Why A Lot Of People Have A Problem Believing NASA

A space dilemma: extend missions or start afresh?, Nature

"Possible extensions are not mentioned in the initial mission planning stages, so designing missions to last longer than planned is an easy way for mission scientists to buy into future budgets. There's also the public-relations value — in terms of public image, it is better to say a mission will last for five years and then extend its life, than to say it will last for ten years and lose it after nine. The latter scenario would be seen as a failure, says Lori Garver, a former NASA associate administrator. She sees no problem with deliberately underestimating the life of a spacecraft. By the time a mission is ready to be extended, the big pots of money have already been spent on it, she says."