The residue of fuel in the tanks of an upper stage booster needs to be dumped for safety reasons. The liquid oxygen and liquid methane will be heated by the sun, over time, and that will turn the liquid to gas that will create an immense pressure. If this were to occur, a violent explosion would put thousands of pieces of space junk out there that would be a threat to satelites.
Also, when the spent upper stage eventually falls back to earth, it will not be carrying hazardous fuel back to the ground.
The liquid fuel, when dumped, quickly froze into this vapor trail we see in the photo.
Thank you for asking, Esther.
—Nature Friend
