Strong gravity and a thick atmosphere allow us to live here on Earth, but also what make it so difficult to leave the planet.
Rockets have to exceed at least 25,000 mph to escape Earth's gravity. That means spacecraft may end up using a substantial portion of their fuel before they even start heading to a far-out destination like Mars.
But what if we could launch from somewhere that doesn't have an atmosphere, and has significantly less gravity?
Astronaut Byron Lichtenberg flew two missions to Spacelab in the 1980s and 90s, and he now chairs a NASA standing review board that oversees the Commercial Crew Program, which works with companies to get supplies and astronauts into space.
And Lichtenberg's dream is to make NASA's next mission to build a rocket factory on the moon.
"Personally, I would like us to go back to the moon first and set up permanent bases up there and develop infrastructure on the moon and actually build the Mars ships on the moon, because it's got all the raw materials to build the big structures," Lichtenberg told Tech Insider.
This isn't some crazy fantasy.
NASA scientists have seriously thought about how we could use the moon as a steppingstone to other planets, and they're actively developing tools to help us do it.
Phillip Metzger, for example — formerly a research physicist at NASA’s Kennedy Space Center — co-authored a paper on how to "bootstrap" across the solar system.
"If we want to want to create a robust civilization in our solar system, more of the energy, raw materials, and equipment that we use in space has to come from space. Launching everything we need from Earth is too expensive," Metzger said in a Q&A with the White House in 2014.
"Ultimately what we need to do is to evolve a complete supply chain in space, utilizing the energy and resources of space along the way."
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Scientists have found silicon, titanium, iron, and oxygen in the lunar soil. All of these elements could be mined and turned into rockets — and rocket fuel.
Today, we use liquid oxygen (and sometimes hydrogen) and highly refined kerosene as rocket propellant. Sticking with this design means we could mine the oxygen from the lunar soil and then melt and refine it. It'd take a considerable amount of energy, but it could be done.
And this mining could be done by robots, as illustrated in this artist's depiction of a moon construction system by NASA:
If we develop new fuel or designs, like the plasma rocket from company Ad Astra, Lichtenberg says, it would accelerate deep-space exploration even further.
Plasma rockets don't have the same initial zip that traditional engines using liquid oxygen do. They burn slow-and-steady, so launching them from the moon makes a lot more sense than using them to overcome Earth's gravity, which is six times more powerful.
That steady burn turns into a high velocity over time: Ad Astra boasts it could get to Mars in 39 days. (Today, it takes at least six months to get there.) Slowing down will be an issue, but we could definitely get there faster.
A lunar rocket factory is still a far-out plan, so there are some naysayers. Some worry that focusing on the Moon could pull our attention from Mars, as William Sweet outlines in IEEE Spectrum, while others dismiss the ease with which we could actually mine its resources.
But Lichtenberg sees a moon base as a steppingstone to the rest of the solar system, which humans have yet to visit.
It would take some more time to set up the infrastructure, he said, but that investment would be worth it for the lessons we can learn on the lunar surface.
"You get to really learn how to live and work in space, in a vacuum, in a kind of radiation environment that you would have going to Mars," Lichtenberg said.
"So you learn a lot, and then once you have that infrastructure in place, you can build as many [rockets] as you want, and go wherever you wanna go."
