm conditions, if the glow were detected, high energy electron fluxes capable of spacecraft surface charging to kilovolt levels would be indicated. If the space plasma were disturbed, and the spacecraft were thus being charged negatively by a multitude of multi-kilovolt electrons, the ongoing charging would be seen as an enhanced surface glow. Although easily seen in the laboratory, this glow is likely to be too weak to be detected in space except for a satellite in eclipse. However, GEO satellites charge more in eclipse anyway. We will estimate whether the glow can be detected from both Earth and space. The second technique depends on the fact that when electrons above about 20 keV strike a surface, x-rays are produced (through bremsstrahlung). If immersed in a very high-temperature plasma (like that of the famous Galaxy 15 event or the ATS-6 record charging event) a spacecraft may thus be seen by the x-rays that are produced. It is generally conceded that in eclipse a spacecraft will charge negatively (in volts) up to the electron temperature of the surrounding plasma (in eV). Again, detection in eclipse is probably necessary, since solar x-rays reflected by spacecraft surfaces might make daytime detection impossible. This method would likely only indicate when the most severe charging conditions were ongoing, and would of necessity require detection by an orbiting satellite. Finally, when spacecraft arc, the arcs produce electromagnetic radiation. On PASP Plus and other scientific satellites, radio waves produced by arcs were used to determine the arc location, for instance. Arcs in laboratory conditions have been detected solely by radio emission, and oftentimes the visible light emitted is used to determine arc location and timing. While the radio noise produced is severe enough close by to produce radio interference in sensitive spacecraft electronics, it is likely to drop off rapidly, and most probably could only be detected by satellites orbiting nearby. However, the light produced may be substantial, and might be detected by a suitably filtered telescope even on Earth. Also, shortly after an arc, solar array surfaces glow for two reasons - firstly, while the arc is progressing, the coverglass surface is positively charged, and glows from electron excitation at its surface. If the arc does not completely discharge the surface, the glow may continue until ambient electrons collected completely neutralize it. Secondly, some of the cells in the array circuit are back-biased by the arc, and act as light emitting diodes. Both of these emissions are broadband and may last for hundreds of microseconds. Possibilities for arc detection from Earth-bound optical and radio telescopes will be discussed.