Today, astronaut Scott Kelly will board a Russian Soyuz spacecraft bound for the International Space Station. He’ll spend a year in low-Earth orbit, in part as a lab rat in a study that looks at how his body responds to life in space. The cool part here is the control group: Scott’s twin brother Mark, also an astronaut, is staying on Earth, making him a genetically matched basis for comparison. It's an intriguing experiment, but as far as human space travel goes, it's no giant leap. Humans haven't left low-Earth orbit—just a couple hundred miles above where you’re sitting right now—since 1972, when astronauts last walked on the moon.

Robots, though? Robots are having all the fun. "Uncrewed" spacecraft have ventured to almost every corner of the solar system, and—at this very minute—are exploring alien worlds from asteroids and comets to planets and dwarf planets. Which makes it tempting to declare that space exploration should be the realm of robots, not humans. People are expensive, hard to maintain, and they can die. Who needs the grief?

Well, we do. The crewed space program and the robot space program are two different things with two different purposes. And we need them both.

Yes, when it comes to science, robots kick butt. They're tough, cheap, and no one besides sci-fi sentimentalists cares if they never come home. Everywhere you look in the solar system, a robot is there. Rosetta is orbiting a comet, waiting for the Philae lander to wake up. Dawn is at the icy dwarf planet Ceres, which might have a subsurface ocean. In a couple months, if all goes well, New Horizons will become the first human-made object to visit Pluto. Juno is scheduled to arrive at Jupiter next summer.

And those are only the recent missions. Cassini has been studying the Saturnian system for more than a decade, and a couple weeks ago found evidence that Saturn's moon Enceladus has hydrothermal vents—a hot environment that could harbor life. The Curiosity rover continues to explore Mars, and its smaller predecessor, Opportunity, passed the 26-mile mark this past week—a marathon that took more than 11 years. Oh, and the Messenger spacecraft, launched in 2004, is wrapping up a mission at Mercury. The Voyager probes are in interstellar space. All these robots have sent invaluable data back home, teaching us about how the universe works.

NASA's Mars rover Curiosity, Feb. 3, 2013. NASA

The human space program, on the other hand, has never been about science. The driving force behind Apollo—the pinnacle of the human space program—was to show up the Soviet Union. The Cold War is over; the human space program no longer has an existential purpose.

Which is why it's struggling. How badly? After NASA retired the space shuttles in 2011, the agency was left without a way to get people into orbit. It became a space agency that couldn’t get to space. Private companies like SpaceX, Orbital ATK, Lockheed Martin, Boeing, and Sierra Nevada are all trying to fill the gap. But they're still just doing what people did decades ago. "Commercial space at this point with respect to human space flight is somewhat a sideshow," says John Logsdon, a space policy expert and historian at George Washington University. "All that's happening is two firms, SpaceX and Boeing, are under contract to develop a taxi to take people to the space station. Other than that, there's a lot of talk."

But critically, while the human space program may not have an overarching mission, it does have a purpose. A 2014 report from the National Research Council cited the economy, science, education and inspiration, national security, and—no kidding—human survival. We humans are perpetually in jeopardy if we stay on Earth, whether from nuclear war, climate apocalypse, or a good old-fashioned killer asteroid (a classic). If humanity is to survive, we have to spread out.

More than that, though, that NRC report also cited a "shared destiny and aspiration to explore." Now, that might sound sort of flaky. Logsdon ranks the idea long with all the other "clichés that one tends to spout when talking about the future of humanity." But even he wants people to boldly go. He remembers when men went to the moon. "Knowing what was happening, knowing here there were true explorers going to a new place—it was about as exciting as you can get," Logsdon says. It's about inspiration, adventure, and pride in what we can accomplish together as a species.

Astronaut Scott Kelly along with his brother, former Astronaut Mark Kelly at the Johnson Space Center, Jan.19, 2015. NASA

Eventually humans will be able to do some exploring, too. We can do things robots still can’t. "The ability to react to surprises or to decisions that need to be made tactically—that's directly in the realm of the human endeavor," says Jim Bell, a planetary scientist at Arizona State University who has worked on every Mars rover mission. A person analyzing the Martian terrain could rely on experience and instinct; all a robot has is software and time-delayed commands.

And eventually, the two programs will reunify. NASA's Deep Space Network was the communications link for the Apollo missions, and now connects a bunch of interplanetary robot spacecraft with home. "We wouldn't have healthy robotic exploration without the human exploration program," Bell says. The robots will eventually be scouts, finding the places where people can and should follow up.

By outsourcing its role in low-Earth orbit to the private sector, NASA can focus on deep space. It has started work on a new Orion spacecraft and the space launch system, the most powerful rocket ever built. They've even souped up the huge crawler transporters used to carry the rocket to the launch pad. This week, NASA announced a new mission—using a robot—to pluck a rock off the surface of an asteroid, testing capabilities the agency says people will need on a trip to Mars. "We're further along the path of making it happen than we ever have been," says Logsdon.

It’s true that NASA doesn't have the money to visit Mars. Under existing budget constraints, it never will. Right now, much of the science astronauts do on board the ISS has to do with keeping astronauts alive on board the ISS. But that’s not recursive. What researchers learn from Mark Kelly and his brother might not apply to more than a handful of people today…but someday it’ll be important to all of us. Spacefaring robots and spacefaring humans have only taken baby steps so far. But even baby steps move you forward.