Sixty years ago today, May 25, 1961, President John F. Kennedy appeared before Congress and argued America “Should commit itself to achieving the goal, before this decade is out, of landing a man on the Moon and returning him safely to the Earth." The goal was achieved, but it is 49 years since the last time any human walked upon the Moon, or indeed any other non-earthly natural body, raising the question of why spaceflight's big dreams have not been realized yet.
The first part of the answer is to recognize that visiting the Moon wasn't an inevitable development, at least in the 20th century. Polling indicated most Americans opposed the idea, despite the president's sky-high approval ratings. Congress was far from convinced to release the funds required to make Kennedy's vision a reality.
Kennedy's even more famous speech more than a year later, including the immortal lines “We choose to go to the Moon in this decade and do the other things, not because they are easy, but because they are hard,” was needed to change the mood.
After his assassination, continuing Kennedy's mission became a way of honoring him, but once the goal had been achieved that impetus was gone.
Meanwhile, the costs of the program mounted: $28 billion was spent on the Apollo and associated missions, and everyone had their own ideas on better uses for the money. Many attacked the program because it was diverting resources from military purposes. The Vietnam War, and more recently Afghanistan and Iraq, drained America's capacities to spend big again.
The Moon missions yielded immense technological benefits and inspired a generation of future scientists, but measuring either to weigh them against alternatives was almost impossible.
The two obvious next moves after the historic Moon landing in 1969, either going to Mars or establishing an enduring lunar base, would each have required a commitment at least as big as getting Neil Armstrong there in the first place. Meanwhile, the tragedy of Apollo 1 and the technical issues of Apollo 13 reinforced spaceflight's dangers.
Kennedy's proposal was very much a product of its time. Russia had bested the United States both in launching the first satellite, and getting the first human in space. America needed a win to restore its prestige, and landing a human on the Moon did that in style. The US coasted on this victory through the '70s and '80s, after which the collapse of the Soviet Union left it without an opponent to spur it on. It's probably not a coincidence that plans to finally go back to the Moon – with NASA's Artemis mission aiming for a 2024 landing – coincide with a multitude of other nations making historic space breakthroughs, not least China, which is the only country to have a lander on the far side of the Moon and recently became the second-ever nation to successfully land a rover on Mars after the US.
The idea of renewed Moon missions is popular, which is why several presidents have proposed it, but paying for it is less so. Advances in computing and rocket propulsion mean going back will be a lot cheaper now than for the original missions, inflation adjusted, but the cost is still immense. Which is where NASA's Commerical Crew Program comes in.
There are plenty of scientific reasons for renewed Moon missions. There are questions about our satellite's geology the rocks Apollo astronauts brought back can't answer, and telescopes located on the lunar surface could have advantages over those in space. Whether these would outweigh the value of all the other things we could do with the same money is a very different question.
A self-sustaining Lunar base that could preserve humanity if something happened to the Earth is so far beyond our current capacity taking the first step always gets postponed. Hopefully, the current planned missions will take us back, but the long delay since we were last there shows we might not have gotten there in the first place had it not been for Kennedy's enthusiasm.