nice pic with the three rovers. i knew curiosity is bigger, but i didn't realize the size difference between the other two. i am curious as to why they went with a reactor rather than solar? was it strictly a weight/size issue?
Mars is marginal for Solar Power even under ideal circumstances, like... in space in Mars orbit where you can get sunlight 24/7. On the surface where it's night half the time, and there's also Martian dust which gets on the panels too. Plus, you can't drive or explore and guarantee the panels are even close to their ideal angle for the sun. And like Earth, Mars has seasons too, so there's a winter (about twice as long) where that hemisphere is getting sunlight at an even lower angle. The earlier solar rovers even got "saved" once or twice by a lucky wind storm (such as it is, about the half a percent the pressure as Earth sea-level..) cleaning the dust off the panels allowing them to get more work out of the rover.
The inverse square law by which light and electromagnetic radiation is a bitch. Mars is half again as far out from the sun as Earth is.
The other problem is the extreme cold. It affects science instruments and electronics, motors etc. and trying to warm things up with solar and batteries (which also take a hit from the cold) is a huge power drain. Or you just don't do it at all. The nuclear RTG allows for a lot more power, and a reliable source of heat to keep things running better. Probes operating around Mars and further often have Plutonium pellets that are not part of the main RTG secreted about their structure so waste heat can transmit through it's mass and keep things just warm enough so they don't seize up.
