High above Earth’s protective atmosphere, spacecraft face a constant barrage of radiation from cosmic rays and solar storms. This space radiation can wreak havoc on onboard computers and electronics, causing anything from minor glitches to catastrophic system failures. While the effects of space radiation on satellites and spacecraft are well studied, few realize that similar radiation effects impact systems much closer to home – even computer systems on the ground.
Space radiation primarily consists of energetic particles such as protons, electrons, and heavy ions. These particles carry enough energy to penetrate deep inside microelectronic components, where they can alter the state of transistors and memory cells. A single particle strike can flip a 0 to a 1 or vice versa, corrupting data stored in memory or even changing a processor’s instructions. These “single event upsets” or SEUs are a major reliability issue for satellites, where they can cause anything from data corruption to loss of orientation to complete system shutdown.
What is less appreciated is that similar SEUs occur in ground-based computers due to “atmospheric radiation.” The Earth’s atmosphere shields us from most space radiation, but secondary particles created by cosmic ray interactions in the upper atmosphere have enough energy to reach sea level. Neutrons make up the majority of this atmospheric radiation. Though uncharged, neutron collisions in microelectronic components can still upset digital states. Atmospheric radiation is hundreds of times less intense than in space, but with microelectronic components packing more transistors into smaller volumes, even sea-level computers exhibit occasional errors due to cosmic rays.
The first evidence that atmospheric radiation could impact computers came in 1979 from tests at IBM. Researchers discovered that special error checking RAM showed periodic bit errors, even though it was stored in a shielded, temperature-controlled vault. The bit error rate increased by an order of magnitude between sea level and 10,000 feet altitude, implicating atmospheric radiation as the culprit. Subsequent tests confirmed that cosmic ray secondary neutrons were responsible.
Researchers now know that nearly every memory chip or microprocessor will experience occasional SEUs due to atmospheric radiation. The average desktop computer likely encounters one cosmic ray-induced error per 256 MB of RAM per month at sea level. The rate is 4-5 times higher in Denver and can be 30 times higher on commercial aircraft. Servers in data centers exhibit higher SEU rates due to greater total memory capacity. Though rare, the billions of computers in use ensures SEUs from atmospheric radiation occur frequently overall.
In most cases, the random bit flips caused by atmospheric radiation do not pose a serious problem for computers. Desktops, laptops, and even servers are designed with multiple layers of redundancy and error checking that prevent minor data corruption from causing crashes or failures. In server farms, redundant servers run parallel computations, allowing comparisons between results to eliminate errors.
As microelectronic components continue to shrink in size, their vulnerability to radiation upsets increases. Chip designers now incorporate redundancy and error checking similar to larger computer systems to mitigate SEU issues. However, atmospheric radiation may prove to be an ultimate limit to the reliability and stability of microelectronic systems. Already, researchers developing systems for high-reliability applications like aerospace engineering or server farms must specifically design systems to account for cosmic ray effects.
So while computer users need not worry about atmospheric radiation on a day-to-day basis, cosmic rays still impact ground-based electronics in subtle but important ways. As engineers design smaller, faster, and more powerful microelectronic systems, accounting for these random errors from space becomes ever more critical. In a sense, atmospheric radiation provides a glimpse into the future – where reliability of global computing infrastructure depends on mitigating the effects of single particles from across the galaxy.
