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.

An index is a statistical composite representing the value of all stocks in a certain market segment. It’s presented as one simple number so investors can track trends in that segment.The broader the index, the more representative it is of the entire stock market. The Dow Jones Industrial is an average of 30 of the most widely held stocks. The S&P 500 Index combines the prices of 500 widely held stocks chosen by Standard and Poor’s. The Russell 2000 includes 2000 small-company stocks; the Wilshire 5000 includes both small- and large-company stocks and is weighted to represent all U.S.-headquartered securities.