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Tholen Takes Sharpest Picture of Pluto System

Pluto system.

The Pluto system. The region around Pluto and Charon (center) was reduced in brightness by a factor of ten so that all four objects could be shown individually in a single image. Nix and Hydra are the two small dots near the top right. The Pluto system moved with respect to the background stars during the one hour of observations, creating the star trails seen in this image.

Almost 30 years after the discovery of Pluto's large moon, Charon, IfA astronomer David Tholen has used a ground-based telescope to take an image of the Pluto system that exceeds the sharpness possible with the Hubble Space Telescope.

"Several favorable factors occurred simultaneously to yield these spectacular images of the Pluto system," said Tholen, who performed the observations during the early evening of September 5 using the adaptive optics system on one of Mauna Kea's twin Keck telescopes. "The atmosphere above Mauna Kea was particularly stable that night, upgraded sensors were installed on the telescope, and Pluto was at its maximum brightness, thereby giving the improved adaptive optics system more light with which to work its magic," he said.

The adaptive optics system uses a deformable mirror and sensors that allow it to compensate for turbulence in Earth's atmosphere that normally blurs the light from celestial bodies. The new images of Pluto are about 20 times sharper than the ones taken 30 years ago, making it possible to see details as small as a dime as seen from a distance of 64 miles.

Altogether, Tholen took 16 images of the Pluto system during one hour on the Keck telescope. When all 16 images were combined to form a single image, Nix and Hydra, Pluto's small satellites discovered with the Hubble Space Telescope in 2005, became clearly visible.

"It is our intent to obtain several more images of the Pluto system, hopefully with this same level of quality, so that we can track Nix and Hydra completely around Pluto several times," said Tholen. "By making extremely precise measurements of the satellites' positions, we will determine their masses by detecting the tiny displacements caused by their mutual gravitational attraction. Once the masses are in hand, we'll be able to say something more definitive about how big these new satellites are."

Astronomers have estimated that Nix and Hydra are less than one tenth the size of Charon, which itself is half the size of Pluto. Scientists planning the 2015 flyby of Pluto with NASA's New Horizons spacecraft are intensely interested in the results. "Something as simple as selecting the proper exposure time to snap images of Nix and Hydra with New Horizons depends on knowing how big they are and how reflective their surfaces are," said Tholen. "One of our goals is to have those answers well in advance of the flyby."

Related scientific article: http://arxiv.org/abs/0712.1261