“We leave as we came and, God willing, as we shall return with peace and hope for all mankind.”

Those were the last words that Eugene Cernan spoke while on the moon, the last person to step foot there. Twelve people total walked on the moon during the Apollo missions: Neil Armstrong, Buzz Aldrin, Pete Conrad, Alan Bean, Alan Shepard, Edgar Mitchell, David Scott, James Irwin, John Young, Charles Duke, Eugene Cernan, and Harrison Schmitt. Cernan was one of only three who went to the moon twice. Of the twelve, six are now still alive.

Cernan and Schmitt were the two astronauts of the Apollo 17 mission, the last mission of the Apollo program, to walk on the moon. They lifted off from the lunar surface on December 14, 1972. That means this year it will be 45 years since a person last walked on the moon. We are fast approaching half a century since the end of the Apollo missions.

I was only 8 years old in 1972, but I avidly followed the Apollo program. From my perspective at that age the Apollo program had existed my entire conscious life. To me it was just something that we did. I remained a fan of NASA, the space program, and all things having to do with space travel since then. I would have considered it unthinkable that we would not return to the moon in the next 50 years. Surely by 2001, which seemed far in the future, Kubrick and Clarke’s vision of a permanent moon base was likely a reasonable extrapolation.

So why haven’t we been back to the moon? Should we return to the moon?

There are some real practical reasons we have not returned to the moon, and there are differing opinions on which are most important. Some argue that the Apollo program was the result of cold war competition with the Soviets. Absent that, there simply isn’t the political will for such an investment.

While this is likely true, there are some technical reasons also. Apollo was a “flags and footprints” program, meaning that the missions were short. It took several days to travel from the Earth to the moon or back, and missions typically spent three days on the surface of the moon. That relatively short duration meant that the astronauts did not have to bring that many supplies, and their exposure to radiation was also minimal.

Even then, the Apollo missions stretched our capabilities. Getting out of Earth’s gravity well and making it to the moon required a massive rocket, the Saturn V. After the Saturn V was retired, we simply no longer have a rocket powerful enough to get to the moon.

NASA’s current plans to return to the moon are not for short missions, but for a sustainable permanent presence on the moon. This will require longer missions, lasting for months. It will therefore require more equipment, food, and water. To shield the astronauts from radiation, it will also require more shielding and the construction of permanent shelters on the lunar surface.

Therefore we need rockets that are even more powerful than the Saturn V, and that is literally a tall order. To get to the moon with more supplies and equipment NASA designed the Ares V, which would have had a lift capacity of 188 metric tons, compared to Saturn V’s 118 metric tons.

Unfortunately, the Constellation Program that would have built the Ares V was cancelled by president Obama in 2009. NASA continues to develop the Orion capsule, which is designed to be a deep space vehicle, but there are no rockets which can take it to the moon or to Mars, as was originally planned.

This gets us back to the political will thing again. The Constellation program was over budget and delayed (of course it was, it was trying to do something no one had ever done before). So, the pulled the funding from NASA and the program is dead.

Constellation was replaced by a plan to rely on private companies to build the rockets. This makes sense for low-Earth orbit, because that is within reach of private companies and there is an immediate commercial application. Space X is making great progress in private space flight. It remains to be seen, however, if the public-private partnership of NASA and Space X or some other private company can get us back to the moon or beyond.

Meanwhile, NASA replaced Constellation with the Space Launch System (SLS). These are heavy lift rockets designed for deep space, such as the moon. They are much more modest than Ares V, however. The first configuration, called Block 1, will have a lift capacity of 77 metric tons. The SLS is designed, however, so that NASA can evolve its configuration for greater lift capacity. Block 2, which will be used to get to the moon, will be a 130 metric ton configuration.

NASA is currently planing Exploration Mission 2 (EM-2) which will use the SLS to send a crew in an Orion capsule to the moon and back.  Right now they are planning just looping around the moon on a trajectory that would automatically return the capsule to Earth. If all goes well, however, they may convert the mission to lunar orbit prior to return. This is primarily a test mission for the SLS and Orion.

The schedule for EM-2 is vague – the first half of the 2020’s. Of course, a lot can happen between now and then.

The question remains, however – should we return to the moon? I think we should. I know that robotic exploration and missions are more practical, and our robotic capabilities are increasing rapidly. I agree that most of what we sent into space should be robotic, not human.

At the same time I think it is important for humanity that we colonize our solar system. We can’t do that with robots alone. There is much to be gained by becoming a spacefaring race. If nothing else, as Carl Sagan noted, it can be a hedge against extinction.

I also think it’s important to feed the spirit of human exploration. To some extent, I think we should send humans into space just because we can. Then again, my sensibilities were formed as an 8-year old watching Cernan and Schmitt walk on the moon and thinking this was a normal thing for humans to do.