In space, the moon was our closest neighbor. There was no water on the moon. If you want a drink, you must bring it with you. There was no atmosphere. You cannot breathe unless you bring your air with you. And there was no protection against radiation unless you bring those shields with you and stay always within their protection. Nothing grows there. If you plant something, it won’t grow because it gets too hot in the sunshine and too cold in the shade. If temperature could be controlled, it was not known that moon dust would provide nutrients capable of producing a nutritious plant like it does here. There was nothing to eat unless you bring it with you. There were no animals, not even insects. You cannot live there unless you bring everything you need with you. When everything you brought was used up, then you must go home. Such a place is a desert. The moon is a desert in space.
The nearby planet, Mars, also was a desert in space. Like the moon, if you want to live there then you must take with you all the air, water and food that you will need. And you must always protect yourself against radiation. When everything you brought was 18 months before being used up, then you must go home. But now going home will take 18 months if you took with you enough air, water, food, radiation shielding, and rocket power for the return trip. Mars was a desert in space, but much more remote than the moon and much more difficult to colonize. As difficult as the repair of planet, Earth, may be, it would be orders of magnitude easier than building a colony in space. Restore planet, Earth. There seemed no practical alternative.