g out of the Earth’s gravitational grasp.Cost is a rough proxy for difficulty, and so we can gain some insight into the challenge by noting that Virgin Galactic is currently charging $200,000 per person for what will be just a few hours of sub-orbital space flight.Of course, Virgin Galactic is a private company that does not have the resources of the federal government, but then, with the Space Shuttle having recently flown its last mission, the United States currently has no operational means of transporting people into space.Gingrich pretty clearly meant to signal that he would like to change that state of affairs, but he has not said that he thinks the government should get back into the business of launching humans into space.Nor has he said how, as a tax-cutter, he would pay for colonization.In his speech, and later, during Thursday’s debate, Gingrich merely offered that government could provide financial incentives for private space-exploration firms.None of the above is to say that colonizing the Moon is technologically impossible.But at the very least it is a large and costly challenge.When NASA most recently addressed the issue, it proposed building an outpost to be occupied for six-month periods, not a whole self-sustaining colony.And even that much more limited idea was subject to withering criticism.Legal Obstacles to Colonizing the MoonSuppose that scientists and engineers were to rapidly overcome the technological obstacles to colonizing the Moon.Still, there remains an important legal obstacle.Some Moon-colonization enthusiasts have noted that the United States is not a signatory to the U.N. Ag