Closing In on Planet Pluto: an Interview with New Horizons PI Alan Stern

Artist's impression of the New Horizons flyby in July 2015. (NASA/SwRI)
Artist’s impression of the New Horizons flyby in July 2015. (NASA/SwRI)

After more than nine years of traveling through space the New Horizons spacecraft is now in the home stretch of its journey, with less than 120 days and 143 million kilometers to go before it makes its historic flyby of the Pluto system on July 14. It will be the first time we get a good close-up look at the distant world which had for over seven decades held reign over the frozen edges of our Solar System as the outermost planet, much like its namesake governed the cold darkness of the mythological Greek underworld.

Discovered on February 18, 1930, the ninth planet Pluto lost its “full” planetary status in August 2006 as the result of a highly-contested decision by the International Astronomical Union (IAU) to create a new class of “dwarf planets,” of which Pluto, Ceres, and the then-recently-discovered Eris became charter members. Seven months after New Horizons launched, the edict did not sit well with both many members of the planetary science community and the public, who were suddenly informed that little Pluto just didn’t measure up and had to be let go… nothing serious, right?

Wrong. It was a serious scientific issue for many people, and especially for Dr. Alan Stern, Associate Vice President of Space Science and Engineering at the Southwest Research Institute in Boulder, CO and principal investigator of New Horizons. And understandably so – Dr. Stern’s planetary exploration mission is on its way to Pluto after all, and what it’s expected to encounter is far more complex than what’s implied by the diminutive moniker of “dwarf planet” (which, oddly enough, was coined by Stern himself in 1990.)

I had a change to talk briefly with Dr. Stern on Friday, March 13 – which, incidentally, was the 85th anniversary of Pluto’s first announcement to the world – and got some insight from him on the mission and what we can expect from the upcoming flyby, as well as his views on the whole “planet/dwarf planet” thing. (And yes, it certainly does still matter!)

Dr. Alan Stern, New Horizons PI, Southwest Research Institute
Dr. Alan Stern, New Horizons PI, Southwest Research Institute

Lights in the Dark: The New Horizons mission is now down to its final AU to go before Pluto… how does it feel to know the long-anticipated visit to Pluto is only a few months away?

Alan Stern: Well you know, when you launch on a mission that’s nine and a half years to reach the primary target, it’s always ‘very far away.’ I remember January of 2013 saying to my team my new favorite acronym was TYAN – ‘the year after next!’ And even then, although close, it was still the year after next. It was still quite a ways away. But it’s really accelerated, particularly since the calendar turned 2015. And now the last bit of sand is running out of the hourglass… soon it will be the month after next. Then it will be the week after next. Then the day after next.

LITD: It’s like a little kid waiting for Christmas; it’s always forever away…until it’s not.

Stern: Yep, I make that analogy a lot. Interesting you thought of it too. I refer to Pluto as a Christmas present that’s been sitting under my tree since 1989!

LITD: Do you expect to find any more moons around Pluto during the flyby? Or do you think Styx and Kerberos were the last ones?

Hubble image of Pluto's system of moons. (HST/NASA/ESA and M. Showalter)
Hubble image of Pluto’s system of moons. Styx was the last one discovered, announced on July 11 2012. (HST/NASA/ESA and M. Showalter)

Stern: I personally would be surprised to find zero more. That said, the whole gestalt of planetary exploration has been first missions always yield surprises. You could fit more moons, smaller than detection limits, and we’re going to find out. And it’s not going to be very long.

LITD: What season will it be on Pluto when New Horizons flies by? I understand that affects its atmosphere… whether it’s expanded out or ‘frozen’ down onto its surface.* Will the spacecraft be experiencing ‘Pluto in the spring?’

Stern: First I have to give you some background. Pluto’s seasons are controlled by energy inputs. And those depend on three things: how far Pluto is from the Sun, which has to do with its elliptical orbit; its polar tilt; and it also has to do with what the Sun is doing – solar minimum versus solar max. Those three things beat together in complicated ways. On Earth the only one that really matters is the second one, the polar tilt at twenty-three and a half degrees. But at Pluto all three are extreme. They can cancel each other out and all three can dominate at different times. So it’s not really a well-posed question to ask what season it is in the atmosphere. But you can ask what season it is on the surface, because that only has to do with the polar tilt. And what season it is has to do with what hemisphere it’s in, and Pluto is currently moving from an equinox to a solstice. It’s about halfway between the two right now.

*Note: Pluto’s atmosphere may actually not collapse at all, based on a recent model. Read more here.

LITD: Are there any specific observations or findings about Pluto that intrigue you most? As principal investigator for New Horizons, do you have a ‘pet Pluto project?’

Stern: I have a lot of pet Pluto projects so there’s no one. But I would say my interest as an explorer is revealing the Pluto system and a Kuiper Belt planet for the first time. And there are so many aspects to that that there’s no single individual research problem that even comes close to the whole package.

Read more: What We’ve Learned from the Kuiper Belt

Artist's impression of Pluto's surface. Image: NASA
Artist’s impression of Pluto’s surface. Image: NASA

LITD: Recently there’s been a lot of news about liquid water oceans being found in the outer Solar System. Do you think that Pluto may have a subsurface ocean of some sort?

Stern: Yes! There are geophysical models of the interior of Pluto and also its companion Charon – bigger than Ceres – which indicates possibilities of subsurface oceans in one or both. We know that Pluto is made of about 30 percent water ice, and Charon has about 50 percent of its mass in water ice. As you go down in the water ice mantle, the deeper you go and the warmer it gets because the overlying burden of ice makes more and more pressure. There’s a depth at which the temperature is warm enough to melt that ice into liquid. Now there are also other factors so that’s why I say there are some models that show it’s possible to have an ocean. Whether Pluto has an ocean may be something [New Horizons] can address… we may see geysers like Enceladus, we could see geological expression on the surface that indicates a soft underlying layer or liquid layer, there could be other tell-tales and we’ll be looking for that.

LITD: Wow. That’d be really fascinating. Now do you think all the events of the next few months will re-open the whole planet/dwarf planet debate anew or tip the public’s perception more strongly either way?

Stern: Well first of all the debate is open, and as you know many planetary scientists – maybe most – don’t buy what the IAU did. Astronomers aren’t very good planetary scientists any more than planetary scientists are very good astrophysicists. So the debate is on, and it’s wide-ranging as we learn more and more about planetary types in our Solar System and others. I think when the public sees the Pluto system they won’t know what to call it but a planet. You have an atmosphere, and seasons, and a core, and a hard surface, and moons… when you see an object in space like that on Star Trek you know what to call it. You don’t have to start calculating orbits and determining what has moved what out of its way, you just look on the viewfinder and you can tell.

Video: What is Pluto?

LITD: So the debate is really more semantic than anything else. (Note: this isn’t to downplay the seriousness of the need for a reliable categorization process, which many believe still hasn’t been found. Read more here.)

Stern: In planetary science we typically characterize objects by their attributes, not their location. The fact that Pluto is far from the Sun means that its zone is much larger than any other planet. In fact the Kuiper Belt is two times bigger than everything from the Sun to Neptune. So expecting a single object to clear that zone – even if the object were as large as the Earth – is an unrealistic expectation. Earth can clear orbits near the Sun because it’s not a very big zone. But put it out where Pluto is and it couldn’t do it. So I think the public will take one look at this and they’re going to feel like the IAU made fools of themselves. I don’t know a single planetary scientist – out of thousands – I don’t know a single one who says that the IAU decision was a good one.

LITD: Well. Maybe seeing what Pluto is really like after so many years of just being a distant ‘light in the dark’ (wink) will make everyone feel the same way. Personally I’m fine with having nine planets again, if not lots more!

Stern: Same here.


Many thanks to Dr. Stern for taking the time to share his thoughts!

New Horizons is currently getting closer and closer to the Pluto system every day. It is now less than one AU from Pluto, or less than the distance between Earth and the Sun, and 32.24 AU from Earth… that’s a light-travel time of almost nine hours! As of the time of this writing New Horizons has been traveling for over 3,342 days, and has recently performed a successful 93-second thruster burn on March 10 to adjust its course – the most distant maneuver of its type to be executed by a human spacecraft. Follow news from the New Horizons mission here (and of course here on Lights in the Dark!)

Timeline of the New Horizons mission. Download a large version here. (NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)
Timeline of the New Horizons mission. (NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)

“This is an amazing project — one that will go down in the history 21st century achievements. And the history-making is just beginning — in July we reach Pluto, humankind’s farthest exploration shore, to explore it and its fascinating system of moons for the first time!”

– Dr. Alan Stern, New Horizons PI, SwRI


  1. KG says:

    Reblogged this on The World Around Us.

    Liked by 4 people

  2. RFGrassi says:

    Reblogged this on The Next Generation and commented:
    The names of my husband and my son, together with mine and other thousands of names are on a golden disc that NASA has placed on the New Horizon that is nearly reaching Pluto. It comes in a moment that our family life is completely changing – possibly in a positive way.
    I think this is really for us a New Horizon. This is why I read with so much interest about this spacecraft I am particularly fond of.

    Liked by 6 people

  3. Jeff Barani says:

    Very interesting interview.
    Thanks for sharing it 😉
    Jeff Barani from Vence (France)

    Liked by 7 people

  4. girlthatsshy says:


    Liked by 4 people

  5. faisalfairuh says:

    well let’s hope the best for rest of its journey!

    Liked by 6 people

  6. thetankiptalk says:

    Reblogged this on thetankiptalk.

    Liked by 2 people

  7. jimtgammill says:

    Very cool, I think that it was great of Dr. Stern to sit down for the interview. The questions were very well thought out and helped shine some light on exactly why Pluto was “downgraded”. I was also glad to find out that I’m not the only one that still thinks of it as a planet!

    Liked by 5 people

    1. Jason Major says:

      Alan was a great interview. Very clear on his mission and passionate about planetary science.

      Liked by 1 person

  8. sonatano1 says:

    Man, I don’t know. Does it really matter what category we put Pluto into? I get that it’s a thing for Dr. Stern because New Horizons is his mission and all, but if Pluto is still classed with the other planets, we’ll have to add Eris, Sedna, etc. to the list, right? And new objects out there, too. That doesn’t seem practical.

    With Pluto, I did grow up learning it was a planet (I think I was in the very last generation that still saw Pluto on the “nine planets of the Solar System” charts in the classroom.) But it seems to me that Pluto getting its own class of dwarf planets puts it first among equals in a sense. Well, maybe there’s some politics here over funding that I don’t get because I’m not involved in this field.

    Liked by 3 people

  9. Bovasjohn says:

    Hope the best from pluto..

    Liked by 3 people

  10. makabba says:

    Reblogged this on makabba.

    Liked by 2 people

  11. Tips Sukses Kerja says:

    I hope one day my family go to pluto…its amazing traveling.

    Liked by 4 people

  12. tausifi says:

    Reblogged this on tausifi and commented:
    Secrets of planet

    Liked by 2 people

  13. bilalsh00 says:

    Reblogged this on billsh00 and commented:

    Liked by 2 people

  14. gymnastmya says:

    Very interesting🙏Well done💖

    Liked by 3 people

  15. zperskin says:

    Reblogged this on Zick for Zack.

    Liked by 2 people

  16. ahoyybrie says:

    Reblogged this on .

    Liked by 2 people

  17. That Pluto should be classified as a planet (speaking generally here) strikes me as more emotional than scientific. Nobody ever demands that Eris be classified as a planet. Not even Eris’ discoverer, Dr. Mike Brown, argues it should be a planet! So why should Pluto be one?

    I, too, had resisted calls for Pluto’s “demotion,” until reading a rational argument on the topic by Neil deGrasse Tyson convinced me that Pluto shouldn’t be thought of as a planet. Ceres was once considered a planet; nobody today would make that argument. Nor do I think Pluto’s current status takes anything away from Clyde Tombaugh’s feat — he discovered a new class of object more than 60 years before any others in the Kuiper Belt were found. That is just as incredible as discovering a new planet.

    As to the argument that Earth would not clear its path where it in the Kuiper Belt, I note that it would be hundreds of times more massive than anything else out there and would likely dominate its immediate surroundings, so I am not sure that is the most solid of arguments.

    Liked by 2 people

  18. aurita100 says:

    ¡Hola mundo! NO tienes dinero buscas una entrada de dolares de verdad sin invertir de tu bolsillo? te tengo la solucion clixsense la mejor pagina del mundo | ganadolaresdeverdad

    Liked by 2 people

  19. mirrorgirl says:

    I don`t know much about other planets, so reading about Pluto was a good experience. There is so many exciting things happening. Maybe we will have the chance to travel to places like that in a 100 years?

    Liked by 2 people

    1. Jason Major says:

      Maybe, but for now we will have to be happy with robot explorers like New Horizons!

      Liked by 1 person

  20. I love that all

    Liked by 2 people

  21. kyomben says:

    Waoh! This is really an interesting piece. The new definitions of the term planet has really put Pluto out to a dwarf planet, which makes wonder why Size was not considered, because I grew knowing Mercury as the smallest planet in the solar system. Dr. Stern has also made a vital point on the space to be cleared by one planet in the kuiper belt(Pluto). I look forward to a new discovery to set things straight.

    Liked by 4 people

  22. iDikko says:


    Liked by 1 person

  23. kachimeziri7 says:

    Reblogged this on chuksville and commented:
    Nice post

    Liked by 1 person

  24. ekavai1976 says:

    Scince for life.

    Liked by 1 person

  25. ekavai1976 says:

    Reblogged this on elviskavai and commented:
    Science for life.

    Liked by 1 person

  26. Joseph's Mommy says:

    Reblogged this on In Memory of Joseph DeNicola and commented:
    Joseph loved space, planets and the stars.. just like his mom…

    Liked by 1 person

  27. theoriginalimperfectwriter says:

    Reblogged this on Imperfect Writer: My Journey to Finding Myself.

    Liked by 1 person

  28. Paulo Silva says:

    I just find it amazing that we are able to send an object the size of a car billions of kms away. And such achievements are not even on the news because people prefer to know why a guy left a boy band.

    Science rules!

    Liked by 2 people

  29. LScott says:

    awesome, thanks for sharing

    Liked by 1 person

  30. rdxrobo says:


    Liked by 1 person

  31. So cool! Amazing to think that we’re capable of doing this and understanding it.

    Liked by 1 person

  32. Jack says:

    Reblogged this on The Missal.


  33. Great Article! Thank you!


  34. revoche says:

    That’s a great update. I don’t follow so much these days and had forgotten all about this mission.


  35. kamikazezealot says:

    Reblogged this on Ravings of a Madman.


  36. noftiana says:

    i want smart speak english, and then i want to moon


  37. Rayan says:

    Reblogged this on Rayan's Blog and commented:
    I’ve watched the BBC documentary about this mission 2 months ago and I can’t wait for July 14th …


  38. emelda200041 says:

    Very interesting!! Excellent I hope the best for rest the journey good luck


  39. davidjohncullen says:

    Reblogged this on davejohncullen.


  40. themachineheadchronicles says:

    Reblogged this on The Machinehead Chronicles.


  41. ingaavenue says:

    Very interesting


  42. dighe31 says:

    Reblogged this on prajaktadighe.


  43. dighe31 says:

    Reblogged on my blog prajaktadighe.


    1. dighe31 says:

      Very interesting !


  44. Arul says:



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