I’m continually impressed by space missions that last several times their original life expectancy - the Hubble was expected to last 10–15 years and yet here we are 30+ years later expecting at least that yet to come.
There is a good reason for that. In order to have an amalgamation of systems with design life of a given duration, every component must have a design life of much, much longer. When you get lucky, a system can go for a really long time relative to its design life.
Great examples of this are Spirit and Opportunity. The latter failed at ~61 times its specified design lifetime.
Having been a small part of some space-mission design and larger science collaborations, I can state with confidence that you really don't want your subcomponent to be the one that causes a failure.
And Voyager 1 and 2 are still zooming as far away from our solar system as they can get and happily sending back teeny bits of data every now and then. Pretty wild to think their voyages are now over 40 years old.
For a lot of stuff there's redundancy too— like rovers with six wheels but that can drive with any four, and the Curiosity and Perseverance missions having dual computers and dozens of cameras. I'm sure they run scenarios where significant chunks of the rover's systems fail and they figure out how to carry on anyway— thinking in part here of the famous HGA issues on Galileo:
People are scared and want to save their asses so they make super low estimates. That way it sounds really good in the media when things keep working.
I used to do the same when I did software development “I will be done with that in 2 days” then proceeded to build the thing in 2 hours chill 3-4 hours and hand it in earlier than expected.
Everyone was happy.
There is truth to this in aerospace: components will be qualified to the design life of the vehicle, but the qualification campaign is intentionally conservative to envelope all possibilities and uncertainty. When the actual conditions are more benign, more life can generally be expected.
Additionally as another case, Hubble has been continuously losing gyroscopes since it was first launched. Some or all gyros were replaced during most shuttle servicing missions. Seeing the writing on the wall for servicing missions post-Columbia, NASA developed software to operate with fewer gyros at a time, allowing for fewer to be spun up at any time, but importantly, allowing for the telescope to continue to operate after more than 3 of 6 gyros had failed. The key here is that engineers can often coax more performance out of a damaged (or otherwise limited) subsystem given the incentive- this has been the case in my aerospace experience, and seemed mirrored with Hubble too.
> People are scared and want to save their asses so they make super low estimates. That way it sounds really good in the media when things keep working.
I think that's an unnecessarily pessimistic take. Any system can have its life expectancy modeled over a range, e.g. a 99% chance it will survive 1 month but only a 50% chance it will survive to 1 year. When you're just giving one number to the media or other stakeholders, e.g. "expected life span", (or, for that matter, a single estimate to your boss), to be able to give a value with high confidence you will have to pick a number that is in the 99%-ish range, which means there is a good chance your system will survive much longer.
Underpromise. Overdeliver. You didn't have to do a crunch and the people waiting got something ahead of the deadline. That's my practice whenever possible as well :-)
As to your related point, I probably wouldn't use the term "scared." But, yes, there are a lot of incentives to meet/exceed goals and strong disincentives to fail by not meeting a somewhat arbitrary lifetime of a probe. Of course, some probes have clear primary objectives and you want to hit those but you may not want to "promise" you can hit a bunch of less important secondary objectives as well even if you think you probably can.
Well, that is one way to do it. I think it was necessary for HST because it was such a complex instrument that had to be capable of doing many things, and reliably -- because specifically you have to have a mirror of a certain size to make it worth it, and so it calls for a certain level of engineering reliability (and cost).
On the other end of the spectrum, the Mars programs (lately) were incentivized to do the opposite -- cheap, fast, and good, even if some of them fail.
Different circumstances may call for different incentives.
It proved Hubble right about the expansion of the universe, so all in all I think we can say with absolute certainty it was a smashing success.
Apparently the total cost for Hubble is on the order of 10 billion. That's just a fraction of what a tech company like Apple makes every quarter, so I think it was a totally reasonable expense relative to its discoveries.
And luckily we learned building giant, precise mirrors with 1980s technology maybe wasn't the best decision. The James Webb design is totally different and hopefully less prone to such problems.
A different mission, but Jake from We Martians did a really great twitter thread[0] about what goes in to the mission length. I'll summarise the thread here (mostly just quoting verbatim):
Firstly, this is about why solar powered Mars missions (specifically InSight) do not have solar panel cleaning devices, but it touches on how missions can be extended and what thinking goes into that.
Robotic missions to Mars are competed like any other; they have to "earn" their chunk of the budget by being effective science missions at effective prices. This causes a very normal tension between "do a lot" and "don't cost a lot".
InSight in particular is part of the Discovery program at NASA, which technically has a cost cap of around $500M. Program managers thus do everything they can to reduce the cost without sacrificing science. You want to avoid taking instruments off, for example.
One really effective way to reduce scope is to shorten missions. This has two big benefits:
1) Every year a mission doesn't operate is a year you don't have to pay salaries for those who operate it
2) Hardware on the spacecraft can be made cheaper if it doesn't have to last
That second point is really critical.
The testing program you have to put stuff through doesn't scale linearly. A mission that lasts twice as long costs more than twice as much to test. It spirals fast.
In InSight's case, they also saved money by reusing a spacecraft bus design from the Phoenix mission, which was also designed for a short mission - just 90 sols in fact.
See the similarity?
So all of these reasons (staying under cost cap, reducing operation cost, reducing testing costs and saving money by reusing spacecraft busses) combined to drive a decision that InSight's prime mission is just 1 Martian Year (about 2 Earth Years), which it completed in November.
So the short answer to "why not add solar panel cleaning devices" is that they don't need them!
By the time the dust takes down the spacecraft, it will have completed its mission. It's an element of disposability, though NASA probably wouldn't characterize it that way.
Now, often the vehicles last longer than their prime mission. Spirit and Opportunity, for example, blew past their prime mission of 90 sols. All of NASA orbiters (ODY, MRO, MAVEN) have gone past their prime mission, too. Last November, InSight also surpassed its prime mission.
When this happens, NASA reevaluates the vehicle's health & cost against further science it can do, and decides to either re-fund or end a mission.
InSight got the funding, and so here we are making the best of that!
NASA isn't scrambling to save a mission hampered by dust they forgot existed. They're doing their best to squeeze out more return on investment for the taxpayer on a spacecraft that has already completed its mission!
> So the short answer to "why not add solar panel cleaning devices" is that they don't need them!
> By the time the dust takes down the spacecraft, it will have completed its mission. It's an element of disposability, though NASA probably wouldn't characterize it that way.
I don't think that's a very compelling answer. The thing to ask, I guess, is whether they would have added them anyway if they were very cheap and lightweight.
If yes then "it was a waste of resources for the designed mission" hits much closer to the real reason. But if no, it means the real reason is "bureaucratic maneuvering caused a longevity problem on purpose" which is really bad.
Sounds like they didn’t do a good estimate of the lifespan. Grossly underestimating it to impress the general public is quite lame if you think about it. If you were 60 times off on your estimate that was a horrible estimate.
