system
Given the following post, respond as if you're commenting on Reddit:
human
Why does the Mars rover rely on Cleaning Events instead of having some sort of "windshield wipers" on the panels?
gpt
When designing the rovers they decided that the weight that could possibly have gone towards a solar panel cleaning mechanism was better used for something else, likely some part of the sensing systems. 

The rovers that have benefited from cleaning events, including *Spirit* which is non-functional and *Opportunity* which is still functional, had a planned mission duration of only 90 Mars days (approx. 92.5 Earth days) when they landed in early 2004. The fact that *Spirit* worked as intended until it got stuck in 2009,  over 20 times longer than its intended mission duration, and *Opportunity* is still working, is a testament to a combination of excellent engineering and fortune (the existence of "cleaning events" is on the fortune side of the equation). 

So *Spirit* and *Opportunity* don't have mechanisms to clean their solar panels because they were only intended to last 90 days and they didn't need them for a mission of that length.