Interview to Alan Stern: what a wonderful world called Pluto

“Who would have expected a blue sky in the Kuiper Belt? It’s gorgeous!”

Alan Stern
© Alan Stern

Pluto has become the most famous Dwarf Planet of the history. New Horizons has been recognized as the most exciting space mission during the past two decades and the exploration of trans-Neptunian minor planet is fascinating for all of us, lovers of the sky.
The enthusiasm that involved the entire world’s population is extraordinary. For example, think about the hashtag #PlutoFlyby; it has been used thousands of times, sparking irony, curiosity, wonder and amazement among social network users.

Cristina Dalle Ore

Now that we are, finally, getting back New Horizons data, we are confirming Pluto’s extraordinariness, day after day. Data-packets take a long time to download, because of both the significant distance (about 5 billion kilometers) and the large size of the files. “Receiving files from New Horizons is like downloading from the web a big file using an old modem.” – Explained Cristina Dalle Ore, NASA principal investigator and deputy leader of the “composition” of New Horizons, in our interview last month. “Each pixel refers to an area several kilometers wide” – she added.

October’s discoveries left everyone breathless: the Pluto’s atmosphere is as blue as Earth’s. In addition, on the surface of Pluto, the New Horizons spacecraft has observed the presence of glaciers. In September, Pluto has been observed for the first time while opposed to the sun, and it attracted the curiosity of scientists and enthusiasts. The photographic processing, though, was possible just in this month, when the color’s addition revealed staining atmospheric.

Pluto's Eclipse
NASA/New Horizons
Description: Portrait of Alan Stern, Div. 15 Associate Vice President-R&D Date photographed: Charge number: Publication: Contact name: Photographed by: Department name: Division:
Alan Stern, credits:

The image of Pluto, wrapped in the charming and mysterious atmosphere blue, makes us perceive this small and distant celestial body like a less hostile world and a more similar one to our planet.

“Who would have expected a blue sky in the Kuiper Belt? It’s gorgeous!” – said Alan Stern, the scientific director of New Horizons. “Pluto is showing us a diversity of landforms and complexity of processes that rival anything we’ve seen in the solar system. If an artist had painted this Pluto before our flyby, I probably would have called it over the top but that’s what is actually there.” – he declared a while ago.

Cathy Olkin
Cathy Olkin, credits:

The latter statement encouraged me to contact him, sparking in me a multitude of questions. After some research on the web, I could trace his email and, at the contrary of what I thought, get in touch with Alan Stern has been easier than expected: he was very nice and helpful. Thanks to the excellent work done by Dr. Cathy Olkin, who collected for me Stern’s considerations, I received the answers in one night record time, between a coffee’s thermos and another one. Thank to Dr. Stern and to Dr. Olkin!


Acqua su Plutone
NASA/New Horizons

Everybody knew Pluto was teeming with surprises. Nevertheless, when we all have seen it with a blue sky as well as the earth, we were astonished. When I saw the famous amazing picture, I told to myself “Holy hell! Did someone have fun using Photoshop here?”. Actually we know that Pluto’s atmosphere is way different from the Earth’s one. Could you clarify it?
Pluto’s atmosphere is much thinner than Earth’s with an atmospheric pressure that is 100,000 times smaller.  We see hazes everywhere on Pluto and they extend more than 100 miles up in the atmosphere.  It is the small haze particles that make the atmosphere on Pluto look blue.

New Horizons localized H2O, precisely, ice. Did you expect to find it on Pluto?
We had been studying Pluto spectroscopically from the ground for decades and one of the things were were looking for was H2O.  But we can’t see the signature of water ice on Pluto from Earth.  We expected that water ice is on Pluto but hadn’t clearly seen the signature of it until New Horizons got high resolution spectroscopy of the surface.  We have detected localized patches of water ice.

It would seem that the ice is present mainly in the reddish zone of the dwarf planet. This could seem unusual for not expert. Could you tell us something more about it? Is there a correlation between ice and tholins?
We see the water ice mainly where there is a distinct red color on the surface, but not everywhere there are tholins. It could be that mixing the water with the tholin produces that distinct red color.

Technically speaking, how are the data analyzed? How is it problematic to lead to scientific conclusions? How is important the risk to be deceived by the appearance?
The Ralph/LEISA instrument is an imaging spectrometer which allows us to record a spectrum of the light from 1.25 to 2.5 microns for each location across the surface of Pluto down to the resolution of the observation.  We have observations with a resolution of less than 10 km/pixel giving us the ability to look at the spectrum and therefore composition of certain landforms. The spectroscopic signatures of certain molecules (like methane ice, carbon monoxide ice, nitrogen ice, and water ice to name a few) are very clear with specific features.  When we see the signature of these ices it is clear.

New Horizons is, surely, one of the most exciting mission of the last twenty years. What will the future goals be?
The New Horizons team will be writing an extended mission proposal to NASA to conduct a flyby of a Kuiper Belt object in early 2019. There is a small Kuiper Belt object, named 2014 MU69, that New Horizons could fly past to gather more information about the bodies in the Kuiper Belt.  This body is much smaller than Pluto, about 30 km across.

Until what date will you be in communication with it?
We will be able to communicate with the spacecraft into the 2030s.

Thanks to Dawn probe we are learning a lot of things about Ceres too during last months. What are the analogies between Ceres and Pluto? Are they similar?
It has been exciting to learn so much about our solar system this year.  When I compare Ceres and Pluto, I see a lot of differences.  They inhabit different parts of the solar system which Ceres in the asteroid belt and Pluto in the Kuiper belt.  And as a result, Pluto’s surface is dominated with ices while Ceres is not.

Space missions, such as New Horizons and Rosetta, are paving the way for long-term space programs. According to your opinion, will the probes contribution help the “human trip” to Mars?
Yes, I think robotic missions like New Horizons and Rosetta pave the way for long-term space programs.  By exploring the worlds in our solar system, we are learning more and more about the processes that occur.  Increasing our knowledge of the solar system helps to understand the issues and concerns that might face humans on a trip to Mars.


Pluto, credits: NASA/New Horizons

Now that we know better Pluto and surroundings (rather lacking of protoplanetary rubble) do you think is still right define Pluto as a Dwarf Planet?
I always call Pluto a planet.  It seems like the right classification for this world with interesting geology, an atmosphere and 5 moons.

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Nato a Foggia, frequenta il corso di laurea in Scienze Geologiche presso l'Università degli Studi "Aldo Moro" di Bari. Appassionato di astronomia e giornalismo si dedica alla divulgazione scientifica intervistando diversi personaggi della scienza come gli astronauti Umberto Guidoni e Maurizio Cheli e l'astronomo Alan Stern della NASA. Scrive per "Le Stelle", la rivista astronomica fondata da Margherita Hack, "HuffPost Italia" e "Il Messaggero". In passato ha collaborato con "BBC Scienze" e "l'Espresso". Nel 2016 il CNR (Consiglio Nazionale delle Ricerche) e l'Associazione Italiana del Libro gli hanno conferito il Premio Nazionale per la Divulgazione Scientifica.