Landing 12 people on the Moon remains one of NASA's greatest achievements, if not the greatest.

Astronauts collected rocks, took photos, performed experiments, planted some flags, and then came home. But those week-long stays during the Apollo program didn't establish a lasting human presence on the Moon.

More than 45 years after the most recent crewed Moon landing – Apollo 17 in December 1972 – there are plenty of reasons to return people to Earth's giant, dusty satellite and stay there.

Researchers and entrepreneurs think a crewed base on the Moon could evolve into a fuel depot for deep-space missions, lead to the creation of unprecedented space telescopes, make it easier to live on Mars, and solve longstanding scientific mysteries about Earth and the moon's creation. A lunar base could even become a thriving off-world economy, perhaps one built around lunar space tourism.

"A permanent human research station on the Moon is the next logical step. It's only three days away. We can afford to get it wrong, and not kill everybody," former astronaut Chris Hadfield recently told Business Insider. "And we have a whole bunch of stuff we have to invent and then test in order to learn before we can go deeper out."

But many astronauts and other experts suggest the biggest impediments to crewed Moon missions over the last four-plus decades have been banal if not depressing.

It's really expensive to get to the Moon – but not that expensive

A tried-and-true hurdle for any spaceflight program, especially for missions that involve people, is the steep cost.

A law signed in March 2017 by President Donald Trump gives NASA an annual budget of about $US19.5 billion, and it may rise to $US19.9 billion in 2019.

Either amount sounds like a windfall – until you consider that the total gets split among all of the agency's divisions and ambitious projects: the James Webb Space Telescope, the giant rocket project called Space Launch System, and far-flung missions to the sun, Jupiter, Mars, the Asteroid Belt, the Kuiper Belt, and the edge of the solar system. (By contrast, the US military gets a budget of about $US600 billion per year. One project within that budget – the modernisation and now expansion of America's nuclear arsenal– may even cost as much as $US1.7 trillion over 30 years.)

Plus, NASA's budget is somewhat small relative to its past.

"NASA's portion of the federal budget peaked at 4 percent in 1965. For the past 40 years it has remained below 1 percent, and for the last 15 years it has been driving toward 0.4 percent of the federal budget," Apollo 7 astronaut Walter Cunningham said during a 2015 congressional testimony.

Trump's budget calls for a return to the Moon, and then later an orbital visit to Mars. But given the ballooning costs and snowballing delays related to NASA's SLS rocket program, there may not be enough funding to make it to either destination, even if the International Space Station gets defunded early.

A 2005 report by NASA estimated that returning to the Moon would cost about $US104 billion (which is $US133 billion today, with inflation) over about 13 years. The Apollo program cost about $US120 billion in today's dollars.

"Manned exploration is the most expensive space venture and, consequently, the most difficult for which to obtain political support," Cunningham said during his testimony, according to Scientific American. "Unless the country, which is Congress here, decided to put more money in it, this is just talk that we're doing here."

Referring to Mars missions and a return to the Moon, Cunningham added, "NASA's budget is way too low to do all the things that we've talked about doing here."

The problem with presidents

The Trump administration's immediate goal is to get astronauts to "the vicinity of the Moon" sometime in 2023. That would be toward the end of what could be Trump's second term if he is reelected.

And therein lies another major problem: partisan political whiplash.

"Why would you believe what any president said about a prediction of something that was going to happen two administrations in the future?" Hadfield said. "That's just talk."

From the perspective of astronauts, it's about the mission. The process of designing, engineering, and testing a spacecraft that could get people get to another world easily outlasts a two-term president. But there's a predictable pattern of incoming presidents and lawmakers scrapping the previous leader's space-exploration priorities.

"I would like the next president to support a budget that allows us to accomplish the mission that we are asked to perform, whatever that mission may be," astronaut Scott Kelly, who spent a year in space, wrote during a January 2016 Reddit Ask Me Anything session (before Trump took office).

But presidents and Congress don't seem to care about staying the course.

In 2004, for example, the Bush administration tasked NASA with coming up with a way to replace the space shuttle, which was due to retire, and also return to the moon. The agency came up with the Constellation program to land astronauts on the moon, using a rocket called Ares and a spaceship called Orion.

NASA spent $US9 billion over five years designing, building, and testing hardware for that human spaceflight program. Yet after President Barack Obama took office – and the Government Accountability Office released a report about NASA's inability to estimate Constellation's cost – Obama pushed to scrap the program and signed off on the Space Launch System (SLS) rocket instead.

Trump hasn't scrapped SLS. But he did change Obama's goal of launching astronauts to an asteroid to moon and Mars missions.

Such frequent changes to NASA's expensive priorities has led to cancellation after cancellation, a loss of about $US20 billion, and years of wasted time and momentum.

"I'm disappointed that they're so slow and trying to do something else," Apollo 8 astronaut Jim Lovell told Business Insider in 2017. "I'm not excited about anything in the near future. I'll just see things as they come."

Buzz Aldrin said in a 2015 testimony to Congress that he believes the will to return to the Moon must come from Capitol Hill.

"American leadership is inspiring the world by consistently doing what no other nation is capable of doing. We demonstrated that for a brief time 45 years ago. I do not believe we have done it since," Aldrin wrote in a prepared statement. "I believe it begins with a bi-partisan Congressional and Administration commitment to sustained leadership."

The real driving force behind that government commitment to return to the Moon is the will of the American people, who vote for politicians and help shape their policy priorities. But public interest in lunar exploration has always been lukewarm.

Even at the height of the Apollo program – after Neil Armstrong and Buzz Aldrin stepped onto the lunar surface – only 53 percent of Americans thought the program was worth the cost. Most of the rest of the time, US approval of Apollo hovered significantly below 50 percent.

Today, 55 percent of Americans think NASA should make returning to the moon a priority, though only a quarter of those believers think it should be a top priority, according to a Pew Research Center poll released in June. But 44 percent of people surveyed by the poll think sending astronauts back to the moon shouldn't be done at all.

Support for crewed Mars exploration is stronger, with 63 percent believing it should be a NASA priority, and 91 percent of people think scanning the skies for killer asteroids is important.

The challenges beyond politics

The political tug-of-war over NASA's mission and budget isn't the only reason people haven't returned to the moon. The moon is also a 4.5-billion-year-old death trap for humans, and must not be trifled with or underestimated.

Its surface is littered with craters and boulders that threaten safe landings. Leading up to the first moon landing in 1969, the US government spent what would be billions in today's dollars to develop, launch, and deliver satellites to the moon to could map its surface and help mission planners scout for possible Apollo landing sites.

But a bigger worry is what eons of meteorite impacts has created: regolith, also called moon dust.

Madhu Thangavelu, an aeronautical engineer at the University of Southern California, wrote in 2014 that the moon is covered in "a fine, talc-like top layer of lunar dust, several inches deep in some regions, which is electro-statically charged through interaction with the solar wind and is very abrasive and clingy, fouling up spacesuits, vehicles and systems very quickly."

Peggy Whitson, an astronaut who lived in space for a total of 665 days, recently told Business Insider that the Apollo missions "had a lot of problems with dust."

"If we're going to spend long durations and build permanent habitats, we have to figure out how to handle that," Whitson said.

There's also a problem with sunlight. For 14.75 days at a time, the lunar surface is a boiling hellscape that is exposed directly to the sun's harsh rays – the moon has no protective atmosphere. The next 14.75 days are in total darkness, making the moon's surface one of the coldest places in the universe.

A small nuclear reactor being developed by NASA, called Kilopower, could supply astronauts with electricity during weeks-long lunar nights – and would be useful on other worlds, including Mars.

"There is not a more environmentally unforgiving or harsher place to live than the moon," Thangavelu wrote. "And yet, since it is so close to the Earth, there is not a better place to learn how to live, away from planet Earth."

NASA has designed dust- and sun-resistant spacesuits and rovers, though it's uncertain if that equipment is anywhere near ready to launch, as some of it was part of the now-canceled Constellation program.

A generation of billionaire 'space nuts' may get there

A suite of moon-capable rockets is on the horizon.

"There's this generation of billionaires who are space nuts, which is great," astronaut Jeffrey Hoffman told journalists during a roundtable earlier this year. "The innovation that's been going on over the last 10 years in spaceflight never would have happened if it was just NASA and Boeing and Lockheed. Because there was no motivation to reduce the cost or change the way we do it."

Hoffman is referring to the work by Elon Musk and his rocket company, SpaceX, as well as that of Jeff Bezos, who runs a secretive aerospace company called Blue Origin.

"There's no question – if we're going to go farther, especially if we're going to go farther than the moon – we need new transportation," Hoffman added. "Right now we're still in the horse-and-buggy days of spaceflight."

Many astronauts' desire to return to the Moon fits into Bezos' long-term vision. Bezos has floated a plan around Washington to start building the first Moon base using Blue Origin's upcoming New Glenn rocket system. In April, he said, "we will move all heavy industry off of Earth, and Earth will be zoned residential and light industry."

Musk has also spoken at length about how SpaceX's in-development "Big Falcon Rocket" could pave the way for affordable, regular lunar visits. SpaceX might even visit the Moon before NASA or Blue Origin. The company's new Falcon Heavy rocket is capable of launching a small Crew Dragon space capsule past the Moon and back to Earth – and Musk has said two private citizens have already paid a large deposit to go on the voyage.

"My dream would be that, some day, the Moon would become part of the economic sphere of the Earth – just like geostationary orbit and low-Earth orbit," Hoffman said. "Space out as far as geostationary orbit is part of our everyday economy. Some day I think the Moon will be, and that's something to work for."

Astronauts don't doubt we'll get back to the Moon, and on to Mars. It's just a matter of when.

"I guess eventually, things will come to pass where they will go back to the Moon and eventually go to Mars, probably not in my lifetime," Lovell said. "Hopefully they will be successful."

This article was originally published by Business Insider.

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