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This question may be a little vague, so I will explain what I mean by this. Most major NASA missions have had a "Mission Life Expectancy". But I've found 8 times out of 10, the spacecraft operated longer than the expectancy (most times by a factor of 2 or more). Here's some examples:

Opportunity: 90-100 days (lasted: 14 years 136 days)

Spitzer: 2.5 years (Lasted: 16 years)

Curiosity: 1 year (10 years, still going)

Hubble: 15 years (30+ still going)

Voyager 1 and 2: 5 years (40+ stunningly, still going!)

New Horizons: 9 years (16 still going)

Sojourner: 7 days (lasted 83 days.)

See what I mean?

This is not a duplicate because I asked WHY they underestimate, not HOW the spacecraft themselves last so long.

Deko Revinio
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    It's the Mr. Scott school of estimation. How else can they maintain their reputation as miracle workers? – GdD Nov 02 '22 at 17:51
  • @GdD but what bothers me is that HUGE margin! – Deko Revinio Nov 02 '22 at 17:52
  • ah I see it now – Deko Revinio Nov 02 '22 at 17:52
  • Surely they have to put some sort of an "expected program life" on it for budgetary purposes? NASA doesn't have open ended funding, it cant say "we will launch this and you pay until it dies", the program has to have funding for a minimum period of time or series of experiments... On that basis, funding will be issued. Anything after that is a bonus and funding depends on how useful the asset is - in all of the above, the additional time was very useful for both science and PR purposes, so they got additional funding. – Moo Nov 02 '22 at 17:58
  • The Spitzer ran out of liquid helium about when it was expected to. The Spitzer Warm Mission ran on for another decade, but the types of imagery returned were not up to par with the primary mission. The Voyagers are running on extremely reduced capabilities. The cameras were turned off decades ago. Robots have all kinds of infant death problems. Think of incandescent light bulbs. Sometimes they fail the very first time you turn them, sometimes the tenth, and other times, the 1000th. Those first few failures are the infant death part of the bathtub failure curve. – David Hammen Nov 02 '22 at 18:59
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    A somewhat simplistic answer would be: if you build something to work for a month with 99.999% probability, then it will also work for a year with 99.9% probability and a decade with 90% probability. (Numbers pulled out of my rectum, but you get the drift.) – Jörg W Mittag Nov 03 '22 at 06:39

2 Answers2

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It's better to think of those as a mission lifetime guarantee to the President, to Congress, and to the American public as opposed to thinking of them as a mission lifetime expectancy. The vehicle has to be designed to do much better than that to meet that guaranteed lifetime. Things do go bump in the night (and even in the broad daylight) in space.

David Hammen
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"Life expectancy" can be interpreted a few ways, and I cannot find details on how NASA defines it. In humans, life expectancy is generally defined as an average age of death, which includes mortality from all causes at all ages. A space probe life expectancy calculated this way would include possibilities of the probe exploding on the launch pad or any other failures very early in the mission. So long as you avoid those very early failures (which are likely low probability and would not be well represented in a small sample size), your life expectancy now exceeds the overall life expectancy. Basically, a probe that makes it to Mars is likely to exceed its expected lifespan, since it has already survived through part of its life.

Another possibility is that this isn't a life expectancy in the traditional "mean age of death" sense, but a design expectancy. A design expectancy could have a much higher threshold of confidence for meeting a particular goal. It might not make much sense to plan a 90-day mission for Opportunity if the designers did not have high confidence that Opportunity would actually survive 90 days. In a traditional life expectancy calculation, you might find that roughly half the samples fail before the life expectancy time. I'm not convinced that NASA would launch a mission expecting a roughly 50% chance that the mission will fail before achieving its planned goals. A design expectancy could have a higher threshold, meaning that most missions should indeed survive to their design goal and beyond.

Ultimately, it might come down to exactly what is meant when discussing "life expectancy", and whether that's the traditional statistical usage (mean time to failure is 90 days) or a more colloquial/design usage (we expect 90 days of failure-free operation with high confidence). If it's intended to represent mean time of failure, it may indeed be an overestimate. If it's intended to represent a time before which failure is very unlikely, it may not be. I lean toward the latter interpretation - in a scenario with a long tail of survival times, more than 50% of samples will fail before the mean failure time. It would be rather poor PR for missions to fail to meet their expected lifespan more than half the time. I would interpret these as design expectancies, meaning it should be highly likely for missions to survive beyond the "expected" time.

Nuclear Hoagie
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    It's the latter, high confidence of so many days of failure free operation. I would say it's not just high confidence but rather extremely high confidence. Failure is not an option. Heads can roll (figuratively) if that minimum guarantee is not met. Congress has become risk averse with regard to where it sends money. – David Hammen Nov 02 '22 at 18:49