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Survivable Systems for Extreme Environments
Survivable electronic and mechanical systems enable reliable operations under extreme radiation, temperature, pressure, and particulate conditions.
The environments for solar system in-situ exploration missions cover extremes of temperature, pressure, and radiation that can far exceed the operational limits of electronics, electronic packaging, thermal control, sensors, actuators, power sources and batteries. At one extreme, Venus lander missions would need to survive at 460 °C (730 K) temperatures and 90-bar pressures, and must pass through corrosive sulfuric acid clouds during descent (the current technology limits the duration of Venus surface exploration to only 1 to 2 hours). At the other extreme, Titan, Europa, asteroids, comets, and Mars missions would require operations in extremely cold temperatures in the range of –180 to –120 °C (~90-150 K). For missions to comets or close to the Sun, high-velocity impacts are a real concern, with impact velocities reaching greater than 500 km/s at functional redundancy. Investments in technologies for developing these systems and for operations and survivability in extreme environments are emerging, and should enable the successful development of future NASA missions.
Spacecraft survival in these environments requires not only that mission designers test and model the effects but also that they develop systems solutions, including fault tolerance, thermal management, systems integration, and 4 solar radii (perihelion for a solar-probe mission). For example, proposed missions to Europa must survive radiation levels behind typical shielding thicknesses combined with very low temperatures in the vicinity of -160 °C (~110 K). As recommended in New Frontiers in the Solar System: An Integrated Exploration Strategy, the National Research Councils’ decadal survey on solar system exploration, such missions would require operations in extreme environments at very high and very low temperatures, high and low pressures, corrosive atmospheres, or high radiation.
Critical environments for planetary missions.
Whether it is temperature, pressure, radiation, or dust, nearly all of the proposed planetary and deep space missions must contend with the various extreme environments of space. Improving understanding of extreme environments
is crucial for the success of future space exploration, and for understanding the solar system as a whole. The ability to design for specific radiation levels allows flying the correct components without excessive shielding. Understanding the behavior of electronics and materials at extreme cold and with large temperature swings allows designers to prepare for reliable extended missions.
Survival in High-Radiation Environments
Improvements in technology for spacecraft survival in high-radiation environments to enable Europa, Titan, lunar, and mid-Earth-orbit missions are crucial and are in development. Many potential missions to Europa (both lander and orbiter) present a challenge of surviving radiation levels behind typical shielding thicknesses. Significant efforts to meet high-radiation challenges include test, analysis, and mitigation of single-event effects for complex processors and other integrated circuits at high device operating speeds. Total dose testing at high-dose and low-dose rates are being performed at high radiation levels to validate test methods for long-life missions. Tests and analysis of device performance in combined environments, total dose, displacement damage dose, and heavy ion dose must be performed to validate radiation effects models. Methodology used in the development of device performance data and worst-case analysis is being developed to support reliable modeling and a probabilistic approach to system survival.
Survival in Particulate and Hypervelocity Impact Environments
An important consideration when building survivable systems is the reliability, extended functionality, and operation of systems in particulate environments; for example, lunar surface missions must operate in the highly abrasive lunar dust, and all missions must penetrate orbital-debris fields. Potential impacts by meteoroids or Earth space debris at velocities in the range of 20–40 km/s short term and > 500 km/s long term (solar probe) are also an issue. JPL has developed a roadmap for impact environments -- including debris, comets, and meteoroids -- that includes modeling, testing, and shielding, as well as some of the leading models for dust environments.
Electronics and Mechanical Systems for Extreme Temperatures and Pressures Over Wide Temperature Ranges
Previous strategies in this area generally involved isolation of the spacecraft from the environment; however, isolation approaches can add substantially to weight, mass, and power. Environmentally tolerant technologies may provide better solutions, particularly in subsystems such as sensors, drilling mechanisms, sample acquisition, and energy storage. In order to get the maximum science return, JPL is developing electronic and mechanical subsystems designed to survive temperature extremes. The challenges, outlined below, may be categorized into the following areas: cold-temperature operations, high-temperature and high-pressure operations, and operations at wide temperature ranges.
The heat shield left behind after MER successfully descended through the Martian atmosphere.
Several targeted missions and mission classes require the ability to function in extreme cold. These include potential missions to the Moon, Europa (lander only), deep-space missions (astrophysics and planet finding), and any mission requiring sample acquisition, as well as actuators or transmitters outside any interplanetary spacecraft. Many of the currently available electronics will not perform at extreme cold. Additionally, many metals undergo brittle phase transitions with abrupt changes in properties, which are not well understood, in the extreme cold environments. Other performance issues at cold temperatures include: the effects of combined low temperature and radiation; the reliability issues of field-effect transistors due to hot carriers; freeze-out of advanced complementary metal-oxide semiconductors at very cold temperatures; severe single-event effects at cold temperatures for silicon germanium semiconductors; and battery operations at low temperatures.
High-Temperature and High-Pressure Operation
To achieve successful missions, previous Venus landers employed high-temperature pressure vessels with thermally protected electronics, which had a maximum surface lifetime of 127 minutes. Extending the operating range of electronic systems to the temperatures (485 °C, ~760 K) and pressures (90 bar) of the Venus ground ambient would significantly increase the science return of future missions. Toward that end, current work endeavors to develop an innovative sensor preamplifier capable of working in the Venus ground ambient, designed using commercial components (thermionic vacuum and solid-state devices; wide-bandgap, thick-film resistors; high-temperature ceramic capacitors; and monometallic interfaces). To identify commercial components and electronic packaging materials capable of operation within the specified environment, a series of active devices, passive components, and packaging materials was screened for operability at 500 °C (~ 775 K), targeting a tenfold increase in mission lifetime.
Survivability and operation of electronic systems in extreme environments are critical to the success of future NASA missions. Mission requirements for planets such as Venus cover the extremes of the temperature spectrum, greatly exceeding the rated limits of operation and survival of current commercially available military and space-rated electronics, electronic packaging, and sensors. In addition, distributed electronics into future missions are rapidly being developed.
Operations at Wide Temperature Ranges
Both lunar and Mars missions would involve extreme temperature cycling. In the case of Mars, temperatures may vary from -130 to +20 °C (143-293 K), with a cycle approximately every 25 hours. For an extended mission, this translates into thousands of cycles. Lunar extremes are even greater (-230 to +130 °C, ~40-400 K) but with a cycle every month. Such extreme cases involve not only extreme temperatures but also fatigue issues not generally encountered in commercial, military, or space applications.
Reliability of Systems for Extended Lifetimes
Survivable systems need to have extensive reliability for extended lifetimes. Electronics are generally not designed to be functional for more than 10 years, unless specially fabricated for long life. Long-life systems ultimately need a 20-year (or greater) lifetime and are critical for extended lunar-stay missions, deep- and interstellar-space missions,
and some Earth-orbiting missions.
Contour plots of Jupiter’s radiation belts based on the JPL Galileo Interim Radiation Electron model. The left panel illustrates the Jovian 10-MeV proton integral fluxes, while the right panel illustrates the 1-MeV electron integral fluxes.
Space Radiation Modeling
The modeling of radiation environments is another important aspect of extreme environments technology. Extensive models have been developed for both the Jovian and Saturnian environments. Measurements of the high-energy, omnidirectional electron environment were used to develop a new model of Jupiter’s trapped electron radiation in the Jovian equatorial plane; this omnidirectional equatorial model was combined with components of the original Divine model of Jovian electron radiation to yield estimates of the out-of-plane radiation environment, referred to as the Galileo Interim Radiation Electron (GIRE) model. The GIRE model was then used to calculate a Europa mission dose for an average and a 1-sigma worst-case situation. While work remains to be done, the GIRE model represents a significant step forward in the study of the Jovian radiation environment, and provides a valuable tool for estimating and designing for that environment for future space missions.
Saturnian radiation belts have not received as much attention as the Jovian radiation belts because they are not nearly as intense; the famous Saturnian particle rings tend to deplete the belts near where their peak would occur. As a result, there has not been a systematic development of engineering models of the Saturnian radiation environment for mission design, with the exception of the Divine (1990) study that used published data from several charged-particle experiments from several flybys of Saturn to generate numerical models for the electron and proton radiation belts; however, Divine never formally developed a computer program that could be used for general mission analyses.
JPL has attempted to fill that void by developing the Saturn Radiation Model (SATRAD), which is a software version of the Divine model that can be used as a design tool for missions to Saturn. Extension and refinement of these models are critical to future missions to Europa and Titan as well as extended Jovian missions.