Ever since the tiny moons Nix and Hydra were found circling Pluto, planetary astronomers have enlisted the largest telescopes on and off the Earth to try to learn more about them. It's painstaking work because the little moons are small and distant, making them incredibly faint. At 23rd magnitude, they're about 1/5,000 the brightness of Pluto — which itself is a challenge to spot even in a large backyard telescope.
But the trio of David Tholen (University of Hawaii), Marc Buie, and Wil Grundy (both at Lowell Observatory) have kept plugging away, recording these elusive blips repeatedly with the Hubble Space Telescope and with an adaptive-optics camera attached to one of the giant Keck telescopes atop Mauna Kea. They even dredged up a dozen revealing Hubble images taken before the moons' discovery in May 2005.
All that effort came to fruition this week with the release of a new view of Pluto's family. It's actually a composite of 16 individual near-infrared images, and both Pluto and Charon have been toned down to a tenth of their true brightness so they don't appear overexposed. You can see how the picture was put together and learn more about it at the website of the University of Hawaii's Institute for Astronomy.
Of course, Tholen, Buie, and Grundy didn't go to all this trouble just to get a pleasing family portrait. It's critical that astronomers determine the exact orbits of these bodies so that instruments on New Horizons can be pointed in the right directions when the spacecraft gets there in 2015.
Tholen presented the latest specs yesterday at the annual meeting of the American Astronomical Society's Division for Planetary Sciences. He had less than 10 minutes to convey all the details — I was scribbling furiously in the darkened room.
The three moons all lie in the same plane, to within a fraction of a degree, which coincides with Pluto's equator. By carefully tracking the motions of all four bodies, the team determined how much they perturb one another's movement. Those little tugs yield their masses and estimates of their diameters. Nix appears slightly brighter than Hydra and still seems to be the larger moonlet; they're about 55 and 45 miles (88 and 72 km) across, respectively.
Nix's average distance from the center of Pluto is 30,600 miles (49,240 km), taking 25.49 days to circle Pluto. Hydra is farther out, averaging 40,530 miles (65,210 km) away and taking 38.72 days to go around. None of the moons are locked in a resonance (that is, one moon's orbital period isn't some simple multiple of another's), though together Nix and Hydra do force Charon's orbit to be slightly out of round.
I've got more scribblings in my notepad, but let's wait to get the full story from Tholen, Buie, and Grundy. They've already submitted their results to the Astronomical Journal. But things are still too dicey to know where to point the instruments on New Horizons — for example, the orbital radii of Nix and Hydra are still uncertain by roughly 100 miles — so the team plans to be back at the telescope very soon.