NASA’s recent discovery of flowing water on the surface of Mars has rekindled a vital debate: What is the proper role of NASA in an era when private companies are actively competing to open more access to space?

The success of commercial space ventures is no small feat. I served as the first private astronaut working on techniques to manufacture new medicines in space on three space shuttle flights in 1984-85. Now companies like SpaceX and Virgin Galactic are building their own rockets, launching satellites and ferrying supplies to the International Space Station.

Some have suggested that we outsource to these companies even bolder space exploration. SpaceX founder Elon Musk has already boasted plans for a new rocket that could send citizen colonists to Mars years ahead of NASA’s schedule and for only $500,000 a ticket. That’s dirt cheap.

The idea is attractive, even if commercial plans for a Mars mission are hypothetical at best. But as much as I support the private space industry, experience and common sense tell me that a commercial Mars human landing won’t ever get off the ground — not unless NASA goes there first.

Businesses are slaves to short-term balance sheets, and private space-industry investors and shareholders are notoriously risk-averse. Even wealthy entrepreneurs won’t throw their money away. They’ll back straightforward missions — like delivering cargo to the space station 250 miles above the Earth using mature and well-tested technologies — if they can turn a profit within a reasonable time with acceptable risk.

But true exploration is, by its nature, risky. Only a nation can marshal the long-term funding and pioneering vision needed to “boldly go where no one has gone before.”

In fact, nearly every great exploration in history has been government-funded or guaranteed, from Magellan’s trip around the globe to the Lewis and Clark expedition. NASA’s own history reads as an improbable list of “firsts.”

When President John F. Kennedy declared that the U.S. would put a man on the moon by the end of the 1960s, no one had the technologies we would need to get there. NASA scientists and engineers led a government-industry team inventing the rocket boosters, space capsule and computer-guidance systems from scratch in just a few years.

Before it succeeded, NASA failed publicly many times. If we had entrusted the project to private industry, shareholders or investors would have pulled the plug long before the Apollo program.

NASA engineers are already developing new technologies for a manned Mars mission like new propulsion systems that produce high velocities at low power, efficient wastewater recycling and deep-space radiation shields needed for humans to survive the 2½-year round-trip mission to Mars.

NASA is also far down the road building the critical rocket needed to power this mission — the most powerful launch vehicle in history, known as the Space Launch System. SLS will have twice the payload mass and six times the volume of any other American rocket, allowing NASA to accomplish the Mars mission with fewer launches. Fewer launches means lower costs, shorter timetables and, most important of all, less risk to our astronauts.

Mars is our era’s moon shot — the difficult challenge we choose to accept not because it is easy, as President Kennedy said, but because it is hard, because it will drive us to invent new technologies, answer the toughest questions and inspire a new generation of American engineers and scientists to carry the torch for decades to come.

We should continue to support commercial space companies as they make spaceflight cheaper and more accessible. But we should not be content to do what we’ve done since the ’60s, only a little cheaper — or to stake our most important space-exploration goal on the whims of the market.

We should continue to push the envelope, to expand the frontier as a top national priority. That’s a job only NASA can lead.

Charles D. Walker is an engineer and a former space shuttle astronaut. He lives in Tucson.