The environments encountered by Solar System in-situ exploration missions cover extremes of temperature, pressure, and radiation that far exceed the operational limits of conventional electronics, electronic packaging, thermal control, sensors, actuators, power sources and batteries. In these studies, environments are defined as “extreme” if they present extremes in pressure, temperature, radiation, and chemical or physical corrosion. In addition, certain proposed missions would experience extremes in heat flux and deceleration during their entry, descent and landing (EDL) phases, leading to their inclusion as missions in need of technologies for extreme environments.
Specifically, a space mission environment is considered “extreme” if one or more of the following criteria are met:
- Heat flux: at atmospheric entry exceeding 1 kW/cm2 at atmospheric entry
- Hypervelocity impact: higher than 20 km/sec
- Low temperature: lower than -55°C
- High temperature: exceeding +125°C
- Thermal cycling: temperature extremes outside of the military standard range of -55°C to +125°C
- High pressures: exceeding 20 bars
- High radiation: total ionizing dose (TID) exceeding 300 krad (Si)
Additional extremes include:
- Deceleration (g-loading): exceeding 100 g
- Acidic environments: such as the sulfuric acid droplets in Venusian clouds
- Dusty environments: such as experienced on Mars
A summary of planetary destinations and their relevant – and sometimes coupled – extreme environments is shown in Table 1. Typically, high temperature and pressure conditions are coupled – e.g., for Venus in-situ mission concepts and proposed deep-entry probe missions to the two gas giants, Jupiter and Saturn. High radiation and low temperature can also be coupled, as experienced by missions to the Jovian system. Low temperatures could be associated with surface missions to the “Ocean Worlds,” the Moon, Mars, Titan, Triton, and comets. Thermal cycling would affect missions where the frequency of the diurnal cycle is relatively short, e.g., for Mars (with a similar cycle to Earth), and for the Moon (with 28 Earth days).
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 (current technology limits the duration of Venus surface exploration to <2 hours). At the other extreme, ocean worlds, asteroids, comets, and Mars missions operate 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/second. Investments in technologies for developing these systems -- and for operations and survivability in extreme environments -- are continually emerging, and are crucial to 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-level solutions, including: fault tolerance, thermal management, systems integration, and effects of four solar radii (perihelion for a solar-probe mission). For example, 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 the National Research Council’s most recent decadal survey on solar system exploration, New Frontiers in the Solar System: An Integrated Exploration Strategy, future missions may require operations in extreme environments at very high and very low temperatures, high and low pressures, corrosive atmospheres, or high radiation.
Survival in High-Radiation Environments
Improvements in technology for possible missions to Europa, Titan, the Moon, and mid-Earth-orbit are crucial and are in development. Europa mission concepts (both lander and orbiter) present the challenge of surviving radiation levels behind typical shielding thicknesses. Significant research and development 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. The methodology used in the development of device performance data and worst-case scenario analysis is being developed to support reliable modeling and a realistic approach to system survival.
Survival in Particulate and Hypervelocity Impact Environments
An important consideration when building survivable systems is their reliability, extended functionality and operation in particulate environments. For example, lunar surface missions must operate in highly abrasive lunar dust, and all missions must penetrate orbital-debris fields. Potential impacts from meteoroids or Earth-based 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 mitigating 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 requirements. Environmentally tolerant technologies may provide better solutions, particularly in subsystems such as sensors, drilling mechanics, 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: low-temperature operation, high-temperature and high-pressure operation, and operations at wide temperature ranges.
Several targeted missions and classes of mission concepts require the ability to function in extreme cold. These include 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 located on the exterior of any interplanetary spacecraft. Many of the currently available electronics will not perform in extremely cold environments. Additionally, many metals undergo brittle phase transitions with abrupt changes in properties, which are not well understood in these 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.
Low-temperature survivability is required for surface missions to Titan (-180 °C), Europa (-170 °C), Ocean Worlds such as Ganymede (-200 °C) and comets. Also, the Moon's equatorial regions experience wide temperature swings (from -180 °C to +130 °C during the lunar day/night cycle). The sustained temperature at the shadowed regions of lunar poles can be as low as -230 °C. Mars diurnal temperature changes from about -120 °C to +20 °C. Proposals are being developed for technologies that enable NASA to achieve scientific success on long duration missions to both low-temperature and wide-temperature range environments. Technologies of interest include:
- low-temperature, radiation-tolerant/radiation-hardened power electronics
- low-temperature-resistant, high strength-weight textiles for landing systems (parachutes, air bags)
- low-power and wide-operating-temperature, radiation-tolerant /radiation hardened RF electronics
- radiation-tolerant/radiation-hardened low-power/ultra-low-power, wide-operating-temperature, low-noise, mixed-signal electronics for space-borne systems, such as guidance and navigation avionics and instruments
- low-temperature, radiation-tolerant/radiation-hardened high-speed fiber optic transceivers
- low-temperature and thermal-cycle-resistant radiation-tolerant/radiation-hardened electronic packaging (including shielding, passives, connectors, wiring harness, and materials used in advanced electronics assembly)
- low- to medium-power actuators, gear boxes, lubricants and energy storage sources capable of operating across an ultra-wide temperature range (from -230 °C to 200 °C)
- Computer-Aided Design (CAD) tools for modeling and predicting the electrical performance, reliability, and life cycle for wide-temperature electronic/electro-mechanical systems and components
Research needs to continue to demonstrate technical feasibility (Phase I) and show a path toward a hardware/software demonstration (Phase II), and when possible, deliver a demonstration unit for functional and environmental testing at the completion of the Phase II contract.
High-temperature and high-pressure operation
To achieve successful long-term 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) at the surface of Venus could significantly increase the science return of future missions. Toward that end, current work continues to develop an innovative sensor preamplifier capable of working in the Venus ground ambient and to be designed using commercial components (thermionic vacuum and solid-state devices; wide-band-gap, 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. The technology developed could also be used for Jupiter deep-atmosphere probes, which could reach pressures of up to 100 bars at temperatures of 450 °C (~ 725 K).
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 for future mission concepts are rapidly being developed.
Operations at wide temperature ranges
Both lunar and Mars missions involve extreme temperature cycling. In the case of Mars, diurnal 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.
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 proposed Europa mission dose for an average and a 1-sigma worst-case scenario. 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.
Saturn's radiation belts have not received as much attention as the Jovian radiation belts because they are not nearly as intense; Saturn's famous 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 Saturn's radiation environment for mission design, with the exception of the (1990) Divine 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 possible future missions to Saturn. Extension and refinement of these models are critical to future missions to Europa and Titan, as well as for extended Jovian missions.