Just seeing it was an accomplishment.  The Hubble Space Telescope has spotted a tiny new moon, somewhere between 8 and 21 miles in diameter, in orbit around Pluto. 

Pluto was already known to have three other moons, named Charon, Nix and Hydra; this newest one is, for now, designated P4.

(Hubble's June 28 image.  Click on it to enlarge.) 

"I find it remarkable that Hubble's cameras enabled us to see such a tiny object so clearly from a distance of more than 3 billion miles (5 billion km)," said Mark Showalter of the SETI Institute in Mountain View, Calif., who led the observing program with Hubble.  NASA's New Horizons spacecraft is set to fly past Pluto in July 2015, and mission organizers want to know before then just what they should look for.

P4 first showed up in an image shot by the Hubble on June 28.  Its presence was confirmed by follow-up pictures shot on July 2 and July 18, says the Space Telescope Science Institute.  It may, in fact, have shown up in a long-exposure image in 2006, but until now, nobody was really watching Pluto's environs for anything that reflected so little light.

What might it be like on P4?  Dark and cold, for sure, but beyond that it's dangerous to guess.  It's too small to have much graviational pull, almost undoubtedly too small for its gravity to have pulled it into a spherical shape.  But does it have ice or rock, a shiny surface or a dark one?  Now begin the calculations so that New Horizons, as it races through the Pluto-Charon system, can shoot images of P4 on the way.

Obviously, it needs a more evocative name.  Pluto and its other moons have, by tradition, been named for mythological figures from the underworld.  P4's discoverers get the honor of picking a permanent name for their find; they may welcome ideas.