8) We have maps of its surface, even though telescopes can barely see any features.
At Pluto's closest distance, it's just
barely resolvable (that is, seen as a disk and not a single dot in the sky) even using cameras on board Hubble Space Telescope. So how did we ever get maps of its surface?
By being clever. Imagine I have a ball, and I paint one hemisphere black and the other white. Now I get a friend to hold it a long way off, where to me the ball is so small it looks like a dot. If my friend slowly spins it, I'll see it get brighter when the white side faces me, and darker when the black side does. So I can make a crude map of the ball even though it's unresolved to me!
You can imagine that if you can precisely measure the amount of light you see from the ball, you can make even better maps. You could tell if just one hemisphere is painted, or if it's splotchy, or striped, or lots of other details.
And that's just what astronomers Alan Stern and Marc Buie did back in 1996.
They used a camera on Hubble to precisely measure the amount of light detected from Pluto and its moon Charon. They did this over one rotation period -- a "day" -- of Pluto, about 6.4 Earth days. By measuring the fluctuations in the light received, they were able to make the crude map displayed above.
If Stern's name sounds familiar, it should: he's the Principle Investigator for
the New Horizons Pluto mission, which will fly by the distant orb in 2015 and give us far better maps. Although the encounter will be brief, I expect we'll learn more about Pluto in a few days than we will have in the 85 years since it was first discovered.
Alan is a friend of mine, and we've had some lively discussions this past year over Pluto's planetary status, and what it means to be a planet. Because he's an expert on Pluto, I asked him to give me his opinion on some things you don't know about Pluto. The last two entries in this tour were his suggestions, so I'm indebted to him. Thanks Alan!