Roger Launius has an essay on the decline of significance of the metaphor with regard to space:

The image of the frontier, however, has been a less and less acceptable and effective metaphor as the twentieth century became the twenty-first century. Progressives have come to view the space program from a quite different perspective. To the extent that space represents a new frontier, it conjures up images of commercial exploitation and the subjugation of oppressed peoples. Implemented through a large aerospace industry, in their view, it appears to create the sort of governmental-corporate complexes of which liberals are increasingly wary.

Despite the promise that the Space Shuttle, like jet aircraft, would make space flight accessible to the “common man,” space travel remains the province of a favored few, perpetuating inequalities rather than leveling differences. They also assert that space exploration has also remained largely a male frontier, with room for few minorities.

In the eyes of progressives, space perpetuates the inequities that they have increasingly sought to abolish on Earth. As a consequence, it is not viewed favorably by those caught up in what political scientist Aaron Wildavsky has characterized as “the rise of radical egalitarianism.” The advent of this liberal philosophy coincides with the shift in ideological positions on the U.S. space program in the late 1960s and early 1970s.

Frankly, I don’t give a damn what regressives think about space any more than I do about their thoughts on any other subject. Dr. Launius does attempt to defend the metaphor, though, at the end:

I would like to suggest that the frontier myth is an incomplete but uniquely understandable way of looking at the space program. From the beginning of the space age the U.S. effort has been motivated by essentially three priorities. The first was Cold War rivalries with the Soviet Union and the desire to demonstrate the technological superiority of a democratic state over a communist dictatorship. The second was the lure of discovery of the unknown. The third was adventure. The first priority, oriented toward national security, has ceased to be important in this post-Cold War era. But the second and third priorities lie at the heart of the frontier myth and are still just as attractive as they were more than 40 years ago at the creation of NASA.

He misses a key priority, though, that is quintessentially American: liberty.