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.

Stem cells are undeveloped human cells that can grow into many or all of the different specialized cells in the human body. Although stem cells have been harvested from adults, the most versatile stem cells come from embryos. The main source of embryos is fertility clinics. Vitro fertilization, the process that produces test-tube babies, involves the fertilization of eggs to produce embryos outside the woman’s body.