What they are promised will determine what the mission must “prove”.
If you approach a donor and say something to the effect of “Will you give us money to try to go to Mars? It will benefit all mankind. Can you help?” You could do pretty much anything with whatever funds you raised. You could claim success for almost anything, and cite “funding” as your limiting factor.
If you approach an investor and say something such as, “We are looking to create new materials and technologies for the aerospace market. We plan to demonstrate these new technologies with a highly publicized manned Mars flyby. We anticipate opening markets of $XXXX over YY years. We also expect $ZZZZ in revenue from mission television rights.” Now you are sort of on the hook to produce, and meet tangible, measurable goals. Frankly, this is the approach I favor. Investors bring discipline to ventures, where donations can be more easily wasted.
Right.