Space exploration vehicles like satellites, probes, shuttles and spacecrafts all rely on electronics. These electronic devices have to withstand for their working not only the extreme (very low to very high) temperatures but also the radiation hazards prevailing in the space. In fact, very little of a space system falls within the conventional electronics’ temperature range. Its temperature in space depends on its proximity and orientation to other bodies, absorption and emission of energy, and internal heat generation. Therefore extreme-temperature electronics is a key technology for space exploration.
Use of conventional electronics
Conventional-temperature-range electronics can be used in space (an extreme-temperature environment) by means of insulation and heating (for low-temperature environments) or refrigeration (for high-temperature environments). This can be combined with thermal sinks or thermal sources. For example, the well-logging electronic devices may be placed in a dewar flask (vacuum-insulated vessel) to protect these from the hot environment.
In addition, a thermal sink thermally connected to the electronic devices will absorb a large amount of heat without a substantial increase in temperature. This is usually done by employing the material’s phase change from solid to liquid, which absorbs a large amount of heat (the latent heat of fusion). For low temperatures, the opposite effect may be used to provide a thermal source. Thus the same material may serve both as a thermal sink for high temperatures and a thermal source for low temperatures. It might be as basic as ice/water or less familiar such as a bismuth alloy.
However, in many situations, the techniques described above would be undesirable or impractical due to various reasons. The passive techniques might have a limited lifetime that is insufficient for the appliation, while the active techniques require additional power and subsystems. Also, for some applications, active techniques might be too disturbing to the environment because of the additional heat that needs to be dumped. All such techniques add weight, bulk and some degree of complexity.
Operating electronics beyond the normal limits is thus an option worth considering. This special electronics will be able to withstand the extreme temperatures of space.
The term ‘extreme-temperature electronics’ (ETE) is used for electronics operating outside the traditional temperature range of –55/–65°C to +125°C. It covers both the very low temperatures—down to essentially absolute zero (0°K or –273°C)—and the high temperatures (+125°C and above).
In low-temperature electronics (LTE), operation of semiconductor-based devices and circuits has often been reported down to temperatures as low as a few degrees above absolute zero. These devices are based on silicon (Si), germanium (Ge), gallium arsenide (GaAs) and other semiconductor materials. Moreover, there is no reason to believe that operation might not extend all the way down to absolute zero.
In high-temperature electronics (HTE), laboratory operation of discrete semiconductor devices has been reported at temperatures as high as +700°C (for a diamond Schottky diode) and 650°C (for a silicon carbide (SiC) MOSFET). Integrated circuits (ICs) based on Si and GaAs have operated at 400−500°C. Si ICs have been reported to operate at +300°C for a thousand hours or longer. Covering both extremes, there are reports of the same transistor working at about –270°C to +400°C temperature range. Also, many passive components are usable to the lowest temperatures or up to several hundred degrees Celsius.
However, operation at extreme temperatures is not true for every semiconductor device or passive component; it depends on a number of material and design factors. Practical operation of devices and circuits is reasonably achievable to as low temperature as desired, provided materials and designs appropriate to the temperature are used. However, the various characteristics of the device might improve or degrade. In particular, below about 40°K (–230°C), Si devices often exhibit signifiant changes in characteristics.
High-temperature electronics pre-sents more difficulty.The practical upper temperature limit is determined by many factors and the inherent temperature limit is often not refleced for semiconductor devices. The limit is frequently determined by the interconnections and packaging—both for active devices and passive components. As an indication of the practical upper limit, circuits have been offered commercially for operation at up to +300°C.
Parts availability is a major obstacle to practical ETE. There are few components specifiedfor either low- or high-temperature use. To construct ETE hardware, often the conventional- temperature-range components are selected and adapted. Custom fabrication is done if resources and time permit.
Space electronics over the years
In earlier days, space exploration devices were fragile and required constant human attention. Many of the space exploration missions were unsuccessful due to the failure of the incorporated electronic devices. As space race progressed, hardware had to be put into the orbit as swiftly as possible by making space electronics smaller, more compact and far more reliable.
Vacuum tubes were used as state-of-the-art electronics in the late 1950s. These were well understood and highly developed. Transistors were still pretty new then. Many of the transistors available did not work well at high frequencies as required in satellites. A high percentage of the computers used still depended on vacuum tubes.
Some of the earliest space probes used film cameras and transmitted th processed film in a manner similar to fax machine. Another technique used in some later vehicles was ‘slow-scan TV.’ It involved a TV camera that took a single image and transmitted it, often very slowly.
It was realised further that better testing, redundancy and flexibility prvided the keys to more reliable space electronics demanded by satellite operation.
oday, use of digital tape recorders and digital cameras to save data and pictures and transmit these to earth, or send commands to probes has been replaced with on-board electronic devices and computers. The firs military satellite to have a microprocessor on board was the DSCS-III launched on October 30, 1982. Before that, ground commands were loaded into registers, and simpler circuits such as timers used to execute the commands. All parts must be able to survive launch, have radiation shielding, be able to radiate enough heat in a vacuum to function and have enough computing power to do the job.
Space exploration with space probes, space-shuttles, space crafts and satellites has proved effective with technological advances not only in the mechanical body parts, launch, guiding, and keeping the temperature of the space vehicle in check during the supersonic glide through the atmosphere and onto the runway but also in the electronic components integrated in the space vehicle design. Electronic component suppliers for space electronics must meet stringent temperature standards, along with radiation-hardening specs. These requirements will only get tougher to meet further requirements as scientists plan to send humans beyond the low-earth orbit to explore the solar system.
Faster turnaround times for satellites and other launches are narrowing the delivery window for aerospace electronics, putting added pressure on electronics technology. With greater requirements for testing space electronics ranging from thermal performance to radiation dose and production tests, the pendulum is swinging away from using commercial parts in space. Further, the requirement for space electronics for use in deep-space probes or in instrumentation to be used on scientifi payloads such as earth-imaging and weather satellites is increasing.
Recently, a spacecraft bound for Jupiter got an armour suit to protect it from the fiercest radiation any space probe has ever encountered. The unmanned Juno space probe will face a treacherous environment with more radiation than around any other planet and needs an armoured shield around its sensitive electronics. Juno is basically an armoured tank going to Jupiter; without its protective shield or radiation vault, its electronics would damage on the very firstpass near Jupiter.
The six-sided radiation shield is made of titanium. Lead, an effective shield against radiation, would be too soft to survive vibrational forces and stresses. While the vault is not designed to completely prevent Jupiter’s radiation from hitting the system, it is expected to dramatically slow down the ageing effect that radiation has on electronics for the duration of the mission.
Problems with space electronics
Reliability of components. Space agencies place reliability at the top of their priorities since the failure of just one component can lead to the loss of a multi-million dollar mission. A clear counter trend is the use of commercial off-the-shelf (COTS) components. While these parts are generally more advanced in terms of processor performance and logic or memory density than those designed specificallyfor use in military or space borne systems, COTS devices do not have the background of design and extensive testing that ensures reliability.
Parts designed for military and aerospace often have a wider range of materials at their disposal than COTS components, particularly when it comes to the materials used to connect the chips to the printed control boards. The more restricted materials used in COTS tend to suffer from greater reliability problems under heat stress. Military-grade parts used for space flightare also tested to a much greater degree than their COTS counterparts—not just over a wider temperature range but also with a greater concentration on faults that cannot be picked up by generic wafer probe and package level tests.
Radiation effect. When a space-bound electronic component passes its test, there remains one big problem—radiation. Radiation is one of the main characterstics of space weather. Radiations of galactic and solar origin determine radiation hazards for people and technology, computer and memory upsets and failures, solar cell damage, radio wave propagation disturbances, and failures in communication and navigation systems.
The effect of ionising radiation on hard-wired logic circuits is less pronounced. These errors are typically transient and often non-destructive. A review conducted by NASA in 1996 of a hundred failures and problems on its spacecraft found that one-third of the failures were caused by ionising radiation leading to single event upsets (state changes in logic or memory) or permanent degradation in the performance of onboard electronic devices. Sometimes these single event upsets are even capable of destroying computer memories on the earth. But obviously with a much larger probability in spacecraft systems during periods of large energetic particle fuxes, it is advisable to switch off some part of the electronics to protect computer memories.
High-energy particles ionise the medium through which these pass, leaving behind a wake of electron-hole pairs. These pairs can change the state of a memory cell or a logic flip-flopAs a result, a radiation strike might change not just the state of a memory cell but also the design of the circuit it controls, potentially leading to cata-strophic failure.
Engineers only have the option to use triple modular redundancy within the subsystems they design using these parts. This increases the cost and development time and still leaves gaps in the test methodology. Space-oriented components can provide greater levels of protection; this not only simplifies the system desig but also improves the overall reliability—the key criterion for space agencies and satellite operators to minimise damaging failures.
The electronics aboard Juno is encased in a titanium vault designed to protect components from high levels of radiation. But even with this shielding, the spacecraft is expected to sustain serious damage after a year in Jupiter’s orbit. Any loss of control of Juno could leave the spacecraft in the danger of crash.
Advances being made
A five-yearproject led by the Georgia Institute of Technology has developed a novel approach to space electronics that could change the way space vehicles and instruments are designed. The new capabilities are based on silicon-germanium (SiGe) technology, which can produce electronics that is highly resistant to both wide temperature variations and space radiation.
The team’s overall task was to develop a tested infrastructure that included everything needed to design and build extreme-environment electronics for space missions. The result is a robust material that offers important gains in toughness, speed and flexibility.The robustness is crucial for SiGe’s ability to function in space without bulky radiation shields or large power-hungry temperature control devices. Compared to conventional approaches, SiGe electronics can provide major reductions in weight, size, complexity, power and cost, as well as increased reliability and adaptability.
The Radiation Hardened Electronics for Space Environments (RHESE) project endeavours to expand the radiation-hardened electronics by developing high-performance devices robust enough to withstand the extreme radiation and temperature levels of the space environment. The project is a part of the Exploration Technology Development Program (ETDP), which funds an entire suite of technologies needed for accomplishing the goals of the vision for space exploration.
NASA’s Marshall Space Flight Center (MSFC) manages the RHESE project. RHESE’s investment areas include novel materials, design processes to implement radiation hardening, reconfigurablehardware techniques, software development tools, and radiation environment modeling tools.
Near-term emphasis within the multiple RHESE tasks is on hardening feld-programmable gate arrays (FPGAs) for use in reconfigurablear-chitectures and developing electronic components using semiconductor processes and materials (such as SiGe) to enhance the tolerance of a device to radiation events and low-temperature environments.
As these technologies mature, the project will shift its focus to de-veloping low-power, high-efficienc total- processor hardening techniques and hardening of volatile and non-volatile memories. This phased approach to distributing emphasis between technology developments allows RHESE to provide hardened FPGA devices and environmentally-hardened electronic units for mission infusion into early constellation projects.
Once these technologies begin the infusion process, the RHESE project will shift its technology development focus to hardened high-speed processors with associated memory elements and high-density storage for longer-duration missions, such as the Lunar Lander, Lunar Outpost, and eventual mars exploration missions occurring later in the Constellation schedule.
The individual tasks of RHESE are diverse, yet united in the common endeavour to develop electronics capable of operating within the harsh environment of space. Specifcally, the RHESE tasks include SiGe integrated electronics for extreme environments, modeling of radiation effects on electronics, single-event-effects-immune reconfigurable FPGA radiation-hardened high-perfomance processors and reconfigurabl computing.
Though the tasks are diverse in their specifickey performance parameters, these target to accomplish specific goals—improved total ionistion dose tolerance, reduced single event upset rates, increased threshold for single-event latch-up, increased sustained processor performance, increased processor efficiency, increase speed of dynamic reconfigurability reduced lower bound of the operating temperature range, increased available levels of redundancy and reconfigurability,and increased reliability and accuracy of radiation effects modeling.
The author is in the department of physics at Sant Longowal Institute of Engineering and Technology, Longowal, Punjab