Most likely, this is the best-known picture of a flag ever taken: Buzz Aldrin standing next to the first U.S. flag planted on the Moon. For those who knew their world history, it also rang some alarm bells. Only less than a century ago, back on Earth, planting a national flag in another part of the world still amounted to claiming that territory for the fatherland. Did the Stars and Stripes on the moon signify the establishment of an American colony?

When people hear for the first time that I am a lawyer practicing and teaching something called “space law,” the question they ask most frequently, often with a big smile or a twinkle in the eye, is: “So tell me, who owns the moon?”

Of course, claiming new national territories had been very much a European habit, applied to non-European parts of the world. In particular the Portuguese, the Spanish, the Dutch, the French and the English created huge colonial empires. But while their attitude was very Europe-centric, the legal notion that planting a flag was an act of establishing sovereignty quickly stuck and became accepted worldwide as part and parcel of the law of nations.