Hypervelocity Impacts

One of the most elusive aspects of the space environment relates to the interaction of spacecraft with hypervelocity impactors, which includes both orbital debris and meteoroids. During the past two years, my research group has identified a new spacecraft failure mechanism that is associated with the plasma resulting from a hypervelocity impact. Although hypervelocity impactors routinely hit spacecraft, the physics behind the formation of the plasma and the dynamics of its expansion that can cause electrical damage remain largely unknown. The complexity of this phenomenon necessitates a research approach that includes both experimental studies and numerical simulation in order to understand the underlying physical processes that occur upon formation and expansion of the impact plasma. Our goal is to answer the following three scientific questions:

1. What are the properties of hypervelocity impact plasmas?
2. What conditions cause impact plasmas to create RF emission and what is the power dependence on frequency?
3. How does the ambient environment around the spacecraft, including the background magnetic and electric fields and plasma density, influence the hypervelocity impact plasmas?

Hypervelocity impact model

Impactor characteristics

Meteoroids are small, solid, extraterrestrial objects that range from sand- to boulder-sized.  MEDUSSA will detect impacts from sub-microgram particles moving at meteoroid velocities (typically greater than 11 km/s).  In general, meteoroids will impact the spacecraft from Earth’s direction of travel.

Impact and charge generation

When a meteoroid impacts a spacecraft, the meteoroid is vaporized along with part of the spacecraft surface.  Some of the initial kinetic energy also ionizes the vaporized particles, creating a plasma.  This plasma cloud is modeled as being initially spherical with a diameter equal to that of the impact crater.

Plasma expansion and radiation

Initially, due to high plasma density, there will be many random collisions causing bremsstrahlung radiation.  However, the radiated power in this mode is low.  Because the plasma is highly collisional, we assume that electrons and ions are at the same temperature.

Once the plasma cloud expands and collisions become rare, we assume that the plasma remains isothermal and that there is no energy exchange with the environment.  Electrons, being much less massive than positive ions, will expand faster, until the ions pull them back.  The plasma now has an expanding shell of ions, with electrons that oscillate about the ions.  Underneath this shell, the bulk plasma remains electrically uniform.  As the plasma expands, its density drops and the electron oscillations slow down.  The radiated power spectrum from the expanding plasma has a peak at low frequencies and can have another peak at the plasma frequency if oscillations are fast relative to the plasma expansion.

Ground-based Impact Experiments

Optical, Plasma, and RF Sensor Experiments

Recently, hypervelocity impact tests were carried out at the White Sands Test Facility in New Mexico, Colorado Center for Lunar Dust Acceleration Studies in Boulder, Colorado and at the NASA Ames Vertical Gun Range. Our optical, plasma and RF sensors were deployed at these facilities to carry out impact measurements over a wide range of impact masses and speeds.

Max Planck Institute for Nuclear Physics

We have conducted experimental campaigns at the Max Planck Institute for Nuclear Physics in Heidelberg in 2010 and 2011 using a Van de Graaff dust accelerator in order to characterize the plasma formed by a hypervelocity impactor. Small (< 10^-15 g) particles were accelerated up to 60 km/s and impacted material representative of spacecraft in LEO and geosynchronous orbit (GEO). We designed and built plasma, radio frequency (RF), and optical sensors and positioned these throughout the vacuum chamber. These experiments resulted in what we believe to be the first detected RF emission from plasma associated with small hypervelocity particle impacts.

Our effort is aligned with the National Research Council’s recommendation in 2011 to better characterize the “effects of plasma during impacts, including impacts of very small but high-velocity particles.” By understanding the fundamental plasma processes associated with hypervelocity impacts and the interaction of these impact plasmas with the ambient environment, we will greatly improve our understanding of the physical mechanisms driving the plasma expansion process and provide the first understanding of their role in spacecraft failure. Our modeling and simulation, which includes both particle-in-cell (PIC) and computational fluid dynamics (CFD) efforts in conjunction with experimental validation will lead to better characterization of advanced plasma simulation tools. Our research will also lead to improved reliability for all spacecraft systems, including public services that are dependent on space assets (e.g., GPS, weather forecasting, and telecommunications).

While we have confirmed that RF is indeed emitted by hypervelocity plasmas, we know very little about when or why these sometimes cause satellite failure. In the following years, we will continue to execute experiments at both electrostatic dust accelerators, which can achieve speeds representative of meteoroids, and light gas gun facilities that can accelerate larger projectiles (> 10 g) up to 7 km/s, which is representative of orbital debris.

Space-based measurements

We are working toward sensor platforms that can characterize hypervelocity impact plasma on a CubeSat platform as well as on the International Space Station.