It changed! I assume the change came from today’s update. The mission for pin #5 is:

Here are the final results:

#2: Galileo at 20%
#3: Rosetta at 17%
#4: Viking at 13%
#5: Spirit & Oppy at 10%
#6: Pioneer at 9%
#7: Juno at 4%
#8: Venera at 4%
#9: Messenger at 2%

Okay. Nobody expected Messenger to win, but Juno at 4%?! Wow.

Next update: New Horizons Pin Art