Pluto was still a planet when a spacecraft began its journey nine years ago to that small, cold hunk of rock and ice. This month the nasa probe—the fastest spacecraft ever launched—finally reaches its primary target after a five-billion-kilometer cruise. On July 14 it will fly past what is now classified as a dwarf planet, becoming the first spacecraft to visit that faraway world and in doing so completing the initial exploration of our solar system that was conceived with the first interplanetary missions half a century ago. Already the approaching spacecraft, called New Horizons (above), has snapped unprecedented pictures, spying what looks to be an ice cap at one of Pluto's poles. At its closest, New Horizons's suite of cameras, spectrometers and sensors will scrutinize the body's surface and atmosphere from an altitude of just 12,500 kilometers. Scientific American's Lee Billings and New Horizons's principal investigator Alan Stern, a planetary scientist, discussed this historic, long-awaited mission. Edited excerpts follow.

A common conception of Pluto is that it is an inert snowball. Why send a spacecraft to visit it?

We now know Pluto is a dynamic world. We've seen its brightness changing, maybe because of snow moving around; its surface pressure has tripled since the late 1980s; and its temperature is changing in ways that we don't fully understand. We also now know Pluto has a rich system of satellites, a big moon, Charon, and at least four smaller ones, Nix, Hydra, Kerberos and Styx. We don't know a lot about the smaller ones, but Charon has crystalline ice and ammonium hydrates on its surface that may be related to recent outflows from its interior. So maybe Charon has geysers. We also have predictions that Pluto and Charon might actually share a common atmosphere. Some researchers predict that one or both may have or have had subsurface oceans. We'll know a lot more once we study them up close.

I tend to think of Pluto and its moons as presents sitting under a Christmas tree. They're wrapped, and from Earth all we can do is look at the boxes to see whether they're light or heavy, to see if something maybe jiggles a bit inside. We're seeing intriguing things, but we really don't know what's in there. I've been waiting 26 years to unwrap these presents. This year Christmas comes in July!

What do you expect to find in those boxes?

That's hard to answer. It's not just that no one has ever visited Pluto before. No one has ever visited this type of planet. We began planning this mission back in 1989, after Voyager 2's encounter with Neptune, and back then hardly anyone even knew the Kuiper belt existed. It's a vast region populated by lots of small bodies and a few very exotic, very diverse small planets. New Horizons isn't just visiting Pluto; it's visiting this entire region. Whatever it finds, this will be a signal moment for planetary exploration—the capstone to our first reconnaissance of the planets of our solar system.

What will New Horizons do after the flyby?

We've found two small objects, each roughly 50 kilometers across, for a potential post-Pluto flyby in 2019. They're both about a billion miles beyond Pluto, but they're in different directions, so we have to choose whether to go to one or the other. These are ancient, primordial building blocks of the Kuiper belt planets, and we could see them up close! We're looking forward to writing an extended mission proposal next year to convince nasa to let New Horizons visit one of them. Beyond that mission, the spacecraft is healthy and could run into the mid- to late 2030s.

Do you think we'll send another mission to Pluto or the Kuiper belt?

There is no current plan for that by any space agency. We may never do anything like this again. In fact, whether we go back depends on what New Horizons finds and how it might change our priorities in planetary science. If the Pluto system is sufficiently enticing, then I expect we'll see mission proposals to return. Can you come back and ask me in six months?