Ich würde gerne ein Autogramm vom Astronauten Alexander Gerst (bzw. von anderen ESA - Astronauten) bekommen? An welche Autogrammadresse kann ich schreiben?

Frag' die ESA: 

"How can I get an astronaut autograph?

To request an ESA astronaut autograph, send a return envelope (minimum size 10.5 x 15 cm) with your address on it and the astronaut whose autograph you wish to receive to

EAC

Building 12 - reception

Linder Höhe
51147 Köln
GERMANY

If you live in Germany include a stamp with sufficient postage on the return envelope. Outside of Germany include an international reply coupon (coupon-réponse-international)." 

http://www.esa.int/About_Us/EAC/Frequently_Asked_Questions_European_Astronaut_Centre/(print)

Versuche es mal unter folgender Adresse: 

European Space Agency

European Astronaut Centre

P.O. Box 90 60 96

D-51127 Koln

Germany