The first known fault-tolerant computer was SAPO, built in 1951 in Czechoslovakia by Antonín Svoboda. Its basic design was magnetic drums connected via relays, with a voting method of memory error detection (triple modular redundancy). Several other machines were developed along this line, mostly for military use. Eventually, they separated into three distinct categories: machines that would last a long time without any maintenance, such as the ones used on NASA space probes and satellites; computers that were very dependable but required constant monitoring, such as those used to monitor and control nuclear power plants or supercollider experiments; and finally, computers with a high amount of runtime which would be under heavy use, such as many of the supercomputers used by insurance companies for their probability monitoring.