HOUSTON—Possessed of slight build and modest height, John Grunsfeld is not an imposing man. But this appearance belies a brilliant mind and a considerable resume. A quarter-century ago, Grunsfeld worked as a physicist at the California Institute of Technology, studying X-ray and gamma ray astronomy, before becoming an astronaut. He flew in space five times and made three trips to the Hubble Space Telescope where he performed eight spacewalks to repair and upgrade the iconic instrument.

In 2012, Grunsfeld assumed responsibility for all of NASA’s science missions. He oversaw the dramatic landing of Curiosity on the surface of Mars, and he was in charge during the exploration of Pluto. He wrangled the James Webb Space Telescope toward the launch pad, and he plotted a pair of ambitious flights to Jupiter’s moon Europa.

On a gray day in early December, I met Grunsfeld—who left NASA earlier this year—at a Starbucks across the street from Johnson Space Center. In the shadow of the buildings where he spent nearly 20 years training to fly in space, we spoke for two hours about all areas of spaceflight, from the intriguing oceans of Europa to human destinations.