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Thanks For the Memories: Touring the Awesome Random Access of Old (hackaday.com)
szczys writes: The RAM we use today is truly amazing in all respects: performance, reliability, price; all have been optimized to the point you can consider memory a solved problem. Equally fascinating is the meandering path that we've taken over the last half century to get here. Drums, tubes, mercury delay lines, dekatrons, and core memory. They're still as interesting as the day electrons first ran through their circuits. Perhaps most amazing is the cost and complexity, both of which make you wonder how they ever manage to be used in production machines. But here's the clincher: despite being difficult and costly to manufacture, they were all very reliable.
Alpha Particles from space do not penetrate the building that the computer is in, nor the computer case, nor the plastic package of the memory devices themselves.
Alpha particle bit errors are caused by alpha particle emissions within the memory cell itself, as there is a minute amount of radioactive material in all semiconductor devices, including memory.
However, radiation-induced bit errors are seldom actually caused by package alpha particle emissions. The more likely space-related culprit is neutron flux. It has been found that DRAM bit error rates increase dramatically with altitude, and that solar events increase the rates further.
Fun stuff.
The bit flips aren't due to neutrons, but to other high energy particles (cosmic rays).
And modern memory design tolerates this quite well (on chip EDAC, for instance).
But that's not the dominant source of errors any more. It's more things like electrical noise (signal integrity is another term). As you reduce the size of the device holding a single bit, you're starting to get down to where the thermal noise is a significant fraction of the "signal" (i.e. the presence or absence of charge in that bit storage).