Analysis of a detection concept for impact plasma

Annual meteor showers, especially the Perseids in August and the Geminids in December, cause spectacular luminous phenomena in the nightly sky. When meteoroids enter Earth’s atmosphere, they leave a plasma trace due to their high velocities of up to several tens of kilometers per second. This is visible to us as shooting stars. The meteoroids have their origin from collisions and decomposition of astronomical small bodies, especially comets and asteroids. Their small size and high velocity complicate the investigation of meteoroids. Data about frequency of occurrence and their basic properties such as size, density and chemical composition are very uncertain and not liable as they are very often determined in an indirect way, for example, via radar monitoring of shooting stars. This makes high-speed impact of meteoroids on spacecraft systems one of the most uncertain environmental influences in space.

For years, Fraunhofer EMI has been dedicated to the investigation of such high-speed impacts, which have come to be termed hypervelocity impact in the technical jargon. In trendsetting studies, the phenomena and effects of hypervelocity impacts and their consequences for space systems have been examined here using unique experimental possibilities. In most cases, the focus of these studies lies on the mechanical effects, such as fragmentation and perforation of the space components. Before these mechanical effects show in the impact process, another short-term phenomenon can be observed: the propagation of a plasma cloud, the so-called impact plasma, as shown in Figure 2. Impact plasma results from extreme pressure and temperature conditions, which occur at the impact of a meteoroid on the surface of a satellite. By shock-wave effects, the material of the meteoroid and the surface gets compressed in such a way that the material undergoes a phase change to the gaseous state and plasma state during the subsequent energy release. The ionized gas mixture originating from this quickly expands along the satellite surface and abruptly loses density and temperature.

As the properties of the impact plasma depend on the impact conditions, like impact angle and impact velocity, and on the conditions of the meteoroid, measurement of the impact plasma allows for drawing conclusions about these parameters. For this purpose, charge detectors were used in a wide range of interplanetary space missions. These are well-engineered, complex measurement instruments, which extract charge carriers of impact plasma by electrical fields in order to examine its chemical composition (e.g., [1], [2], [3]). At the same time, impact events are involuntarily detected as interfering signals in radio astronomy and space-plasma experiments. As example, Figure 3 shows spectograms of the WAVES instruments from the STEREO A and B satellites. While both satellites measure similar intensities of the ambient plasma, STEREO A is exposed to additional anomalies, which can be ascribed to the generation and propagation of impact plasma [4].

The fact that interfering signals caused by hypervelocity-impact plasma could be measured on the antennas posed the question if and under which conditions the underlying effects could be used as simple and effective approach for the detection of impact plasma and the properties of the respective impact. Fraunhofer EMI pursued this question within a study funded by ESA. The study is based on numerical simulations examining a quite large parameter space, which far surpasses the experimentally accessible domains. A plasma model developed at Fraunhofer EMI was employed for the plasma simulations. Figure 4 shows various features of the plasma simulations. Starting from the simulation of the shock state, the composition and ionization (a) of the plasma cloud is determined. Within the cloud, density gradients of the different impact-plasma constituents appear (b). With the model, properties of the impact plasma can be presented in dependence of the impact parameters (b). The signals on the antenna caused by expanding impact plasma depend on the impact location, the antenna dimensions and the antenna wiring as well as the satellite potential. Figure 5 shows how these parameters were varied for the simulations in order to specify a practical antenna configuration for the extraction of impact parameters from the measured antenna signals. In this context, different signal generation mechanisms operate as, for example, the direct measurement of charge carrier differences within the cloud or the disturbance of the antenna potential by the cloud.

One important insight of the study is that the electron emission from interactions with the fast cloud ions causes the most distinct signal and is best suited for the detection purpose. The parametric analysis of simulated antenna signals showed that simple antenna arrays admit to draw conclusions about fundamental impact characteristics such as velocity vector, impact location and meteoroid size. The array presented in Figure 6 is composed of six antennas with 40 centimeters length and 30 centimeters distance in between. This array suffices to deduct impact location and velocity with less than five percent and the meteoroid mass with less than 15 percent from the measured antenna signals. A roadmap has been defined for further research work in order to experimentally confirm, optimize and technically implement the detection concept. It impresses due to its attractive simplicity and contributes to better cover and record the particle environment in orbit.