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July 2005

Two for the Price of One

Hard to Study

Charon is so far away and so small in our telescopes that it has been hard to study, even with modern technology telescopes. Indeed, in the 27 years since Charon's discovery, only a few key facts have been securely determined about it.

Strikingly, Charon (some 1,200 kilometers across in its own right) is half the size and about 10% the mass of Pluto, making the pair a true double planet. Equally striking is the fact that, like the Earth-Moon pair, the two bodies in the Pluto-Charon pair are quite different from one another. Like Earth, Pluto is larger, more reflective, possesses a complex surface composition rich in volatiles, and an atmosphere. Charon, like Luna, is smaller, less reflective, devoid of the exotic ices (like methane and nitrogen ice) on Pluto's surface, and Charon has no atmosphere. The table below summarizes the key facts we know about Charon and how Charon compares to Pluto in these regards.

Ten years from this month, we hope that New Horizons will expand our knowledge of Charon dramatically, for the complement of sensors New Horizons carries will make the reading of Charon's story as easy, as they say, "as taking candy from a baby."

Indeed, the study of Charon by New Horizons will be fascinating. We plan to make detailed geologic maps, fine scale composition maps, thermal maps, and a very tight search for even the faint trace of an atmosphere. In fact, if all goes as planned, we will know more about Charon than about any of the moons of Uranus or Neptune.

Among the many scientific questions we want to address on Charon, are the following:

  • Was Charon geologically active, and if so, for how long?
  • How old is Charon's surface?
  • How does Charon's surface age compare to Pluto's?
  • Is Charon internally differentiated into layers, or not?
  • Does Charon show evidence of a past atmosphere?

And there are many more things we want to learn about Charon from New Horizons. One twinkle in my own eye is to use the Pluto-Charon system as a template of sorts to clarify what we know about other primary-satellite pairs in the Kuiper Belt. We also hope to see what the Pluto-Charon system can tell us about its origin, and by inference, the origin of the Earth-Luna system.

With Charon being large enough to be considered a dwarf planet in its own right were it in orbit around the Sun, it is no exaggeration to say that the New Horizons Pluto-Charon mission will very much provide a "two for the price of one" flyby reconnaissance mission.

So stay tuned: If all goes well, that flyby will occur 10 years from this month. That may seem like a long way off, but it's not so bad. 1995 doesn't seem so far ago to me; heck, by then many of us had been at work to get a Pluto mission for 7 or 8 years!

-Alan Stern

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