Space / Space Flight

This summer, construction will begin on a probe to Pluto and the Kuiper belt. Planetary scientist Alan Stern, director of the Southwest Research Institute in Boulder, Colorado, is the principal investigator for the mission. He is also a licensed commercial pilot and was once a finalist for the position of space shuttle mission specialist. Stern has made a career of investigating the solar system’s frontiers, from a mysterious band of asteroids that may be orbiting the sun inside Mercury to the vast Oort cloud far beyond the known planets, the source of all comets.

Did you ever think that a mission to Pluto wouldn’t happen?

S: I had a lot of doubts through the 1990s, during Dan Golden’s NASA administration, because NASA repeatedly cancelled missions to Pluto, despite the fact that the scientific community kept saying it was top priority, crucial to getting an understanding of our solar system. Now I am fairly convinced that it will happen.

Is Pluto a planet?

S: Of course it is a planet. The generally accepted definition of a planet is very simple: It is a body that orbits its star, and it has to be large enough to become round under self-gravity but not so large that hydrogen fusion takes place in its center. If the object is too large and fusion takes place, we call it a star. And if it’s too small and its own gravity wouldn’t make it round, we call it a rock. Pluto is about 10 times the size of the smallest object in space that would become round due to gravity, so it easily qualifies.

Why should we go there?

S: In part, we should go because exploration is part of what makes us human. Beyond that, we should go because it turns out that Pluto is at the nexus of four key scientific themes that will lead us to a better understanding of our solar system. One theme is that Pluto was the first to be discovered of an enormous collection of trans-Neptunian objects called the Kuiper belt. These bodies are part of a previously unknown portion of the solar system—what I like to call the third zone of the solar system. This third zone was on its way to growing a very large planet, but something—we don’t know what—stopped the process. Instead we have a collection of miniature planets. That means that in this zone we can find planetary embryos that were frozen in time during their gestation. That gives us a window into the past. That’s the second theme.

The third theme is that Pluto and its moon, Charon, which is half Pluto’s size, constitute a binary planet. We think binary planets are common in the galaxy, just as we know that binary stars are common in the galaxy, and we have even begun to find binary asteroids. The New Horizons Pluto mission will be the first mission to a binary object and will help us understand everything from the origin of Earth’s moon to the physics of mass transfer between binary stars.

Finally, Pluto has a knock-your-socks-off atmosphere that’s escaping rapidly like a comet’s, but on a planetary scale. As a result, the planet has shrunk in size over billions of years because of the same processes that shaped the early evolution of Earth’s atmosphere and very likely that of both Mars and Venus. We have never been to a planet where this kind of rapid escape is taking place. By going to Pluto we have a chance to anchor, with real data, models of the early evolution of Earth’s atmosphere.

What would people see if they went to Pluto and stood on the surface?

S: The surface is bright and covered in a fresh, pinkish snow. People commonly think that it would be dark on Pluto because it is so far from the sun, but it is actually about as bright as dusk here on Earth, with enough light for you to very easily read a book and see what’s going on around you. On the Charon side of Pluto, you’d see a big old moon up in the sky, appearing 10 times as wide as Earth’s moon and twice as bright. You might see mountains on Pluto. You’d certainly see craters. There may be volcanoes and geysers. You would from time to time see atmospheric phenomena such as fog, clouds, or hazes. If you were there long enough, you would see it snow.

What is the third zone of the solar system like?

S: The Kuiper belt region, which I call the third zone because it lies beyond the rocky terrestrial planets and beyond the giant planets, is a bizarre frontier. It is dotted by more than 100,000 miniature frozen worlds. Based on data already in hand, we suspect that most are made of rock and ice with a liberal dash of organic molecules. It appears that thousands of these bodies have moons. Some may even occasionally grow atmospheres. And the latest news is that the king of the Kuiper belt, the king of the third zone, Pluto, may even sport an ocean under its icy crust!