I just want to relate a story of the Soviet space program today. There's no original thought here, but that's okay; it's a story worth hearing.
While the Viking program was zipping about Mars, sending back celebrated discoveries, the Russians were having a much harder time of things on Venus. I don't know why they decided to work on Venus, particularly, but I suspect the decision was as much one of pride as anything. Perhaps they didn't want to be upstaged in the implicit competition of simultaneous exploration of Mars; perhaps they wanted to prove they could handle a more difficult target.
The fact that nobody really knew what Venus was like until the Russians went and looked didn't make the target any easier. It took the destruction of Venera 1 through 5 just to learn what was killing the landers. Despite the comparable mass and orbit. Venus is not Earth's twin, as astronomers of the day guessed. Surface temperatures approaching 500°C, pressures near 100 atmospheres and an atmosphere of sulfuric acid play merry hell with electronics, and mechanical control systems are doomed to a painfully short lifespan. In theory, one could protect the gear with heavy plating and pack in an over-powered antenna to signal past it, but the practice of space exploration is way behind theory. Heavy plating and a huge power plant would add a lot of mass to the craft. Even small weight increases demand exorbitant fuel expenditures, due to the diminishing returns of thrust: fuel itself has mass, requiring even more fuel to lift it. So building a craft capable of surviving on Venus was impractical. Scientists were reduced to one-way trips onto the surface, hoping the craft would survive long enough to send back meaningful data.
When Venera 7 finally made it to the surface intact, Russian astronomers were surprised to discover that Venus seemed to have a viscous surface, at least where the probe was unfortunate enough to touch down. All the pictures had a peculiar, gloppy distortion. It took a while to realize the lens cap had melted onto the camera lens. Back to the drawing board. The new lens cap was made of a more heat-resistant substance, and given a mechanism to pop it off before it could gum up the works, just in case. Designers began to rely on redundancy and variety. If something went wrong with the camera, Venera 14 had an X-ray spectrometer. And a seismograph. And a mechanical arm to collect soil samples for on-board analysis.
The arm collected and analyzed the ejected lens cap. Venera 14 travelled tens of millions of miles to learn the electrical conductivity of heat-resistant plastic.
There's an object lesson here. The ways in which our space probes, removed from immediate human guidance and cut off from all hope of repair, can screw up are awesome in their variety. Engineers can plan for rough conditions, like acidic atmospheres, but how can they anticipate ridiculous difficulties like collecting the ship's own lens cap? The Challenger's O-ring, the Hubble reflector, the Apollo 13 air filtration, are single components in mind-bogglingly complex machines, individually crafted, which must be made perfect with one attempt. The wonder is not that so many space projects return unsatisfactory results, but that so few do. I lose patience with critics who lose patience with NASA.