Why have the Americans developed space shuttle according to the passage?

Ever since the Apollo moon landings, the idea of human expeditions to the Mars has come up repeatedly as the kind of grand adventures that would give focus, purpose, and inspirational lift to the space program. But confronted with the cost — estimated at $ 150 billion last week — presidents starting with Richard Nixon and others have flinched and resorted to halfway measures like the shuttle.

It’s amazing what you can see when you don’t look. Astronomers often practice a technique known as averted vision to see very faint things. By looking to the side rather than straight on, they avoid using receptors in the eye’s center, which are more sensitive to dim light. So canny stargazers try to see the wonders of the cosmos out of the corners of their eyes.