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Imagine a spacecraft which orbits or follows an object like Dawn, Rosetta and Cassini do today, not being decommissioned and deliberately crashed, but instead put in hibernation. With enough fuel and other consumables left in store for this purpose: To be reactivated for a while whenever useful. For example as the seasons vary or when the next spacecraft approaches, or as has happened on Jupiter and the Sun when a comet strikes. Imagine reactivating the Galileo orbiter to have a look at those European plumes observed from Earth.

Are very long term intermittently activated space probes feasible?

Meaning that they could work on and off during many decades. JPL already designs some die hard missions. But it seems almost unintentional, with 3 years or so planned lifetimes, but still working a decade and more later. Why not plan for multi-decadal probes, to have every orbiter and lander of object "X" pile up with time, each with its specialized instruments activated briefly as needed. Especially for minisats with limited resources.

What would be the problems with this kind of concept? Long term cosmic radiation exposure? Storage of the consumables? Cost of Earth management? Instruments becoming outdated? Could the current mode of "throw-away" space missions be replaced with "sustainable" dito?

I completely stole and vandalised this question from Robert Inventor asking it here on the Space Show blog, to give credit to where it belongs.

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It has been attempted, but is very difficult. The ISEE-3 mission was put into a hibernation mode where it made a close Earth approach last year. When that close approach happened, it was determined that the spacecraft's thruster did not work, which resulted in a failure to anticipate.

More practically, such missions are not usually attempted because hibernation is tough. Getting the location of the spacecraft is difficult, as they tend to drift with time. The electronics degrade, as does the propulsion. Things get more efficient in operations with a continuing operation, as does getting science out of the mission. It's more practical to get as much science out of the mission as can be done.

Bottom line, putting a satellite in hibernate that is at a potentially interesting science objective isn't very useful. Hibernate is only done between interesting targets. A mission is only destroyed at the end of the life if it doesn't have enough fuel to continue a useful mission beyond the useful lifetime of the mission.

In a more practical sense, how many people know how to use a slide rule today? Or paper tape, or punch cards? We don't know what technology will be like in 20 years, using that technology could become quite tricky. Better technology comes out all the time as well, making it more useful to send updated missions in 20 years if required.

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  • $\begingroup$ ISEE was sooo 1970s. Oops, that's what everyone will say about us in 2050! :) But the Voyager probes have done good science with their outdated and degraded instruments decades overtime. The ground team still seems to know how to use the slide rules by which they were constructed. There's nothing else out there. Space travel takes time, maybe we should get used to multi-decadal missions, since it seems as if we are already finding ourselves improvising in that situation anyway, as an afterthought. $\endgroup$ – LocalFluff Dec 10 '15 at 14:12
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    $\begingroup$ ISEE-3 was sent a shutdown signal which it failed/refused to complete. $\endgroup$ – SF. Dec 10 '15 at 18:19
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Spacecraft are designed for a mission: "We want to measure X, Y and Z". The spacecraft is designed to fulfill this objective. For many missions, once you've completed that, the mission is over and there's little point in doing it again. E.g. if the objective is to provide a surface map, photographing the entire surface once is enough, doing it again won't yield new information in many cases (unless the planet has weather or active geology). The exceptions are e.g. the Voyagers, which continue to discover new territory.

Many spacecraft are in unstable orbits. Missions like Cassini and Rosetta require the spacecraft to correct its course at regular intervals, or it's going to drift away (Rosetta) or crash somewhere (Cassini). So a decades-long mission requires decades worth of fuel and constant monitoring.

Then there's the logistics and funding. If you hibernate a spacecraft for a decade, you need to find something else to do for the science team for that decade, and then be sure the team has something to come back to. I.e. you need continued funding to keep the operations center intact, to keep receivers around for the next time you activate the spacecraft. You need a core team to continue convincing your political masters that funcing this mission remains a good idea, and that is hard when there have been no new results for years because the spacecraft is in hibernation.

The spacecraft becomes more expensive (an RTG that can power all the instruments for 30 years is much larger than one that only needs to work for 5 years, for example.)

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    $\begingroup$ That is of course all true for today's missions. But the question is about from the beginning designing a mission for intermittent hibernation during many decades, activated briefly as needed. After its primary mission has been accomplished. Placed in stable orbits. With appropriate power budget. After the primary mission of, say, surface mapping is over, there are other long-term benefits of having an orbiter out there. The Jupiter system has several examples in the last decade (comet, plume, red spot shrinking). The operating team's salaries don't compare to a billion dollar mission. $\endgroup$ – LocalFluff Dec 10 '15 at 14:56
  • $\begingroup$ @LocalFluff: Energy budget for that is expensive; NASA is scrapping the bottom of the barrel for RTG batteries. OTOH I believe sending the "shutdown" signal is a pure waste. Build it for the primary mission, then let it free. If it survives, good. If it dies, no big loss. $\endgroup$ – SF. Dec 10 '15 at 18:23
  • $\begingroup$ @SF. And as ISEE-3 demonstrated, others are willing to take over even if the original financing goes away. $\endgroup$ – LocalFluff Dec 10 '15 at 19:26
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The primary issue is that most missions, such as Cassini, are not particularly useful once consumables such as propellant or RTG lifetime run out. Adding consumables that enable use after the mission extensions are over means redesigning the mission, and in that case you might as well make the main mission longer if that was feasible in the first place...

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  • $\begingroup$ A RTG for a much longer lifetime should use an isotope with a much longer half-life than Plutonium 238 with 87.7 years. Americum 241 has 432.2 years, but the power density is only about 1/5. A RTG with Am 241 would be much heavier than a RTG using Pu 238 with the same initial power. $\endgroup$ – Uwe May 27 '17 at 9:12
  • $\begingroup$ @Uwe exactly. It's possible to design missions that last long much longer, but if the significant tradeoffs are justifiable, then the mission design is going to incorporate that in the primary mission. $\endgroup$ – Deimophobia May 27 '17 at 21:22
  • $\begingroup$ @Uwe, the isotope isn't the problem with the RTG. The problem is that the thermoelectric generator that turns the RTG's heat into electricity is destroyed by the RTG's radioactivity. $\endgroup$ – Mark Jan 21 at 6:25

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