Hope did some research in a college library and discovered the 1967 Outer Space Treaty, a pact in which dozens of nations, including the United States, laid out the basic legal guidelines for dealing with celestial bodies. Hope thought he saw a loophole: The treaty declares that no nation can assert sovereignty over the moon, but it fails to say clearly that individuals can’t.

So Hope sent a note to the United Nations, laying claim to the moon as well as the other planets and moons in the solar system, and went to work. In the years since, Hope has made a tidy fortune selling deeds to plots on the moon and other celestial bodies; he estimates around $12 million so far. A typical moon acre costs $24.99. The whole of Pluto is going for $250,000 — a good deal, but a tough commute.

Even if you don’t seriously credit the idea that one man in Rio Vista, California, is sole proprietor of the moon—and most experts don’t—Hope’s quixotic career attests to the legal vacuum surrounding real estate that doesn’t happen to be located on this planet. And as humans prepare to return to the moon, with a dozen or more nations and private firms eyeing the lunar surface for everything from mining to scientific research, the arguments over ownership are expected to become only more complicated.