Ten @ESAGaia #SpacePostcards with #GaiaStamp are ready to be sent to YOU! Just answer this: precisely how many stars are there in the first Gaia sky map? First 10 correct replies win! (We will ask you to DM your address). Clue 🔎 https://t.co/J0fURpyt7A pic.twitter.com/Fbz0B2qFF6

— ESA Science (@esascience) December 7, 2017