Detection and characterization of meteoroid and orbital debris impacts in space [electronic resource]
Keywords: Aeronautics and Astronautics.

Meteoroids and orbital debris are critical components of the space environment, threatening both Earth-orbiting and interplanetary satellites. While an impact from a large (> 1 mg) object is likely to cause catastrophic damage, spacecraft are more likely to be hit by dust-sized particles in the nanogram to microgram mass range. Traveling at speeds in tens of km/s, when these particles hit the satellite surface, they vaporize, ionize and produce a radially expanding plasma plume that can potentially damage electrical sub-systems on the satellite. While there is some empirical and statistical evidence of impact-induced electrical anomalies on spacecraft, the properties of the impact plasma and the mechanism by which it induces anomalies in spacecraft are not well understood. From a science perspective, measuring the properties of the impact flash and impact plasma can help us estimate the mass distribution, velocity distribution and composition of these particles. Space-based measurements of the properties of these particles have been limited and the information gained from ground-based radars and sky cameras is riddled with modeling uncertainties. To estimate the masses and velocities of the particles hitting the satellite surface, an optical sensing system comprising photomultiplier tube and high speed amplifiers was developed. To characterize the plasma that is created upon impact and to gain a deeper understanding of the impact phenomenon, a novel plasma spectrometer, called the Transient Plasma Analyzer (TPA), was developed for measuring the energy distribution of the impact plasma. The design of the sensor was optimized through particle tracing simulations using COMSOL Multiphysics software. An innovative approach of fabricating the sensor using printed circuit boards was implemented, following which, calibration was done using an electron source in a vacuum chamber. These sensors were deployed during ground-based hypervelocity impact tests at the Max Planck Institute (MPI) in Germany, Colorado Center for Lunar Dust Acceleration Studies (CCLDAS) and NASA Ames Vertical Gun Range (AVGR). At MPI, optical flash was measured from impacts on a variety of spacecraft surfaces. A new data-driven scheme was developed for estimating the mass and velocity of the particle from the strength and temporal evolution of the optical signal. Our algorithm performed better than the heuristic of using just the rise time of the optical signal as an estimator of the impact speed. Using machine learning techniques, we were able to estimate the impact speed of particles with a mean estimation error of 6.40~km/s. This opens up the possibility of using impact flash measurements for distinguishing between meteoroid and orbital debris impacts and assessing which of the two sources of particles is a bigger threat to satellites in low Earth orbit. At CCLDAS and AVGR, the TPA was used to carry out measurements of impact plasma across a wide range of masses and velocities. A plasma evolution model was developed to infer properties of the plasma from the measurements made in Colorado. By comparing the output of this model with the sensor measurements, the initial plasma temperature and bulk expansion speed were estimated. Our results showed that the temperature and bulk speed values are in agreement with results obtained through hydrocode simulations and ground-based experiments by other researchers. The model also adds the unique capability of tapping into the measurements of plasma geometry for estimating the properties of the plasma immediately after impact. In conjunction with models based on the temporal evolution of the signal, our model can help probe the impact plasma close to the target surface, which is otherwise very difficult to measure directly. A Monte-Carlo elastic collision model was also developed to understand the role of neutrals in the response of the TPA for the high-mass impact experiments at NASA Ames. It was found that the presence of a neutral background modifies the behavior of the sensor, yielding a flat response for high energy particles. The flatness of the energy spectra measured at NASA Ames thus indicates that during high-mass impacts, the plasma gets energized to several tens of eV. It also gives us an idea of how signals scale across different mass and velocity ranges. These developments in sensors and techniques bring us closer to the goal of building a "black-box" to diagnose spacecraft anomalies and failures in future space missions. It also gives us the set of tools we need to explore various meteoroid and dust streams in the solar system to understand their origin, their composition and to make predictions about their future.