In four years of reading Cassini updates, they have never once mentioned worrying about software errors due to cosmic rays. And this is without any atmospheric protection at all; we have 100 miles of atmosphere.
They have mentioned that their solid state relays trip 2 or 3 times a year due to cosmic rays. I'm not sure how comparable those are to DIMMs, but it does suggest that the author's claim of one error per day is a bit off...
Pretty much anything launched into space uses radiation-hardened electronics. I don't know what Cassini has on board, but this is popular: http://en.wikipedia.org/wiki/IBM_RAD6000
They have mentioned that their solid state relays trip 2 or 3 times a year due to cosmic rays. I'm not sure how comparable those are to DIMMs, but it does suggest that the author's claim of one error per day is a bit off...