Resiliency of Electronics

Overview

This mission requires electronics to be subject to various different stressors and extreme conditions. The electronics on the lander must be subject to many months of travel through the vacuum of space at frigid temperatures, and then last for 90 days on the hostile surface of Venus. The first step in designing the system to withstand these extremes is to define parameters of each environment. Second, the devices can be chosen so they may operate within these defined parameters.

Environmental Requirements

The journey through space and the stay on Venus each have their own unique environmental challenges to overcome. The initial trip to Venus will subject the satellite to various forms of radiation such as electrons, protons, and to a lesser extent heavier charged particles. On the surface of Venus, the lander must endure high pressures and temperatures for the three month duration of the mission. This section will define the radiation, temperature, and pressure during both phases of the mission.

Radiation

Radiation is defined as the transit of energy in the form of high-speed particles and electromagnetic waves. Electromagnetic radiation is very common throughout Earth and space in the form visible light, radio and television waves, and microwaves [1]. The space radiation consists primarily of ionizing radiation which exists in the form of high-energy, charged particles. There are three naturally occurring sources of space radiation: solar cosmic ray, trapped radiation, galactic cosmic ray [2].

Solar Cosmic Ray

Solar cosmic rays consist chiefly of protons with some alpha particles ejected sporadically from the sun during some solar-flare events. These are blocked by the Earth’s atmosphere but are the major contributors to radiation in space.

Trapped Radiation

Trapped radiation, consisting of magnetically trapped protons and electrons in the vicinity of the earth and other planets, exist above the atmosphere and within the envelope of the magnetic field. For this mission the effect of this radiation is too small to constitute a significant hazard because the Venus has no significant magnetic field [3].

Galactic Cosmic Ray

Galactic cosmic ray provide a continuous, essentially isotropic radiation source consisting of about 85% protons, 14% alpha particles, and less than 1% heavier nuclei. This effect is negligible because its intensity is so small.

Ionizing radiation such as solar cosmic ray, trapped radiation and galactic cosmic ray loses energy primarily by creating electron-hole pairs on the satellites surface. It is associated with breaking bonds and creating point defects which accumulate with radiation exposure. Defects in any active device or electronics trap charge which may affect its performance characteristics [4]. Therefore, the successful use of electronic devices in radiation environments requires that all flight parts must operate within the limits of radiation specification. Figure 1 outlines the environmental specifications for various missions throughout the solar system.

Figure 1: Critical Planetary Missions [5] 

The unit of an absorbed radiation dose is a rad, where 1 rad = 0.01 J/kg. The VISTA mission to Venus experienced 0.02 Mrads / 100 mils aluminum. This mission lasted for 2 years in space and a single day on the surface of Venus. The use of a 100 mil aluminum box shielding was deemed appropriate for this and similar missions. The proposed mission will be approximately two years and can utilize a similar box shield of 100 mils Al to protect the orbiter in a similar manner to the VISTA mission.

Temperature and Pressure

Venus has a remarkable distribution of temperature and pressure throughout its atmosphere that is unique amongst the other planets. The heat and pressure at a point is highly dependent on the altitude of that point. Figure 2 maps the temperature and pressure as a function of altitude.

Figure 2: Temperature and Pressure in the Venus [6]

The electronics in the lander on the surface of Venus must be able to operate normally while subject to 462 degrees Celsius and a pressure of 90.8 atm for 103 days considering an additional 14% knockdown factor.

Radiation Hardening

Radiation shielding provides protection from particles trapped in planetary magnetic fields and emitted from the Sun. There are two general types of shielding applicable to protection against the charged particles that make up the space-radiation environment: active shielding and passive shielding. Active shielding uses electric or magnetic fields deflecting the charged particles away from the spacecraft. Passive shielding simply places a mass between the radiation source and the receptor [2]. Based on the analysis of the results above, our electronics will be affected by solar radiation throughout the course of the mission. The total dose of the solar radiation is 20k rad. This section outlines the materials and electronics that can withstand this radiation.

Recommend Electronics

There are two core electronic elements of our satellite and lander systems that are potential victims of radiation. The first is the CPU, and the second is the low noise amplifier. The following commercially available products would satisfy the mission requirements.

Figure 3: Power PC Processor

Figure 4: Low Noise Amplifier 

Total dose of Power PC processor is 200krad, which is 10 times the expected mission radiation. The total permissible radiation dose for the LNA ranges from 50 krad to 100 krad.

Points of Failure

Solar cosmic rays are highly variable, time-dependent, and not always predicable because little data exist to make reliable, long-term predictions. In addition, similar estimates for solar alpha particles would involve much more uncertainty. Therefore, the provided estimated total dose for the mission is smaller than actual dose we can expect from the environment, and by overdesigning we expect to minimize the risk of failure.

Heat Hardening

Most commercially available electronic device have rated operating temperature limits of 125 degrees Celsius, as seen in Figure 3.This specification is far below the requirement of the Venus environment (462 degrees Celsius). Conventional silicon (Si) devices cannot be used at temperatures exceeding 200 degrees Celsius, due to increased leakage current and latch-up at reverse bias junctions. For functionality up to 300 degrees Celsius, electronics may utilize silicon-on-insulator (SOI) technology, where the integrated circuits are dielectrically isolated from the base substrate [7]. Figure 5 and Figure 6 show high temperature electronics currently in development and/or use by NASA. 

Fig. 5. Example for High Temperature Electronics[8]

Fig. 6. Example for High Temperature Electronics[8]

Current estimates suggest that these electronics can survive 500°C for at least tens of hours. NASA GRC has demonstrated that a packaged SiC transistor amplifier circuit operation for 1000 hours at 500 degrees C. This group has also demonstrated 1GHz Oscillator antenna transmission with CREE MESFET at 270 degrees Celsius. From above information, the current state of the art technology does not satisfy the mission temperature requirement 462°C for the mission duration of 3024hrs. As such, the lander will require an active cooling system to lower the temperature in our lander to a manageable level. This cooling system is described in the power and cooling systems section of the mission report.

Points of Failure

Active cooling system determines the normal operation of all electronics. So, for successful mission, it is critical to guarantee high reliability of active cooling system. 

[1] "What is space radiation?," Space Radiation Analysis Group, Johnson Space Center, Last updated: April 2013. http://srag.jsc.nasa.gov/SpaceRadiation/What/What.cfm

[2] "Space Radiation Protection," NASA SP-8054, June 1970.

[3] Luhmann, J.C.,  and Russell, C.T., "Venus:Magnetic Field and Magnetosphere," Encyclopedia of Planetary Sciences, 1997.

http://www-ssc.igpp.ucla.edu/personnel/russell/papers/venus_mag/

[4] Petkov, M.P., "Effects of Space Environment on Electronic Components," NASA, Jan 2003.

[5] "Survivable Systems for Extreme Environments," Jet Propulsion Laboratory, NASA, Retrieved: July 2013. "http://scienceandtechnology.jpl.nasa.gov/research/ResearchTopics/topicdetails/?ID=57

[6] Dyson, Rodger W., and Bruder, Geoffery A, “Progress Towards the Development of a Long-Lived Venus Lander Duplex System,” NASA Glenn Research Center Thermal Energy Conversion Branch, 8th AIAA EICEC Session 126-APS-4, July 27, 2010.

[7] "Extreme Environments Technologies for Future Space Science Missions," JPL D-32832, September 2007.

[8] Hunter, Gary W., "Long-lived VENUS Lander Technolgies: A Brief Discussion Of Technologies Relevant To Long-lived Landers For Venus Exploration,"  NASA Glenn Research Center, Cleveland, OH, October 2009.