It's probably public domain in USA due to their weird copyright laws that differate between year of creation, was the copyright assinged to company or to a natural person at start etc. I wonder what the situation is in EU: Herge died in 1983, it wasn't work-for-hire, and EU has the copyright set as life+70 years.
(I understand Mickey Mouse is different for being assinged to Disney Corp, and due to it being created in USA from where the copyright period carries to EU.)