The MEDUSSA CubeSat
Introduction
Meteoroids cause both mechanical and electrical damage to spacecraft. Large ones can penetrate through spacecraft components but are relatively rare. Small ones impact much more frequently, and can generate electromagnetic pulses (EMPs) or trigger electrostatic discharges (ESDs) that can also effectively destroy a satelite.
Despite the probable loss of spacecraft to electrical damage from meteoroids, there is still debate as to whether a meteoroid-induced EMP or ESD can cause satellite failures. MEDUSSA intends to answer this question by flying a CubeSat that will characterize the effects of impacts from meteoroids and from energetic particles.
Hypervelocity impact model
Meteoroid 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.
Depiction of plasma expansion process
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.
Power spectrum of expanding plasmas at different ratios of plasma frequency to expansion rate.
MEDUSSA spacecraft
Configuration
The MEDUSSA spacecraft.
The MEDUSSA spacecraft conforms to the CubeSat 3U form factor (10 x 10 x 34 cm). It has four deployable solar panels providing approximately 20 W of power. On the other end of the spacecraft, a 1 × 1 m screen is attached, providing a surface that should yield nearly one impact event per hour. The screen is held in place by metallic booms that double as antennas. An Attitude Determination and Control System (ADACS) uses reaction wheels and magnetic torque coils to orient the spacecraft.
Sensors
Impacts on the meteor screen will be detected and localized using an optical camera. The RF signal from the plasma will be picked up by antennae tuned to a range of frequencies from VLF (3-30 kHz) to S-band (>2 GHz). Tape-spring material is used for the VLF antennae so that they will self-deploy and remain straight. Plasma detection panels mounted on the chassis sides and on the solar panels will be used to detect electrons and ions from the plasma, and localize the point of impact using photodiodes.
Screen deployment
The meteor screen is housed in the bottom end of the spacecraft, and is held in place by the folded solar panels and a detachable panel underneath. The plasma detection antennae are also coiled and stored with the screen spool. Upon deployment of the solar panels, the screen will deploy using stored strain energy in four steel tape-springs. A spring-plunger will push the rear panel downward, releasing the antennae from their housing.
Operations
MEDUSSA is designed to operate in any available low Earth orbit to maximize launch opportunities. The spacecraft will be operated from a ground station based at Stanford University, with the potential for remote ground stations to be set up at other locations.