These are incredibly ex­citing times for space exploration. NASA currently operates more than 50 robotic spacecraft that are studying Earth and reaching throughout the solar system, from Mercury to Pluto and beyond. Another 40 unmanned NASA missions are in development, and space agencies in Europe, Russia, Japan, India and China are running or building their own robotic craft. With such an armada at our disposal, delivering a stream of scientific data from so many distant ports, you might think that researchers like me who are involved in robotic space exploration would dismiss astronaut missions as costly and unnecessary. To the contrary: many of us embrace human exploration as a worthy goal in its own right and as a critically important part of space science in the 21st century.

Although astronaut missions are much more expensive and risky than robotic craft, they are absolutely critical to the success of our exploration program. Why? Because space exploration is an adventure—a human adventure—that has historically enjoyed broad public support precisely because of the pride we take from it. President John F. Kennedy committed the U.S. to sending astronauts to the moon to make a statement about the power of democracy and freedom, not to do science. As a by-product, some outstanding lunar science was done, leading ultimately to an understanding of the moon’s origin. What is more, the Apollo moon program trained and inspired an entire generation of researchers and engineers, who made the breakthroughs that paved the way for robotic missions, as well as much of the tech­nology that we take for granted today.

Letting the Apollo program end prematurely was a phenomenal mistake. NASA’s subsequent strategy for human exploration, focused on space shuttle missions and orbital space stations, turned out to be uninspiring and tragically flawed. The recent successes of the Mars rovers, the Cassini probe to Saturn and other robotic missions may signal a renaissance, but the situation is still precarious. Indeed, the post-Apollo decline in public interest in space exploration reverberates today in the debates over NASA’s budget and the general skepticism about the agency’s future relevance, especially among the generation now entering the workforce. Further triumphs of the robotic missions will be possible only if public and political interest is rebuilt and sustained by a reinvigorated program of human exploration.

What is more, human brains will be vitally needed in many future missions. Although robots have proved their worth in documenting and measuring the characteristics of distant places, they fall far short of humans when it comes to making judgments, incorporating broader contexts into decision making and learning from their experiences. Some of these capabilities can be programmed, and so-called machine learning has advanced considerably in the past few decades. But the neural complexity that is so often needed to make discoveries—the same combination of logic, experience and gut instinct required to solve a mystery—cannot easily be distilled to a series of “if-then” statements in a computer algorithm. Robotic brains will lag far behind in these kinds of abilities for a long time to come, perhaps forever, thus placing severe constraints on the science they can do on other planets.

Robotic craft have worked well for the first age of space exploration, when simply flying a probe past a planet or landing on an alien terrain was enough to make dramatic discoveries. That era, however, is coming to an end. Now we are entering a new age of space exploration in which we must look more carefully at such planetary landscapes, as well as at what lies underneath them—analyzing the rocks, soils and gases of distant worlds in greater detail to flesh out the history of our solar system. This kind of science absolutely requires human explorers. In this new era, we will need brave people with brains to boldly go where no robot can take us.