Their budget is determined by the elected representatives in congress, and influenced by the executive branch.

As a percentage of the national budget, NASA's funding is half of what it was twenty years ago (and dropping), and 1/9th of the peak 20th century value. There is a historical precedent for NASA receiving a much greater degree of federal funding, which would certainly increase the number of active projects and possibly increase the rate at which we gain an interplanetary foothold.