Why not leave interplanetary probes on standby after primary mission, activating them at will decades later?

Question

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 Instruments becoming outdated?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.

Answer 1

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.

Answer 2

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.)

Answer 3

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...