We Americans are an optimistic bunch. Just compare Hollywood movies with foreign films, and you’ll see a big difference in worldview — we love it when the good guys win. I believe this difference goes all the way back to “Manifest Destiny,” the 19th Century belief that American settlers were destined to expand across the continent.

But when it comes to space exploration, Manifest Destiny doesn’t apply. And if we choose simply to rest on the laurels of being the first nation to send humans to the moon, or on the achievements of the Space Shuttle and International Space Station (ISS), we will be surpassed by nations whose people are “humble and hungry.”

As a former NASA astronaut who is troubled by our ever-shifting goals for space, I don’t want this to happen. If we don’t have a destination, we'll never get anywhere. A decade ago, NASA was pursuing the Constellation program, whose goal was to develop a new space capsule and related systems that would ferry humans to the ISS before taking us to the moon and then to Mars and beyond. But Constellation was cancelled in 2010, ostensibly for budgetary reasons, and since then the U.S. has lacked a coherent strategy for human spaceflight. So I am proposing the following plan that ultimately would send humans to Mars. This plan sets concrete goals, and would inspire future generations of scientists and engineers and bring nations together to solve the many technological and political challenges we face here on Earth.

With these modest goals in mind, let us begin with past as prologue...

NASA’s moon program of the late 1960s actually played out over three distinct programs: Mercury, Gemini, and — last and most famous — Apollo. The initial phase, Mercury, proved that we could fly humans in space. Gemini, the least well known of the three programs, was even more critical. It created and tested the technologies that would be needed for the moon landings that were to follow. These included long-duration missions, spacewalking, the development of computers and software, and protocols for the rendezvous and docking of spacecraft flying in formation at thousands of miles per hour.

Finally, of course, Apollo was the fulfillment of President Kennedy’s famous charge that we should “land a man on the moon and return him safely to the Earth.” But again, Neil and Buzz and the men who followed them would never have made it to the moon without Mercury or Gemini.

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The original seven Mercury astronauts. NASA

I give this brief history because I believe the next strategy we pursue in space should parallel the Mercury/Gemini/Apollo model. First and most important, we need a vision. In the 1960s it was to put a man on the moon. Now it should be to send humans to Mars and back beginning in the 2030s, with increasingly long-duration missions to the planet’s surface. This vision is clear, and it goes beyond mere “boot prints and flags.” Long-term goals should be to understand the environmental, geological, and biological history of Mars, but also to set the stage for human settlements on the red planet.

The ISS has been the equivalent of Project Mercury, proving that humans can live and work in space for long periods of time. (I myself recently spent 200 consecutive days in space.) But the next phase of our space program — like the Gemini program — must be used to develop and test the critical technologies that will be needed for eventual missions to Mars. These technologies should be demonstrated on the ISS as well as on the moon.

Gemini IX spacecraft as seen on June 5, 1966. Eugene Cernan / NASA

The most important of these critical technologies is advanced space propulsion. With existing rocket technology, a round-trip to Mars would take three years. The astronauts aboard would need a huge amount of supplies — and would be exposed to dangerous radiation for the entire trip. What’s more, ensuring the reliability of critical equipment for a three-year mission would be costly, if not impossible.

Electric space propulsion engines could send a crew to Mars and back in roughly one year, dramatically lowering the radiation risk and circumventing the other problems associated with a three-year mission. A one-year mission would also cost a lot less, since it would take hundreds of thousands of dollars per pound to send stuff to Mars.

Electric engines have been used for decades on small satellites but have never been scaled up to a size capable of sending humans into space. The elephant in the room is that a 50-megawatt nuclear reactor would be needed for such massive engines. And the political challenges of sending such a reactor into space are probably more difficult than the technical challenges.

Apollo 11 lunar module on the surface of the moon. NASA

There are other “Gemini” technologies we will need to develop before travelling to the red planet. These include reliable life-support systems, such as CO2 removal, and water and oxygen recycling; equipment capable of launching and recovering crews at the highest speeds humans have ever flown; habitats and rovers that could be landed and robotically assembled on the Martian surface; space suits that would let astronauts live and work on Mars without frequent servicing; a Martian navigation and communication satellite network; innovative ways to protect crews from radiation; and finally, surface-based nuclear power to provide enough electricity for human survival.

This is a lot of expensive work, and making this project international would make it more affordable. But there are other, more important reasons to make this a multinational mission. Astronauts from many countries working aboard the ISS have demonstrated a level of cooperation that is often missing down here on Earth; I recently commanded the ISS during the height of U.S.-Russian tensions, and I am proud of how well my international crew of astronauts and cosmonauts worked together. And without the stabilizing influence of partners, it’s doubtful that the U.S. would have the political attention span to stick with a long-term Mars program.

Finally, this project must have bipartisan buy-in here in the U.S. The president should unveil and promote this vision alongside the minority leaders of the House and Senate. If not, it will be doomed to cancellation once his administration ends.

If we do these things — promote a coherent vision for space exploration, build international cooperation, and pursue a Gemini-style program of technology development — we’ll be flying our next “Apollo” missions to Mars in the near future. And if we fail to do these things? There will be no 21st Century Manifest Destiny for the U.S. Instead of securing our place in history, we’ll watch as other, more forward-thinking nations secure theirs.

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