Course: ELEC3225 Spacecraft Systems Engineering
Topic: Space Environment and Spacecraft Charging
Professor: S.B. Gabriel
Institution: University of Southampton
Date: October 2021
Basics of Space Environment
Understanding the differences between surface charging and internal charging.
Overview of design practices related to charging.
Examples of charging effects.
Surface:
Thermal Plasma: Ionized particles influencing surface charging.
High Energy Electrons: Ranges between 1-100 keV impacting the spacecraft.
UV/EUV Radiation: Ultra-Violet and Extreme Ultra-Violet photons affecting charging properties.
Magnetic Field: Interaction with the spacecraft's materials can alter charge distributions.
Neutral Particles: Contributes to the atmospheric interactions that can result in charging.
Internal:
High Energy Electrons: Energies greater than 100 keV can cause internal charging issues.
Surface Charging Mechanisms:
UV/EUV Photons: Lead to the emission of photoelectrons, affecting charge on surfaces.
High energy photoelectrons can accumulate charge.
Thermal plasma electrons contribute to surface charge but can help reduce accumulation through secondary electron ejection.
Magnetic Fields: Can inhibit the escape of secondary electrons, leading to altered properties.
Contaminants: Change surface charging properties by affecting secondary emission coefficients.
Internal Charging Mechanisms:
Trapped charge can occur due to high energy electrons,
Effects of ungrounded cables or isolated conductors create electric fields leading to breakdown.
Cable Insulation: Lack of proper shielding can exacerbate this phenomenon.
Charging Equation:
JE - JI - JPH = 0
Where:
JE: Electron current density
JI: Ion current density
JPH: Photoelectron current density
Interpretation of the equation reveals balance conditions for current within the spacecraft system under sunlight exposure.
Potential Equation: CAdV/dt = J(V) + σV = 0 where ΣI = 0
Notable Challenges: Capacitance (CA) and conductivity (σ) are difficult to determine accurately.
Current density (J) also presents complications due to its non-linear and non-local nature.
Surface Potential Calculation Components:
J = - Je + Jb + Jse + Jsi + Jp ± Jc + Ji
JE: incident electron current density
JB: back-scattered electron current density
JSE: secondary electron current due to JE
JSI: secondary electron current due to Ji
JP: photo-electron current
JC: conduction current
Ji: incident ion current density
In eclipse, V ~ - Te for high temperatures (Te > ~ 1000eV).
Key Points:
High fluence of 0.5 – 5.0 MeV electrons (> 1011 MeV cm-2) leads to significant internal charging.
These energies can penetrate thin shielding materials (<1.5 mm Al equivalent).
Thin insulating dielectrics with low conductivity can lead to electrostatic discharge paths.
Connection between internal charging and sensitive circuits.
Impacts on Spacecraft Operations:
Distortion of plasma measurements.
Enhanced risk of contamination.
Potential for arcing, resulting in circuit failures.
Loss of power and logic upsets.
**Recent Studies: **
Comprehensive analyses of ESD-related anomalies reviewed various notable spacecraft incidents (e.g., TELSTAR-401, INTELSAT-511).
Highlighting that many failures remain unexplained and require better monitoring and reporting.
Grounding:
Ensure all conducting elements are connected to a common ground via charge bleed-off resistors.
Exterior Surface Materials:
Employ at least partially conductive materials for exterior surfaces to control differential charging.
Shielding:
Design spacecraft to have continuous shielded structures around electronics (Faraday cage principles).
Filtering:
Implement electrical filters to protect circuits from discharge-related upsets.
Procedures:
Establish rigorous procedures to maintain electrical continuity in the vehicle grounding system.
Challenging Environments:
ESD risks identified in polar-aurora orbiting, influencing space suit charging conditions.
Testing environment outlined with electron beams assessing arc discharge risks.