New Horizons will reach Pluto in July, despite being cancelled twice during development. Alan Stern's determination was crucial to making the mission happen. In this illustration, Pluto's moon Charon is the crescent in the background. (Credit: JHUAPL/SwRI) You don't have to wonder what is on Alan Stern's mind. The planetary scientist and former NASA associate administrator is a relentless champion of all things Pluto; he is both the principal investigator and the prime mover behind the New Horizons mission, which will fly past Pluto and its moons this July 14. In advance of the encounter, Stern's passion is building to a white heat, and he is letting everyone know it. The excitement is infectious. Pluto is looking far more interesting than researchers realized just a few years ago. Ironically, its scientific importance has skyrocketed in the years since the International Astronomical Union demoted Pluto to "dwarf planet." Recent theoretical models indicate that the Kuiper Belt--the population of objects, including Pluto, that orbits beyond Neptune--is key to understanding the early evolution of the outer solar system. It is home to multiple big, round objects that record the movements of water and organic chemicals at the time when Earth was forming. Call these things in the Kuiper Belt dwarf planets, call them planets (or call them "Plutoids" and duck before Stern comes after you), whatever. They are major players in the sun's family, many of them larger than any asteroid, and Pluto is the brightest and most complex of them all. Stern is a Pluto obsessive, but more and more it looks like the science is on his side: Pluto really is something special, and the New Horizons encounter promises to be a unique experience. Here, Stern makes his case--and reveals surprising details about another great mission that almost happened.

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You describe New Horizons as the first mission to the outer solar system—a description that would surprise a lot of people who work on, say, the Cassini mission at Saturn.

Alan Stern: One of the implications of the discovery of the Kuiper Belt and its many small planets is that many scientists now think of the solar system as having not two but three zones. These are, as you go outward from the Sun, the inner rocky terrestrial planets [including Earth], the ice and gas giants [Jupiter and its kin], and the Kuiper Belt—the largest of the three zones, and the one with the most planets. When you think of the architecture of the solar system this way, you see that the missions that explored Jupiter, Saturn, Uranus, and Neptune were really missions to the middle solar system. Then New Horizons becomes the first true mission to the outer planets, the first probe to explore the third zone.

The whole “is it a planet” debate keeps coming up, but it seems more confusing than enlightening. I liked the recent essay by William McKinnon of Washington University, who defined Pluto in terms of its scientific significance. He called it “a beacon to an unexplored solar realm" and "sentinel of the third zone."

No question, Pluto is the belle of the ball. It’s got everything! There are lots of really interesting little planets out there in the Kuiper Belt, but Pluto’s the only one that’s got all the cool attributes. It’s the only one with an atmosphere that we know of, it’s a binary planet, it’s got seasons and global change, it’s got more kinds of volatiles on its surface than any other planet out there, and it’s got a really complicated satellite system. I think that if you asked 100 leading planetary scientists and said ‘make a table, list the interesting aspects of dwarf planets in the Kuiper Belt’, the list for Pluto would be longer than for any other object on every person’s list. That’s not me cheerleading, it’s just a statement of fact. We know more about Pluto, but it seems to have all the goodies. It’s the whole package.

Alan Stern: Pluto evangelist, private spaceflight entrepreneur, pilot, self-described sonofabitch. (Credit: Dan Durda) Since Pluto is the biggest and most interesting of the objects in the third zone, why not let its name define the population? Why not simply call them all “Plutoids,” with Pluto as the type specimen? Please don’t go there, I hate it. It sounds like something else—it sounds like a hemorrhoid. How about we call them ‘the most populous class of planets in the solar system’? [Note: in 2000, long before the “dwarf planet” controversy, Stern and Hal Levison published a thoughtful paper on the best way to define a planet. In it, the two authors referred to Pluto as an “unterplanet.” PDF link here.] Still, we need a collective name for those things; “Kuiper Belt Objects” and “Trans-Neptunian Objects” are awfully clunky terms. And isn’t Pluto the archetype of that whole group? Actually, it’s not the archetype of Kuiper Belt Objects. People, including astronomers, often mix together everything in the Kuiper Belt as if they’re all identical. You have objects that are the size of city blocks, others the size of mountains, others the size of asteroids, and others much, much larger. Pluto’s circumference is the same as a drive from Manhattan to Moscow. Given this range in sizes, and the diversity of their attributes, Kuiper Belt Objects are just not all the same. Lumping them into a single category is detrimental to good science. For example, people were saying, “Oh there are going to be thousands of these KBOs” [when arguing that Pluto shouldn’t be called a planet]. Well, that’s got nothing to do with Pluto, any more than the thousands of near-Earth asteroids should be confused with Earth or the other terrestrial planets that orbit in the same region. There are 6 or 16 or 60, depending on how you count it, small planets in the Kuiper Belt. If you go down to hydrostatic equilibrium, the number is in the dozens. Pluto is the archetype of that population, but not of the entire Kuiper Belt, which is dominated by comet-sized objects. Let’s talk about those things that set Pluto apart. Its atmosphere, for instance, seems unique in the Kuiper Belt. Even Eris doesn’t have that, right? What else is unique about Pluto? There’s currently no atmosphere on Eris to our knowledge. An atmosphere makes for volatile transport, and that in turn makes for interesting surface effects. And then you have this big satellite system. Eris apparently has only one moon. Pluto has five [and New Horizons may well discover more]. On its own, Pluto’s main moon Charon is one of the largest objects in the Kuiper Belt. It’s quite a bit bigger than Ceres.

The Kuiper Belt and its surrounding region, the Scattered Disk, make up the largest zone of the planetary solar solar system. Pluto is in the middle of the action. (Credit: NASA) After the July encounter with the Pluto system, New Horizons will continue on to another target in the Kuiper Belt. Have you decided yet exactly where you will go? Probably to the one we call PT1 [Potential Target 1, a 40 to 70 -kilometer wide object] but we could also go to PT3 [Potential Target 3, a little bigger]. We have to decide in August. Once we do--and if NASA approves the mission extension--we plan to fly as close to that object as we will fly to Pluto, or even closer. The practical constraints on how close we go will center on how well we can navigate to it. That will have to do with how quickly we can home in, and how much fuel we have to home in, during the “end game” weeks of the encounter with the target. We will necessarily have only a short arc on its orbit, 5 years out of a 400 to 500 year orbit, so the homing navigation to the chosen KBO will be one of the most challenging and exciting parts of plotting out the flyby. For PT1, we can probably acquire it with the LORRI camera [on New Horizons] 5 to 6 weeks out. That means, in the space of 5 to 6 weeks we have to do the optical navigation and get the images to the ground when both our power and communications limitations are greater than at Pluto; it’s harder to transmit, calculate the answers, design burns, fire the engines, and then do that again two or three times to home in. We’re going to be successful at it, but it’s going to be…very exciting, very challenging. Ultimately, our ability to home in is going to limit how close we can come. We may have to design two flyby options, a really close one and a more Pluto-like flyby. Then based on what we end up with after all the targeting and homing in, we’ll select option A or B and upload it to the spacecraft to execute. What about a follow-on mission to the Kuiper Belt? How do we go about building a whole exploration program for the outer solar system—for instance, what if we wanted to go to Eris next?Eris is particularly difficult because it’s so far away. But if you want to go to Orcus or Ixion or Quaoar and others, I have an idea in mind about how you do it. You put some sonofabitch in charge—probably someone even persistent more than me--who’s willing to say no to cost overruns Then you stamp out four or five New Horizon II’s, and you use giant-planet flybys to target different small Kuiper Belt planets with each. One might be a Saturn gravity assist, another might be a Uranus gravity assist. Giant planets give you the speed to go there, and you get a giant planet flyby out of each one, so there’s some science in that—especially with Uranus and Neptune [which have not been seen up close since the 1980s]. And each would reach a different Kuiper Belt dwarf planet. If you have identical spacecraft, you buy all the parts with one design, only have to construct one set of flight and ground software, and you have one mission control. You don’t have to launch them all at once, you can launch them over a decade to peanut-butter-out the budget. You buy all the parts together, but you build them serially, so you have a fairly small team that never really becomes a huge part of the budget. As soon as you let then evolve the cost goes through the roof. They all have to be the same, and that means you have to make compromises. So you think slow and steady is the best way to go? There’s another way to explore the Kuiper Belt, which is with a flock of New Horizons-like spacecraft all launched on a single SLS [Space Launch System rocket] to Jupiter. Then from there you have different aim points at Jupiter, to send each of the spacecraft to different KBOs. That approach costs a lot more up front, because SLS is expensive, you have to build the spacecraft all together, and you have to get all the plutonium ready together. But it’s definitely a second way that probably can save money in the long run.

New Horizons will visit a 50-kilometer-wide Kuiper Belt Object around January 2019, assuming all goes well with the Pluto flyby. From that dim, dark location, the probe will fill in our picture of the true outer solar system. (Credit: JHUAPL/SwRI)

You actually had a plan to do this kind of bundled mission by creating a companion to New Horizons. What happened there?

We wanted to build New Horizons II from the spares left over from New Horizons. It didn’t happen, obviously, but we got pretty close. We had enough parts, including an RTG [radiothermal generator] and plutonium, to launch. The mission was Jupiter-Uranus and a big Kuiper Belt Object. We had two or three targets to choose from; we could have done it for something close to $400 million, which is about half the cost of NewHorizons. It was to be built to the same design, and to use the same launcher. We’d already done most of the nuclear launch approval. The idea was that if New Horizons had failed, we’d go to Pluto [with New HorizonsII], and if New Horizons succeeded, we’d go with a launch to a different big KBO. And we’d have reached Uranus at equinox, which means you could see all of the system—the satellites, the planet—instead of doing another solstice mission [as with Voyager 2, when half of the planet and its moons were in constant shadow]. New Horizons II would have arrived at Uranus about now…in 2017, actually, then it would have gone on to another Kuiper Belt planet like Pluto. But it was shot down by people who had other plans.

I’m excited that it looks like Europa Clipper is really going to happen, but I’m worried about the looming drought of new planetary missions at NASA.

I am too, but every decade we go through this [kind of concern]. I guess I’m just impatient. I want to go to Titan and Enceladus, I want to see more of the Kuiper Belt. I want to live to see all these things. I’m with you!