Pluto: The Last Picture Show

In his second "insider blog" about the New Horizons mission, principle investigator Alan Stern offers a look at what we might find at Pluto.

It sounds like science fiction, but it's not: NASA's New Horizons spacecraft is on final approach to the Pluto system! After 112 months in flight, the fastest spacecraft ever launched is now barely 60 days from its destination. It will be closest to Pluto on July 14th at 11:49:58 Universal Time.

So what might we find when the spacecraft finally gets there?

Before answering that, I have to admit that this kind of speculation is both dangerous and oftentimes wrong. Remember: New Horizons is a "first-reconnaissance" mission to a wholly new kind of planet. As history has shown, that just increases the opportunity for me to be wrong!

After all, consider: No one expected to find large numbers of dry riverbeds and fluvial features on Mars when it was first explored by NASA's Mariner spacecraft. Few expected either volcanoes or internal oceans on the satellites of Jupiter, or geysers on Triton, or a planet stripped of its mantle at Mercury. The hallmark of first-reconnaissance flybys has been their tendency to surprise and make advance predictions look quaint — usually because we underestimate the richness and variety of expressions that nature presents in our solar system. I hope, and I bet, that the same will be true when the Pluto system is revealed in July.

And revealed it will be. New Horizons is a highly capable, 21st-century spacecraft, and it carries an amazing battery of advanced scientific instruments to conduct the first close-up reconnaissance of the planet and its five already known moons. This payload includes:

• Ralph: a color and panchromatic imager and infrared spectral composition mapper to map the surfaces, surface compositions, and temperatures across Pluto and its satellites.
• LORRI: a long-focal-length visible panchromatic imager to make both distant and hi-res maps (including stereo topography) of Pluto and all its satellites.
• Alice: an ultraviolet imaging spectrometer to determine the composition and density of Pluto's atmosphere and to search for an atmosphere around Charon.
• REX: two radio-science payloads to probe the vertical pressure and temperature profile of Pluto's atmosphere and to make thermal measurements of both Pluto and Charon.
• SWAP and PEPSSI: charged-particle detectors to sample material from Pluto's escaping atmosphere and to detect any magnetic fields associated with Pluto or Charon.
• Venetia Burney Student Dust Counter: to assay the dust density in the Pluto system; built by students at the University of Colorado.

Having worked toward getting to the Pluto system explored since 1989, I can tell you that I often think of Pluto and its moons as holiday presents under a tree I've been waiting to open and reveal for a very long time — but, this year, the holidays come in July!

Pluto: Frigid and Mysterious

Since its discovery 85 years ago, we've learned surprisingly little about Pluto (and, in time, its moon) from afar. About all we know is as follows:

Pluto has very distinct surface markings, including apparent polar caps, and it has an atmosphere (mostly nitrogen). We know that Pluto's interior is primarily made of rock — about 70% by mass. Also, Pluto-Charon constitute a true binary planet, with a barycenter (center of mass) situated in the open space between them. We know Charon is a "rising star" among the solar system's icy bodies, with evidence for recently created surface ices, possible internal activity (hinted at by the spectroscopic discovery of ammonium hydrates a few years ago), and some likelihood of an atmosphere itself — perhaps gas that was siphoned off Pluto! As for the small satellites — Styx, Nix, Kerberos, and Hydra — we know very little about them beyond their orbits and crude colors. Soon all six of those points of light, planet and moons, will be real worlds thanks to NASA's New Horizons.

So, we're just two months out — it's nearly show time. What will we find? Not to tweak you, but I don't know. No one does. That's what makes this distant exploration so very exciting, so suspenseful, and so wonderful!

But even if we don't know, what might we look out for? I'd say: look for more moons, possibly even rings, and look for a complex geology story on Pluto that perhaps involves the migration of ices and gases from one place to another. You might even seek evidence of liquids — inside or just possibly on — the surfaces Pluto or Charon or both. Clouds in Pluto's atmosphere? Maybe. What about impact basins and fresh craters, revealing new insights into the Kuiper Belt's history and population? I'd say that's a good bet. What about aurorae, geysers, or volcanoes? Perhaps. All of these are possible. But who knows whether we'll find most or very few of them . . . or even something more wonderful?

What I do know is this: We haven't done any interplanetary exploration like this since Voyager 2 reached Neptune in 1989 — and nothing like it is planned, ever again, by any space agency.

So tell your friends, your family, your neighbors, and your coworkers to come witness historic exploration in the making. Tell them to tune in and turn on to "The Last Picture Show." It's coming, in mid-July, to a screen near you.

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Planetary scientist S. Alan Stern of the Southwest Research Institute is the Principal Investigator of NASA's New Horizons mission to Pluto. He is writing a series of blogs for Sky & Telescope before and during the spacecraft's historic encounter.