The late great British engineer and test pilot Geoffery deHavilland (sp) dictated "Simplificate and build in lightness." The late Bill Lear implied something similar when he told his engineers when they were designed the first LearJet, "If you don't need it, then you won't have to build a backup for it."

Spacecraft have to be built as light as possible, with structures just strong enough to withstand launch forces. Redundancy can add to reliability but at the expense of extra weight and cost. Budgets and payload margins are always a factor. Developing new technology can add capability and cut weight, but R&D isn't cheap. Everything becomes a compromise and tradeoff. Like Scotty said, "the more complicated it is, the easier it is to break."

Some spacecraft like the Voyagers, Galileo, and Cassini were built hell-bent for stout, but that resulted in multi-billion dollar vehicles that took very large (and expensive) launchers. Sometimes, even massively expensive space vehicles are crippled or destroyed by seemingly simple things and the US has certainly had our share of failures. Galileo's total scientific return was seriously reduced when one of the simpliest things (the high gain antenna) failed to deploy properly. Back in the early 1990s, the billion dollar Mars Observer was lost shortly before it was to perform an orbital insertion burn. It's believed the reason was that an off-the-shelf propellant valve failed. It had been designed for use in Earth orbit and may have failed in the colder Martian space environment. Mars Polar Lander was a much less expensive mission but it failed, most likely due to a problem with a microswitch and software during landing. Mars Climate Observer failed due to a navigation error caused by the failure to properly account for brief thruster firings during momentum wheel unloads (the infamous metric conversion error).

The Japanese accomplished some wonderful things with Hyabusa. It appears the mission won't be fully successful but let's celebrate the data they returned. The only people who never fail are the ones who never try. Kudos to Japan for trying.