Is this a run-on sentence?
In 2014, researchers discovered forgotten satellite images of Earth from the early 1960s that, when compared with recent satellite photographs, reveal how humans have altered the face of the planet.

 - no
 - yes
This is not a run-on sentence. It is not formed from two sentences that have been run together without appropriate punctuation.
In 2014, researchers discovered forgotten satellite images of Earth from the early 1960 s that, when compared with recent satellite photographs, reveal how humans have altered the face of the planet.