Discover Interview: Planetary Scientist Alan Stern

The planets are the obvious next frontiers for human exploration

By Kathy A Svitil
Jan 5, 2004 6:00 AMNov 12, 2019 6:32 AM


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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?


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!

How much is out there at the edge of our solar system that we have not yet discovered?

S: The short answer is—a lot. The Kuiper belt is probably littered with hundreds, if not thousands, of ice-dwarf planets like Pluto. NASA has explored all four terrestrial planets and all four giant planets. But the number of bodies we’d classify as planets in the solar system is probably closer to 9,000 than it is to nine, and we haven’t been to the most populous class of bodies at all—the ice-dwarf planets of the Kuiper belt. Even farther out, beyond the Kuiper belt, lies the Oort cloud, 1,000 times farther away. The Oort cloud consists of objects ejected from the region surrounding the giant planets during and after their formation. In the Oort cloud there may be large planets that were ejected from the solar system in the early days when Jupiter, Saturn, Uranus, and Neptune were muscling out their rivals.

Could there be objects as big as Jupiter or Saturn in the region beyond Pluto?

S: Not objects the size of Jupiter or Saturn because Jupiter and the other giant planets couldn’t have ejected objects that large, but there certainly could be a handful of Earth- and Mars-size objects. There could be hundreds of things the size of Pluto in the Oort cloud, and a number of objects the size of

the Earth or Mars.

How do we find them?


You have to look! Actually, anything in the Oort cloud is too faint to see with the technology we have today. For example, NASA’s Spitzer Space Telescope, which launched in August

, can see an object the size of Pluto located a few hundred astronomical units away [

one astronomical unit is 96 million miles—the distance from

the Earth to the



, and Spitzer can detect a planet the size of Earth out to about 1,000 astronomical units. However, the Oort cloud is between 10,000 and 100,000 astronomical units away. So an Oort cloud survey will have to be the project of a future generation.

Can studying Pluto and these other objects tell us anything about Earth’s formation and its history?

S: Yes, it can. The standard model for the formation of the Earth-moon system is that a huge, Mars-size object hit Earth and spun off material that coalesced in orbit to become the moon. The Pluto-Charon system is the only other place in our solar system where we believe this happened on a planetary scale. By going to a system like Pluto-Charon, we’ll better understand how the Earth-moon system formed.

After Pluto, where else in the solar system would you like to explore?

S: I would like to see a robotic return mission to Uranus and Neptune. I’d like to see further robotic exploration of the Kuiper belt, and I’d like particularly to see humans go back to the moon for a serious exploration and then on to asteroids and Mars.

What are the vulcanoids?

S: The vulcanoids are a putative population of asteroids that may circle the sun inside the orbit of Mercury, like a little Kuiper belt, if you will, on the inside of the solar system. Instead of being icy, however, they are expected to be rocky because it is so hot. Although no object on such an orbit has been detected just yet, there is good reason to expect that they might exist. For example, the surface of Mercury is severely battered, and many of the projectiles that hit it may have been vulcanoids. The question is whether there are any left or whether they’re all gone. We really don’t know because it’s a very, very difficult observational problem to detect these bodies, even with modern technology. The easiest way to look is from space, but that’s very expensive. Doing it from the ground is extremely hard because one has to look for faint points of light against the twilight sky. Our group has taken a middle-ground approach by using high-altitude aircraft that can fly up into the high upper stratosphere where the sky is much darker, making the vulcanoids easier to detect. We haven’t finished our search yet, so I can’t yet tell you if there are vulcanoids. Stay tuned!

You’ve done these vulcanoid surveys and studies of other asteroids, meteors, and comets from the backseat of a high-performance aircraft, like a WB-57 or a F/A-18 Hornet fighter jet. Is there really a scientific benefit to working out of an F-18, or is it just a lot of fun?

S: It’s both. I have to say that the grants that paid for the studies came through peer-review panels that throw out five of every six proposals. If this was purely so that I could have a good time in the backseat of a fighter, it wouldn’t survive for 10 minutes in a peer-review panel.

My colleague Daniel Durda and I discovered a way to do certain kinds of astronomy from a jet that you just cannot do from the ground. You can do these studies from space, but this is 1,000 times cheaper, and 10 times cheaper than using a big aircraft like those NASA typically has flown around. And for the particular niche we are exploring, the jets we use do the trick.

As a pilot, do you ever get the itch to take the controls?

S: Every flight. Sometimes we even get to. But the mission isn’t to let the astronomer fly; it’s to accomplish specific observing goals with the gear we bring along.

Why do you focus on the solar system’s small, low-profile bodies? Why not big-ticket items like Mars?

S: The smaller objects are big-ticket items! Understanding the architecture of our solar system is a pretty big-ticket item. Discovering whether or not there is an asteroid belt interior to Mercury’s orbit, finding out whether or not the comets were crucial to the formation to life on Earth, doing the first mission to the last planet, Pluto—I consider these all big-ticket items. Just because Pluto or comets aren’t as big as Jupiter doesn’t mean they are not scientifically important—indeed, just the reverse is often true. Sometimes great things come in small packages.

But, honestly, the public just isn’t that interested in comets or the Kuiper belt. Most people have never heard of vulcanoids.

S: I think you’re right about the vulcanoids, but there are a lot of things people have never heard of that eventually come to be very important. Six hundred years ago, most people never heard of North America. Being a researcher doesn’t mean that you follow what is publicly appealing, because public interest generally lags scientific understanding. It’s our responsibility as research explorers to find out the lay of the land, not to follow a popularity contest about where our research should go.

What’s the most important thing about astronomy that everyone should know but don’t?

S: I’d say how ancient virtually everything is in space. Almost all of the galaxies, the stars, the planets, are billions of years old—a million times older than the Parthenon, and tens of millions of times older than the oldest human being who has ever lived. To me, that really puts a lot of things that happen in day-to-day life, or the news, in perspective. No matter how old you are, it makes you feel young. And—perhaps this is the best part—it makes you realize how audacious we are as a species that was “born yesterday” to think we can understand the universe!

What are we learning about comets?

S: Comets are very important in terms of the hazard they pose to Earth’s ecosystems over geological time, and they’re the best samples we have of the primordial material that formed the solar system. We know that comets bombarded Earth after its formation, and they brought a lot of water and more complex stuff to the young planet. Although I personally discount it, it is entirely possible that comets actually brought life to Earth—microspores or something from other systems where life had evolved. We won’t know until we bring back samples.

Many researchers who work on unmanned space projects take issue with all the money that goes into manned missions. You’ve worked both sides of the street. What’s your take?

S: NASA’s human exploration program did historic, even epic, things in the 1960s and early 1970s, but it has been on a leash ever since. We have the capability to do so much more, to do real geological field exploration of the moon and the asteroids and field expeditions to Mars. Instead, for the past three decades we’ve been relegated to nothing more than trips to low Earth orbit in space shuttles and a space station going around in circles growing plants and taking pictures of the weather. I hope that will change soon.

Why aren’t we doing more?

S: The political will hasn’t been there, and it’s so unfortunate. I think human beings are truly explorers at heart. The planets are the obvious next frontiers for human exploration. There’s no reason that we shouldn’t have a significant number of people living and working on the moon, doing geological studies of asteroids and pioneering the path to Mars. The technology is well in hand.

You’re not concerned about the dangers?

S: Of course I am, but danger is an integral part of true exploration. If you’ve ever been around aerospace vehicles, you know that a human being can get hurt in those big machines. Explorers 500 years ago faced a similar question: Is it too risky to sail to some unknown land in a rickety boat at the mercy of the wind? But look at what those daring explorations brought us in terms of the changes to the world. To be a great nation in the 21st century, the United States needs to explore the space frontier. If we choose this course, the road will be long and hard—and yes, dangerous. But so were the frontiers that this great nation took as previous challenges during the 18th, 19th, and 20th centuries. We should make a lasting commitment to the exploration of the moon and planets by both brave humans and sophisticated robots. We should inspire the world, and we should make history again. It’s something America does extremely well.

If you could go anywhere in the solar system for one week, where would you go?

S: I’d like to spend a week exploring Neptune’s giant moon, Triton. The Neptunian system is a scientist’s playground. Triton seems to be geologically active like there’s no tomorrow, even though it’s only 40 degrees above absolute zero there. To conduct a field operation on Triton would be beyond my wildest imagination.

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