On Wednesday 4 November, SpaceX CEO Elon Musk took to Twitter to explain how the spaceflight firm’s latest launch would help humanity protect Earth against asteroids. The long-term goal of his project, however, is to build a self-sustaining city on Mars that would ensure humanity’s survival in case of a catastrophe on our home planet. “If we are able to make life self-sustaining on Mars, we will have passed one of the greatest filters,” Musk wrote. “That then sets us up to become interstellar.”

If we are able to make life self-sustaining on Mars, we will have passed one of the greatest filters. That then sets us up to become interstellar.

Earth is ~4.5B years old, but life is still not multiplanetary and it is extremely uncertain how much time is left to become so.

— Elon Musk (@elonmusk) November 24, 2021

“The comments come after SpaceX launched NASA’s Double Asteroid Redirection Test (DART). The mission launched on November 23 at 10:21 p.m. Pacific time from Space Launch Complex 4 East at Vandenberg Space Force Base in California,” wrote Inverse when reporting on the news.

Billed as “humanity’s first planetary defense test mission,” the goal of the mission is to note whether crashing a spacecraft into an asteroid could help change its course. The current target is the smaller of the Didymos binary asteroids, measuring 160 meters (524 feet) across. DART will strike the moonlet in September 2022, around 11 million kilometres from Earth at a speed of 6.6 kilometres per second.

For Musk, this mission showcases the fragile nature of life on Earth—if we don’t act fast, he believes humanity could run out of time. The business magnate’s plan is to transform humanity into a multi-planet species. It all starts with SpaceX’s Starship, a fully-reusable rocket currently under development in Texas. It’s capable of sending over 100 people or 100 tons to space at a time. SpaceX further aims to host the first orbital flight sometime in 2022.

From there, the company would be able to send the first humans to Mars at some point in the 2020s. This could be the first step toward establishing a full-size city of 1 million people on Mars by 2050. Musk’s goal is for the city to be self-sustaining, so even if regular supplies from Earth stopped one day, the city could function just fine. But that’s not all.

In one way or another, Musk aims to answer the million-dollar question: ‘Are we alone in the universe?’. Well, sort of. Enter the Great Filter, known as an answer to the Fermi Paradox. Coined by nuclear physicist Enrico Fermi, the paradox questions why we haven’t come across any evidence of extraterrestrial life if it really is so common in the universe. In other words, where are all the little green guys we oh-so-often hear about?

The concept of the Great Filter originates from Robin Hanson’s argument that the failure to find any extraterrestrial civilisations in the observable universe implies the possibility that something is wrong with one or more of the arguments from various scientific disciplines that the appearance of advanced intelligent life is probable. Either this ‘filter’ is still stopping us from exploring the stars, or we’ve finally moved past it—which means we might soon be able to encounter extraterrestrial life somewhere out there.

Musk suggests that one of the great filters—he believes we’re facing more than one—could be the inability to become multiplanetary. Moving past that hurdle would allow humanity to overcome one of the most significant barriers to achieving an advanced stage of development. Pretty exciting, right?

Yes. Probably more accurate to say Great Filters plural.

— Elon Musk (@elonmusk) August 16, 2020

Sadly for him though—we’re still debating whether it would be something incredibly good or devastatingly bad for humanity as a whole—it looks like we probably won’t reach another star in Musk’s lifetime. That’s why, as of now, his biggest goal is to establish the Mars city, which could then pave the way for further advancements.

SpaceX designed Starship for full reusability and refuelling using in-space resource utilisation. Its liquid oxygen and methane fuel mean humans could refuel using Mars’ resources and either return to Earth or venture out further, establishing a planet-hopping network along the way.

Sure, the ship won’t take humanity to other stars any time soon, but Musk suggests that establishing a second home for humans would increase the chances of reaching other stars in the far future. It would mean that if a catastrophe wipes out humanity on Earth, work could continue on the Mars base or any other established settlements.

But he’s not the only one looking to save humanity. Jeff Bezos, Musk’s infamous arch-enemy, also has big plans in the making: to have us orbit Earth in giant space stations. In May 2019, he explained how humanity could live in orbiting space colonies similar to those envisioned by physicist Gerard K. O’Neill. The physicist claimed that these would be better than planetary surfaces because they enable real-time communication with Earth and it’s easier to return home.

With human energy use doubling every 25 years, Bezos believes that in around 200 years, humanity will need to cover the Earth in solar panels to meet its needs. And with no room for us on the planet, orbiting it instead sounds like the perfect solution. Expanding into space would ensure the survival of our species, potentially to a population of one trillion humans. It could support, Bezos said, “a thousand Mozarts, or a thousand Einsteins.”

So, which idea is the best one? Who should you trust out of those two (slightly dreadful) options? In another article, Inverse asked space entrepreneur and consultant Rand Simberg to weigh the pros and cons of both propositions and share his opinion on which one represents a more attractive vision of the future of space.

Simberg argued that while Musk’s vision receives a lot of publicity, Bezos’ goal is “more expansive.” To put it simply, Earth is pretty good, so why not stay close to it? “This is the best planet in the Solar System,” he said. “The thing that Elon doesn’t seem to appreciate […] it’s not obvious he’s given a lot of thought about what you do when you get there.”

Musk likes to think that Mars could support a thriving economy but for Simberg, there’s a lot of unknowns around human physiology and partial gravity. Not enough research has been done into the long-term effects. “There’s a huge ethical issue right now with sending people to Mars to live… We don’t know if they can have healthy kids,” he told Inverse.

Funnily enough, when speaking about his vision at the SXSW 2018 conference, Musk explained that the first visitors to reach Earth 2.0 (Mars) would not be living luxuriously, “Like Shackleton’s ad for Antarctic explorers: difficult, dangerous, good chance you will die.” Oookay then.

In comparison, in the coming decades, a Bezos-spearheaded economy could start to thrive in low-Earth orbit. “We’re going to see large rotating space facilities, space hotels,” Simberg said. “We are going to see thousands of people going to space, visiting, living, working [and] playing in space. Barring some planetary catastrophe, I think that’s what we’re going to see.”

While having to pick between the plague and cholera is no easy task, it’s somewhat reassuring to hear what an expert like Simberg has to say on the matter. Long story short, compared to the unknown of expanding to other planets, nearby space stations could prove a more attractive option. There you have it—now we stay tuned for Musk to work on a comeback, on Twitter obviously.