Is this a run-on sentence?
In 2004, the Cassini spacecraft arrived on Saturn and began photographing Saturn's moon Titan, revealing a number of lakes, rivers, and seas on the moon's frozen surface.
A: no
B: yes
This is not a run-on sentence. It is not formed from two sentences that have been run together without appropriate punctuation.
In 2004, the Cassini spacecraft arrived on Saturn and began photographing Saturn's moon Titan, revealing a number of lakes, rivers, and seas on the moon's frozen surface.