Developing Spacecraft Reliability for Lunar Missions—Radiation Shielding

Radiation exposure from SPEs presents a significant hazardous concern for exploration missions outside the protection of the Earth’s magnetic field, which could impair their performance and result in possible mission failure.The impracticability of Earth-Moon transits for repairs, replacements affords a need for greater self-reliability of lunar spacecraft. Radiation environment consists mainly of solar wind (including electrons and protons) to cosmic protons and ions as primary space environment. And, radiation secondary space environment of electrons and neutrons influences spacecraft as well as spacecraft electronic devices; results in spacecraft single event effect; causes displacement damage dose.The objective is to show distinct radiation-mitigating strategies and how they may be designed into different hierarchical levels of a system with the greater functional reliability as well as quicker resilient-like post-fault recoverability.The following radiation-mitigating strategies will be discussed and compared.<br/>1. The satellite structure is the first radiation shield layer. A novel material known as boron nitride nanotubes (BNNTs) per their mechanical properties, suggest replacement of aluminum alloys as structural components for spacecraft, while also providing radiation shielding. More enhanced BNNT performance may be achieved when loaded with hydrogen. A comparison may be made with another shielding material under review, polyethylene composites. Moreover, technologies under review enable greater hydrogen storing capacity in BNNTs. 2. Radiation flux to dose conversions from OLTARIS and HZETRN afford efficacycomparative analyses for different novel radiation-mitigating strategies. 3. Electronic equipment used in the satellite missions is another layer of vulnerability with ionizing particle encounters, causing problems in normal operations. So, the combination of high-density shielding materials such as tantalum and tungsten and low-density ones such as polyethylene can be considered as an ideal strategy.4. Space computers suffer from the unwanted effects of cosmic energy. A single-event upset (SEU) occurs when high-energy particles or heavy ions strike a complementary metal oxide semiconductor (CMOS) device and cause unintended, logic-level transition. 5. In partial reconfiguration (PR), a partial bitstream is loaded into a select section of configuration memory to drive a change in logic. When a fault occurs in either the foreground circuitry of the FPGA or in the configuration memory region corresponding to the foreground circuitry, PR will restore that region to its original state. Detection and repair of SEU-induced faults are accomplished through a comprehensive strategy including partial reconfiguration (PR) of impacted cores, error correction codes (ECC) for memory that cannot be partially reconfigured, and soft error mitigation monitoring for the configuration memory of the FPGA.<br/>6. As radiation-based faults are a great concern to spaceflight hardware, the concept of shielding, in which some sort of material serves as a boundary between cosmic radiation and the hardware at risk. Alternatively, radiation-hardened semiconductor manufacturing modifications are another method of mitigating radiation-induced faults in system hardware and have shown effectiveness. Radiation hardened by design (RHBD) methods will be discussed and comparatively analyzed.<br/>7. Other approaches include fault-mitigation strategies based on a Redundant Multiprocessor System implemented within the FPGA fabric; improved characterizations of the radiation environments to which RadPC would be exposed, and a numerical modeling tool known as the Cosmic Ray Effects on MicroElectronics (CREME96) would be used to model the radiation environment. RadPC contributes to the field of space computing by providing a cost-effective, radiation-tolerant design using a commercial-off-theshelf FPGA to actively respond to radiation-induced faults in the device.