@book { Weltraumtourismus,
    title = "Weltraumtourismus",
    author = "Romy {Freitag}",
    year = "2009",
    publisher = "Diplom.de",
    address = "Hamburg, Deutschland",
    doi = "10.3239/9783836628808",
    url = "https://m.diplom.de/document/226739"
}