When Spirit and Opportunity landed in 2004, I was with the science team in charge of a group of high-school students called the Red Rover Goes to Mars Student Astronauts. We're coming up on the 10th anniversary of the landings -- what have those "kids" grown up into?
Waaaay back when Spirit and Opportunity landed on Mars in January 2004, the Planetary Society helped the public participate in the missions with a number of projects, including one where we printed "secret codes" around the edges of the two names-bearing DVDs that were bolted to the Mars Exploration Rover landers.
The Japanese space agency's science missions have an abundance of names. They start out with a programmatic name, like MUSES-A, PLANET-A, etc. -- which might be like calling NEAR "Discovery-A" and Mars Pathfinder "Discovery-B" and so on.