Posts tagged: Rad hardening of equipment

Rad Hardening

By , September 14, 2009 3:23 AM

When your computer behaves erratically, mauls your data, or just “crashes” completely, it can be frustrating. But for an astronaut trusting a computer to run navigation and life-support systems, computer glitches could be fatal.

Unfortunately, the radiation that pervades space can trigger such glitches. When high-speed particles, such as cosmic rays, collide with the microscopic circuitry of computer chips, they can cause chips to make errors. If those errors send the spacecraft flying off in the wrong direction or disrupt the life-support system, it could be bad news.

To ensure safety, most space missions use radiation hardened computer chips. “Rad-hard” chips are unlike ordinary chips in many ways. For example, they contain extra transistors that take more energy to switch on and off. Cosmic rays can’t trigger them so easily. Rad-hard chips continue to do accurate calculations when ordinary chips might “glitch.”

NASA relies almost exclusively on these extra-durable chips to make computers space-worthy. But these custom-made chips have some downsides: They’re expensive, power hungry, and slow — as much as 10 times slower than an equivalent CPU in a modern consumer desktop PC.

With NASA sending people back to the moon and on to Mars–see the Vision for Space Exploration–mission planners would love to give their spacecraft more computing horsepower.

Having more computing power onboard would help spacecraft conserve one of their most limited resources: bandwidth. The bandwidth available for beaming data back to Earth is often a bottleneck, with transmission speeds even slower than old dial-up modems. If the reams of raw data gathered by the spacecraft’s sensors could be “crunched” onboard, scientists could beam back just the results, which would take much less bandwidth.

Objects, particularly spacecraft structures, antennas, solar arrays and other spacecraft equipment,  are shielded against damage from momentary exposure to high energy electromagnetic radiation in the form of high energy optical (laser) radiation or nuclear radiation by a radiation barrier or shield constructed of fibrous silica refractory composite material like that used for the heat shield tiles on the shuttle spacecraft.

Major radiation damage sources

Typical sources of exposure of electronics to ionizing radiation are solar wind and the Van Allen radiation belts for satellites, nuclear reactors in power plants for sensors and control circuits, residual radiation from isotopes in chip packaging materials, cosmic radiation for both high-altitude airplanes and satellites, and nuclear explosions for potentially all military and civilian electronics.

  • Cosmic rays come from all directions and consist of approx. 85% protons, 14% alpha particles, and 1% heavy ions, together with ultraviolet radiation and x-rays. Most effects are caused by particles with energies between 108 and 2*1010 eV, though there are even particles with energies up to 1020 eV. The atmosphere filters most of these, so they are primarily a concern for high-altitude applications like stratospheric jets and satellites.
  • Solar particle events come from the direction of the sun and consist of a large flux of high-energy (several GeV) protons and heavy ions, again accompanied with UV and x-ray radiation. They cause a scale of problems for satellites, ranging from radiation damage to loss of altitude by heating up the upper regions of the atmosphere, causing them to raise up, and decelerating the low-orbit satellites by friction.
  • Van Allen radiation belts contain electrons (up to about 10 MeV) and protons (up to 100s MeV) trapped in the geomagnetic field. The particle flux in the regions farther from the Earth can vary wildly depending on the actual conditions of the sun and the magnetosphere. Due to their position they pose a concern for satellites.
  • Secondary particles result from interaction of other kinds of radiation with structures around the electronic devices.
  • Chip packaging materials were an insidious source of radiation that was found to be causing soft errors in new DRAM chips in the 1970s. Traces of radioactive elements in the packaging of the chips were producing alpha particles, which were then occasionally discharging some of the capacitors used to store the DRAM data bits. These effects have been reduced today by using purer packaging materials, and employing error-correcting codes to detect and often correct DRAM errors.

Thomas Challenger Thomas Challenger