NASA is very failure adverse and tends to over-engineer hardware. These interplanetary probes almost always live way beyond their designed life expectency. Also, the "designed lifed expectency" is really just a dodge to get around the bean counters. If they claim a short life span then more preallocated money can go into hardware rather than a mission operations budget. After the vehicle is on site or orbit, they can go back and ask for more money. This tactic can sometimes lead to ludicrous situations. The Pioneer 12 spacecraft orbiting Venus ran out of money. It was still beaming back useful science but no one was listening. They assigned one civil servant to look at its telemetry once in a while just to see if it was still there. The same was true with Pioneer 10 and 11. Those vehicles lasted years beyond their original life spans. They would check up on them once every few months. Having only one guy watch over a spacecraft can be a dodgy business. Supposably one of the Viking landers was killed by accident when the guy looking after it sent up a bad command sequence. It'll be interesting to see what they do with Cassini. Cassini is built like a battleship and will orbit Saturn which is a very interesting planet. Cassini could conceivably produce useful data for half a century. The bean counters will NOT be pleased.

Gary