Why are rock stars contributing to a soundtrack? Mostly for fun. This mission has inspired laughs and smiles since the day it was conceived (in addition to the very important and awesome science that it is engaged in). See this well-traveled, unofficial explainer for one example.
While we're thrilled NASA is promoting the idea of mission soundtracks now, there's one great artist who wasn't able to contribute anything to this landmark mission: the one that started the whole "space is cool" musical phenomenon in the 1960s. We'll let him play us out: