Just logged N747NA (NASA) aircraft over Durham.
The aircraft is operating from CGN airport several times until mid of March for observation flights.
See the news here (in German)
Boeing 747 SP Sofia: Fliegende Sternwarte startet 20 Mal in Köln | aeroTELEGRAPH
Many thanks Foxhunter, it’s an interesting article and something I didn’t know.