Resiliency of Electronics
The major elements that extraterrestrial spacecraft must be designed to withstand are extreme temperatures, high levels of radiation, and the physical stress caused during launch. Radiation poses the most unique design challenge for these spacecraft compared to typical terrestrial applications. It can cause minor or even fatal errors in integrated circuits. Significant sources of radiation for which space missions must account include:
- Cosmic rays - There is a background level of radiation that is present throughout the universe. It represents the cumulative average of all radiation producing sources, such as stars and cosmic events. The primary components of cosmic radiation are:
- 89% - Protons
- 10% - Helium nuclei (Alpha particles)
- 1% - Electrons and heavier nuclei
- Solar events - Solar flares and Coronal Mass Ejections can be a significant source of protons, ions, and heavy ions, collectively known as Solar Energetic Particles. These events can produce particles with several GeV of energy.
- Van Allen belts - The magnetosphere causes a donut shaped belt of electrons and ions to be trapped around the Earth. The Van Allen belts extend from 100 km to 65,000 km above the surface of the Earth and have particles up to several hundred MeV of energy.
- Secondary particles - Radiation impinging on the spacecraft from the sources mentioned above can sputter particles away from the spacecraft, resulting in secondary radiation.
- Nuclear sources - The Space Buster spacecraft will be equipped with Rapid Thermo-electric Generators and a nuclear propulsion system. Both of these systems will be immense sources of damaging radiation.
- Chip packaging - Traces of radioactive elements found in the packaging of integrated circuits have been known to cause malfunctions in electronics. This problem has greatly subsided as microelectronics fabrication technology has improved and integrated circuits can be made more pure. However, it could become an issue for a mission lasting millennia.
Galactic cosmic rays represent the most significant threat to the spacecraft. The Space Buster probe must traverse 10.5 light years, which is plenty of time to accumulate errors.
Radiation can causes both transient and permanent errors. If the radiation particle has sufficient energy, the crystal lattice of the semiconductor can become permanently damaged. This is known as lattice displacement. Severe lattice displacement can break transistor by forming recombination centers in pn junctions, depleting minority carriers, and causing parasitic capacitances. Latch up refers to short-circuiting between circuit elements. It can be a significant risk for integrated circuits, due to the close proximity of components. Permanent errors are caused by ionization effects. Charged particles can cause the voltage of a bit register to pass a decision threshold, resulting in a bit in error. While a bit error is typically a transient, soft glitch, it can sometimes trigger a chain reaction of errors resulting in system failure.