What is a "white paper"?
Some funding agencies (e.g., DARPA, AFOSR) use the term "white paper" to mean a brief synopsis of a project idea; this is similar to NSF's "Letter of Intent".  White papers help funding agencies evaluate project ideas quickly, and provide feedback to the proposer about whether or not they're interested, without having the proposer go through the (considerable) hassle of preparing a complete proposal. 

Wikipedia shows other meanings of the term (link), but given the context my guess is that NASA is looking for a synopsis of your idea.  You might look at this DARPA solicitation for an idea of what sort of information an agency might want to see in a white paper (see Section 4, and note the length limit).