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Given that as all probes have solar panels attached to them why do they have to shut down at the last stage? I googled and found that dust sticks to panels, because of which, they cannot function properly.

But suppose a glass/any material covering was put around solar panels, would this be a more power-consuming task to clear the glass again so that again sunlight would pass to the panels? I know it kind of sounds stupid but I am not able to swallow that solar panels cannot function eternally for probes?

According to me at least they should function for 100 years :P

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    $\begingroup$ Nothing is eternal ;). $\endgroup$
    – Magic Octopus Urn
    Oct 22, 2019 at 20:53

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A glass cover isn't enough. The damage is caused by micrometeorite impacts. These are tiny particles that hit at high speeds (more than 1 km/s). You can't build solar panels to withstand that much energy.

Also, it's rare for a mission to end because the solar panels no longer provide enough power. Other factors end the mission long before the solar panels degrade that much:

  • component failure (especially mechanical components like reaction wheels)
  • fuel depletion
  • end of mission funding: science missions are usually designed to last a predefined period. Some missions are extended, on others the plug is pulled when the original mission ends. Commercial satellites are replaced on a schedule.
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  • $\begingroup$ Thanks, @Hobbes just a question For fuel depletion, why not just shut down the system and let solar panels get enough energy and after a year/month or something solar panel will generate significant energy turn it on now we don't need fuel power? just use solar power now and repeat this step till our solar panels are completely dead? $\endgroup$
    – Sudhanshu Gaur
    Oct 17, 2019 at 17:03
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    $\begingroup$ The fuel is used for attitude control. Without fuel, you can't point the solar panels at the Sun, can't point the instruments etc. Without fuel the satellite becomes useless. $\endgroup$
    – Hobbes
    Oct 17, 2019 at 17:07
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    $\begingroup$ @Sudhanshu fuel on spacecraft isn't really for energy; it's 'reaction mass', something to eject to push you. Exerting a force with just energy breaks newton's laws. $\endgroup$
    – 0xDBFB7
    Oct 17, 2019 at 19:13
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    $\begingroup$ It's possible to use weird effects like solar sails or photon engines to produce a tiny force, but in general these systems aren't yet practical. $\endgroup$
    – 0xDBFB7
    Oct 17, 2019 at 19:19
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    $\begingroup$ @SudhanshuGaur MMOD=MicroMeteoroid and Orbital Debris so natural and human-made "debris" in space. Story about hole on ISS thought to be by MMOD but actually drilled on the ground. $\endgroup$
    – astrosnapper
    Oct 18, 2019 at 1:02
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Not all spacecraft have solar cell arrays. Many sent to Jupiter and beyond use generators powered by radioactive decay because sunlight is very weak beyond Mars's orbit. Some spacecraft use full-blown nuclear reactors - such as various old Soviet spy satellites.

Your reference to "dust" makes me think you actually mean Mars rovers, because Mars (unlike space) is a very dusty environment. The Curiosity Mars rover is powered by radioactive decay and has no solar cells. The Opportunity and Spirit Mars rovers operated for years despite having no means of cleaning the dust off their solar cells. It turns out that Mars winds did a good job of keeping their cells reasonably clean (at least in the locations those rovers were deployed in). Contact was only lost with Opportunity after 14 years of operation in 2018 as sunlight was blocked by a Mars-wide dust storm. As the storm's dust was in the air, and not on the solar panels, cleaning apparatus would not have helped. In addition, Opportunity died because it could not keep its batteries and other systems warm during the storm - post-storm its solar cells may well have been working OK.

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    $\begingroup$ "In addition, Curiosity died because..."?? Shouldn't that be Opportunity? $\endgroup$
    – Vishnu
    Oct 18, 2019 at 12:26
  • $\begingroup$ Corrected, thanks. $\endgroup$
    – Ags1
    Oct 18, 2019 at 13:43

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