Consider the three options:
- Source: NASA TM X-2525
- Source: NASA TM X-2525: Atlas-Centaur AC-17 Performance for Applications Technology Satellite ATS-D Mission
where in each case the linked url is less than self-explanatory
I tend to use option #3 because it provides the most information to our customers the readers, but that's just me. However I've probably used
[Source](url) before as well with Wikimedia urls and I'll have to curtail/undo that now.
I'd like to suggest that at least option #2 is much better for the answer's integrity and value to the site than option #1, based on the following reasoning:
Links rot in general, and NASA has at least in the past done major and minor revisions to it's websites and filing systems. I've experienced that myself, seeing swaths of links disappear or end in 404's, and over the last few years seen it referenced (lamented) more than once in other, more experienced users' comments.
Without at least the minimal
NASA TM X-2525 reference, if the link breaks all you have is the following cryptic dead url:
19720017275 may not be sufficient to repair the answer.