Space tourism has arrived, coming into its own with the launch, orbit and safe return of civilians to Earth on SpaceX’s Inspiration4 mission in September. Watching that mission was gratifying. It was American ingenuity at its finest and showed that space missions have evolved from a public enterprise to a private one. But it also reinforced the urgency of reasserting broader American leadership in space.

Nearly 25 years ago, when I testified before Congress on space-related developments—and delays—my primary concern was that...

The SpaceX Falcon 9 rocket launches from NASA's Kennedy Space Center in Cape Canaveral, Fla., Sept. 15.

Photo: chandan khanna/Agence France-Presse/Getty Images

Space tourism has arrived, coming into its own with the launch, orbit and safe return of civilians to Earth on SpaceX’s Inspiration4 mission in September. Watching that mission was gratifying. It was American ingenuity at its finest and showed that space missions have evolved from a public enterprise to a private one. But it also reinforced the urgency of reasserting broader American leadership in space.

Nearly 25 years ago, when I testified before Congress on space-related developments—and delays—my primary concern was that we were failing to approach the future with the same energy, ambition, vision and execution that characterized our moon missions. Areas of particular concern included American leadership in near-Earth orbit, heavy launch, reusability, public-private partnerships, international cooperation, pioneering civilian space tourism and getting back to the moon and on to Mars.

Consistent with those missions, NASA, Congress, multiple administrations and private industry have made new investments. But American leadership in space is growing more urgent. Successfully advancing science and engineering, putting civilians in orbit on private rockets, widening launch options, and encouraging public-private partnership are good for the future.

Needed now are two other factors, which seem contradictory but are not. First, America must again lead—with the enthusiasm of recent missions—in returning to the moon and then taking humans to Mars. Other nations, not least China, are on that trajectory.

For reasons tied to national pride and security, natural expansion of science, engineering, exploration, environmental preservation and setting goals fitted to the next generation, we must shift deeper into space, elevate our eyes and press forward and outward.

Second, we must apply lessons of the past. After Apollo, America and the Russians cooperated in space, first with Apollo-Soyuz and later shared Space Station missions, by putting U.S. astronauts on Soyuz spacecraft and using Russian RD-180 engines in our Atlas rockets. Competition evolved to cooperation.

With a dozen nations aiming for bigger roles in space, ample room exists to rethink international cooperation. Competition may always exist, but militarizing space would be our loss.

Recent launches have awakened America’s latent interest in space travel, including our legacy of pioneering human space exploration. America should assert itself in getting back to the moon, exploring that orb, and from there staging for permanent human presence on Mars. These are achievable goals that can be pursued through a mix of cooperation and competition.

Fifty-two years ago, Neil Armstrong, Mike Collins,

and I were part of humankind’s first moon landing. That event excited the world and inspired countless human advances. We are now at another inflection point, a chance for American leadership in space. Let’s take it.

Mr. Aldrin is a former astronaut.

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