To Pluto and beyond

The New Horizons probe has provided vast quantities of data on Pluto, but what has it told us that we didn't alread know? Alan Stern is the Principal Investigator of the probe and told Graihagh Jackson how difficult it was to get the mission off the ground in the first place...

Alan - Well, New Horizons and a mission to the Pluto system spent about a dozen years seeking funding. The scientific community backed it very strongly, but a whole series of previous missions, some of them very fine missions collapsed for one reason or another. It wasn't the right NASA administrator. It wasn't the right funding time. But fortunately, people really persisted, the scientific community really stood up for this over and over, over again. In the end, because the National Academy of Sciences ranked this mission as the number one priority, we were funded in 2003 and that concluded that long battle from the late 1980s into the early 2000.

Graihagh - That must have been really hard for you. Why did you and your colleagues keep persisting?

Alan - Really, there were two reasons in my case. One was the science. It was so convincingly luscious and secondly, the opportunity to do a first exploration mission is essentially unheard of in our time. It's something out of the '60s and '70s, and '80s, and that was very attractive to many people to have a chance to be on a mission like that to be able to make really big discoveries. And so, people were really willing to back it and to persist to back it.

Graihagh - And why in particular Pluto?

Alan - Even in the late '80s and '90s, we knew that the Pluto system was a binary that it had formed very likely, very much like the earth-moon system so it gave us a chance to see an analogue of that giant collision process. We knew that Pluto had multiple ices on its surface, we had discovered Pluto's atmosphere at that time, and discovered that it had complicated vertical structure. Some evidence for hazes and some evidence for changing conditions, changing pressure in particular. These are three examples of quite a number of others that just convinced us that this was really quite a rich system. So, it was all quite promising.

Graihagh - Promising indeed and your persistence obviously paid off because you got the yes for the mission and it all took off splendidly.

Alan - So, it did.

Graihagh - Not long after New Horizons left Earth, Chris Smith interviewed Alan about the mission...

Alan - New Horizons is a NASA planetary mission or robotic mission that was just launched on the 19th of January to make the first reconnaissance of Pluto and then hopefully, to Kuiper Belt objects as well. The mission is the fastest spacecraft ever launched but because of the great distance, it will take about 9.5 years to reach Pluto before going on into the Kuiper belt. So, it's arrival will be in the summer of 2015 and when it arrives, it will be studying Pluto's system with cameras, spectrometers, and other instruments to give us a very good view of what kind of a system this is and how the different bodies are put together, what they're made of, their geology, and to study their atmospheres.

Chris - What does this actually add in addition to obviously some interesting and intriguing findings of Pluto? What will it add to our understanding of that segment of our Solar System?

Alan - Well, I think most importantly, we've discovered in the last decade something that was completely unexpected and that is, there's a whole new class of object out there, these miniature planets, these so-called ice dwarfs which vastly outnumber the rocky terrestrial planets the four gas giants instead of four of each of those. We think that there are hundreds of these ice dwarfs, Pluto being the first discovered and probably the best known example. So, this is going to give us our first handle on what this very populous, in fact, the most populous class of planetary body in our Solar System is all about.

Chris - Is it just because they're so far away, they're so difficult to see even with telescopes like Hubble that you need to send a craft there to look at them more closely?

Alan - That's exactly right. Pluto, the brightest of this population is itself 50,000 times fainter than Mars and 100 times smaller on the sky. But even over 75 years, we've only been able to eek out a very small amount of information.

Chris - Do you think there are any surprises lurking out there?

Alan - I think the lesson of planetary science is that we'll be quite surprised and it's embarrassing but true statement that across the Solar System, as we visited new types of bodies, we've typically found that our expectations way underestimated the richness of nature. So, I expect very much to be surprised.

Graihagh - The New Horizons probe has finally reached Pluto and we've got this data back. Were you surprised?

Alan - I was surprised, pleasantly so. I was surprised on several grounds. First, we didn't find any more satellites and we found that surprising because we had discovered satellite after satellite, after satellite from the Earth every time we looked harder. So I think, in the science team, there was an expectation that we would find one to many more moons in orbit in addition to the 5 that are already known, but it's clean as a whistle. We found none. We were surprised at the stunning complexity of Pluto's surface including units that are devoid of craters and telling us that they have been created in the very recent past that Pluto is still geologically active today on a massive scale.

Graihagh - What does that mean?

Alan - What it means is that there's still a source of energy inside the planet that drives the building of new surface units. You know, it is a fairly small planet. Let me make an analogy that if you have a small cup of coffee and a big vat of coffee, the small cup of coffee will cool off well before the big vat will because the small cup of coffee has a higher ratio of surface area to the mass of the coffee. And that lets it cool more quickly, in the same way small planets cool off - we think - more rapidly than big planets. So, mathematical models that describe the physics of these worlds predict that Pluto should have run out of that energy much earlier than today. And yet, we see evidence that it hasn't and we don't understand the source of that energy. Fortunately, 95 per cent of all the data that we took is still in the spacecraft. Meaning, there are many more clues and many more discoveries to be made to hopefully, guide us to whatever we're missing, that had been missing for decades and understanding the workings of small planets.

Graihagh - Why is it taking so long for all this data to be sent back?

Alan - Well by design, New Horizons had a big challenge and that was to create a mission of scope like the Voyager programme, but on a budget that was only 20 per cent as large. So, we had to make compromises. We elected at the very beginning of the project not to compromise on the amount or the quality of data, but to make our savings in other places. One example was that we used spacecraft hibernation to cut mission operations costs. Another is that we saved money on our telecommunication system. As a result, it took 9.5 years to get to Pluto and it will take us about an extra year to get all the data back to the ground. I don't find that extra 10 per cent to be daunting at all.

Graihagh - No, not at all. And now that it's flown by, what's going to happen next? Where's New Horizons going?

Alan - Well, we hope to choose a target about a billion miles further away in the next matter of weeks and to fire the engines or fly by in 2019.