Spacecraft computers must keep working far from repair shops, often for years in harsh radiation environments. High-energy particles from the Sun, cosmic rays, and trapped radiation belts can strike tiny electronic circuits and change stored data or damage components. Radiation-hardened computers are designed to survive these hits while controlling navigation, communication, life support, instruments, and power.
They matter because one flipped bit in the wrong command can threaten an entire mission.
Key Facts
- Radiation dose measures absorbed energy per mass: 1 Gy = 1 J/kg.
- A single event upset can flip a memory bit without permanently damaging the chip.
- Triple modular redundancy compares three processors or circuits and uses majority voting to choose the correct result.
- Error correction codes can detect and correct some memory errors: stored data = information bits + check bits.
- Shielding reduces particle flux, but it cannot block all high-energy cosmic rays.
- Reliability improves with redundancy when independent units are used: P(success) = 1 - P(all units fail).
Vocabulary
- Radiation-hardened computer
- A computer built with special materials, circuit designs, shielding, and software to keep operating in high-radiation environments.
- Single event upset
- A temporary error caused when one energetic particle changes a stored bit or logic state in an electronic circuit.
- Total ionizing dose
- The accumulated radiation energy absorbed by electronics over time, which can slowly degrade semiconductor performance.
- Triple modular redundancy
- A fault-tolerant design that runs the same task on three units and accepts the answer that at least two units agree on.
- Error correction code
- A method of adding extra check bits to data so a computer can detect and often correct bit errors.
Common Mistakes to Avoid
- Assuming shielding alone solves the problem is wrong because very energetic particles can pass through shielding or create secondary particles inside it.
- Treating all radiation errors as permanent damage is wrong because many single event upsets are temporary bit flips that can be corrected by memory scrubbing or rebooting.
- Using faster commercial chips without considering radiation tolerance is wrong because smaller, denser circuits are often more sensitive to particle strikes.
- Forgetting to separate redundant units is wrong because one radiation event, power fault, or thermal problem can affect nearby components at the same time.
Practice Questions
- 1 A memory system stores 8.0 x 10^9 bits and experiences an average upset rate of 2.0 x 10^-12 upsets per bit per day. How many bit upsets are expected in 30 days?
- 2 A spacecraft computer uses three independent processors. Each processor has a 0.02 probability of giving a wrong result during one operation. Assuming independent failures, what is the probability that triple modular redundancy gives a wrong majority result?
- 3 A mission team can choose either a very fast commercial processor with little radiation protection or a slower radiation-hardened processor with error correction and redundancy. Explain which choice is usually better for a deep-space mission and why.