• Humans have long imagined life on Mars, though our understanding of the planet has changed a lot.

• Some of the US's earliest plans assumed humans could reach the Red Planet by the 1980s.

• Over the decades, technology and funding challenges have hampered the nation's hopes of crewed flights.

Earlier this month, NASA announced it was funding a revolutionary high-thrust rocket — called a Pulsed Plasma Rocket — that could make crewed missions to Mars in just two months.

That's seven months faster than it'd take with current technology and would drastically reduce the risk and cost of a crewed Mars mission, according to Howe Industries, which is developing the concept. It "holds the potential to revolutionize space exploration," NASA said in a statement.

The PPR is just one of the latest developments in the US's decades-long discussion to send humans to Mars. In the early '60s, for example, nuclear-bomb-powered spaceships were proposed for the trip.

Since well before NASA landed the first humans on the moon, the US has poured money and time into proposals for a crewed Mars mission, only to see its attempts never leave the ground. But technology isn't the only thing standing in the way. Politics also plays a big role.

"That's kind of like a joke within the space community or the Mars community," Matthew Shindell, a curator with the National Air and Space Museum, told Business Insider. "Putting humans on Mars is always 20 years away."

It's short enough to seem tangible, he said, but long enough that the political situation will change before it can be realized.

To fully understand why the US hasn't sent humans to Mars, despite sending more robots there than any other country, it just takes a trip down memory lane. Here's a history of the US's most promising crewed Martian missions that never were.

1950s: The Mars Project

In the '40s and '50s, no one really knew what they might find on Mars, but they knew getting there would be tricky. One of the first to seriously tackle the problem was Wernher von Braun.

During WWII, von Braun was a member of the Nazi party and created V-2 missiles. After the war, he continued his work on missiles with the US Army as part of Operation Paperclip while also working on a novel called "The Mars Project." In it, he laid out the first detailed plan to send humans to the Red Planet.

He envisioned a 260-day mission that would launch in 1985 with 10 spaceships and 70 crew members. "He sat down and did the math and created a whole story around it," Shindell said.

In the late '50s, von Braun consulted on NASA's very first 10-year plan, which included sending the first probes to Mars. (Sending humans to Mars would come later.) What started as fiction got closer to reality when von Braun started working at NASA a couple of years later.

1960s: Mars by 1965

In the late 1950s, Theodore Taylor, who worked on nuclear weapons at Los Alamos, and theoretical physicist Freeman Dyson embarked on an ambitious plan to build a nuclear-explosion-powered spaceship.

Called Project Orion, the resulting ship would take 12 years to develop, cost $100 million per year, and comfortably hold 150 people. Their motto was "Mars by 1965, Saturn by 1970."

However, NASA was concerned about what would happen if any of the hundreds of bombs required to fuel the rocket exploded.

By 1963, the team was having trouble getting increased funding. That same year, the nuclear test-ban treaty was signed, hampering the team's ability to test its vehicle.

The project was canceled a year later.

1965: Mars's first close-up

Though NASA was feverishly working toward the moon in the '60s, it didn't fully abandon its plans for Mars.

In 1962, German rocket scientist Ernst Stuhlinger was working at NASA on a project to get five crewed ships to the Red Planet by the early 1980s.

Stuhlinger's planned ships were huge, almost 500 feet long. For comparison, NASA's Space Shuttles are under 200 feet. But as NASA raced to land the first humans on the moon, it shifted focus to smaller, lighter spacecraft. This helped speed things along toward the moon, but it was a step back for Mars.

This pivot "reduced Apollo's utility as a technological stepping stone to Mars," David S. F. Portree wrote in "Humans to Mars: Fifty Years of Mission Planning, 1950–2000."

In the meantime, NASA knew it needed more information about Mars before it landed humans there. So in 1964, NASA's Jet Propulsion Laboratory launched the very first probe to fly by Mars: Mariner 4.

The images the probe transmitted to Earth were fuzzy and showed a desolate, barren planet. But they were the first close-up images of Mars's surface that anyone on Earth had seen.

1970s: The post-Apollo plans

NASA had just landed the first people on the moon in 1969 as part of its Apollo Program and was ready for the next big step. That same year, a Richard Nixon-appointed Space Task Group issued a report that supported human flights to Mars in 1982.

However, President Nixon ignored most of the 1969 report's suggestions in favor of what would become the Space Shuttle program, which didn't involve going to Mars. It was a turning point for NASA.

During the height of the Apollo era, NASA didn't have to compete for funding, Shindell said. Now, Nixon's administration started cutting its budget.

This was in the midst of the Vietnam War, and many Americans wanted the government to focus on poverty, the environment, and other domestic issues.

"If you're a proponent of human Mars exploration, this is the problem you've faced ever since the 1970s," Shindell said. Sending humans to the moon was already incredibly expensive, and it's a lot closer than Mars.

1980s: Sally Ride's report

In 1985, President Ronald Reagan appointed the National Commission on Space to envision the next 50 years of space travel, which involved the possibility of piloted vehicles to Mars.

But then NASA's Space Shuttle Challenger exploded. The disaster affected how the agency thought about human space travel as a whole.

"In general, there was a great deal of soul-searching within NASA about the use of expensive and risky human-rated launch vehicles like the shuttle," William Sheehan and Jim Bell wrote in "Discovering Mars: A History of Observation and Exploration of the Red Planet."

Just a year later, though, NASA's administrator tasked astronaut Sally Ride with laying out the agency's future space explorations. In her report, she explained what it would take for the US to land an astronaut on Mars by 2005.

To meet that timeline, NASA would need to triple its current budget in the next decade. That didn't happen.

1989: 20 years to Mars

By 1989, a crewed mission to Mars seemed back on the table, according to a speech by the newly elected President George H.W. Bush.

"Why Mars?" he asked. "Because it is humanity's destiny to strive, to seek, to find. And because it is America's destiny to lead."

NASA's response was the Space Exploration Initiative, an analysis of Bush's space exploration goals, which would cost an estimated $400-$500 billion.

At that point, Mars was still a long way off. The missions weren't expected to begin until after 2010.

However, according to Sheehan and Bell, a lack of Congressional funding and political support led to the demise of Bush's Martian mission a few years later in 1993.

1990s: "Better, faster, cheaper"

By the 1990s, Mars enthusiasts were dreaming of getting humans there by the end of the millennium. Aerospace engineer Robert Zubrin formed the Mars Society, an advocacy group pushing for the planet's exploration and eventually establishing a human settlement there.

Meanwhile, NASA was trying to figure out how to study Mars after losing contact with the robotic probe Mars Observer in 1993. With so much still unknown about the planet, uncrewed missions continued to be the focus.

The agency's new administrator Daniel Goldin was pursuing a new mantra for the robotic missions: "better, faster, cheaper."

This decade saw success with the uncrewed Pathfinder and Mars Global Surveyor missions. Pathfinder delivered Sojourner, the first operational Mars rover, while MGS sent back incredible images and data from the planet.

Just a couple of years later, though, NASA lost two more uncrewed spacecraft, the Mars Polar Lander and the Mars Climate Orbiter (MCO).

2000s: But first, the moon

Despite the setbacks of the Polar Lander and MCO, NASA again had success in 2004 with rovers Spirit and Opportunity.

Though NASA had recently suffered another tragedy with the loss of the Space Shuttle Columbia and its crew in 2003, the agency's rovers seemed to reignite some of the desire for human missions to Mars.

In 2004, 15 years after his father's space speech, President George W. Bush announced what would become the Constellation Program. The ultimate goal was to put people on Mars, though there was no exact date given for this part of the plan.

A large part of Bush's vision involved returning to the moon before heading to the Red Planet. In 2010, President Barack Obama canceled Constellation but set a timeline of getting astronauts to Mars by the 2030s.

2010s: Mars goes commercial

In the 2010s, private space companies — like SpaceX and Virgin Galactic — started planning projects to get crews to Mars.

SpaceX founder Elon Musk said in 2016 that he would get people there in less than a decade. He later revised the date to 2029 with robust colonization by 2050.

Equally ambitious was Richard Branson's space company Virgin Galactic. In 2019, the company announced it planned to send an uncrewed spacecraft to Mars by 2022. But so far, neither SpaceX nor Virgin Galactic has sent anything to Mars.

Meanwhile, President Donald Trump reversed the Obama administration's space exploration plans. NASA was again planning for a moon-first agenda.

Established in 2017 under the Trump Administration, NASA's Artemis Program is its latest and current mission for crewed deep space exploration. It aims to return humans to the moon and create a lunar space station where astronauts can live for weeks or months, at a time.

However, this moon-first agenda doesn't completely rule Mars out. Dayna Ise, who leads NASA's Mars Campaign Office, said it will actually help us get to the Red Planet.

"You learn a lot by going to the moon, but you learn even more by staying at the moon," she said. "And so whatever we learn there will help with Mars."

She also thinks private space companies have a role to play. "It's all hands on deck," she said. "It is such a difficult engineering problem that we cannot exclude anybody from helping."

2020s: Simulating life on Mars

The private space companies have been busy this decade. In 2024, SpaceX had its first mostly successful Starship launch after several fiery attempts. The mega-rocket will play a huge role in Musk's plans to colonize Mars.

Meanwhile, the Biden Administration has continued to support the Artemis lunar missions. There have been a few setbacks, though.

Citing safety and technical challenges, NASA recently pushed back its first crewed Artemis mission to the moon, which is now slated for 2025.

Artemis IV, NASA's mission to deliver part of a lunar space station to the moon, is still scheduled for 2028.

Having a long-term presence on the moon will help experts learn more about how crews can survive on a different world for longer than a few days, Ise said.

Meanwhile, the agency is also studying how people will fare in isolation. NASA's CHAPEA missions put volunteers in a simulated Mars habitat for a year. The "analog astronauts" follow strict schedules, have limited contact with loved ones, and are closely monitored. The first crew will emerge from the habitat this year on July 6.

2030s and beyond: Getting humans to Mars

Despite its moon-first agenda, NASA knows Mars has its own challenges that the lunar surface can't prepare them for. In addition to taking a lot of time and fuel to get there, the trip will mean communication delays of at least 20 minutes between the crew and Earth.

The travelers will need to be able to take care of their own health emergencies and fix hardware issues, Ise said. But NASA is also working on making some systems more autonomous. "If there is an issue, they don't have time to troubleshoot with someone on the ground to fix their life support system," she said. "So we need those life support systems to be smarter."

Other problems include keeping the crew safe from radiation, dealing with the planet's skin-irritating dust, and developing a food source. "We have to build an ecology inside a transit vehicle to keep everyone alive and healthy," Ise said.

All that will take time. NASA administrator Bill Nelson has said there's potential for the agency to send humans to Mars by 2040. Ise compared it to eating an entire elephant. "We're doing it one bite at a time and building on everything that we learn," she said.

It remains to be seen whether private US companies will reach Mars first.

Read the original article on Business Insider