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!)
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?
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.
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.
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!)
“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