Almost 30 years after the discovery of Pluto's large moon, Charon, a University of Hawaii astronomer 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 Dr. David 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 natural seeing was better than average that night, more sensitive wavefront 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, which have a resolution of about 35 milliarcseconds, are nearly diffraction limited at the 1.6-micron wavelength used for the exposures. They are about 20 times sharper than the images of Pluto taken 30 years ago.

Altogether, Tholen took 16 images of the Pluto system during one hour on the Keck telescope. When he combined all 16 images to form a single image, Nix and Hydra, Pluto's small satellites discovered with the Hubble Space Telescope in 2005, became clearly visible. The new satellites are both about visual magnitude 23.5 compared with 14th magnitude for Pluto.

"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 100 km in diameter, compared with 1212 km for Charon and about 2300 km for 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."

PICTURE CAPTIONS

Figure 1: Nix and Hydra. This image combines all 16 exposures, with the contrast adjusted to show Pluto's new satellites Nix (left) and Hydra (right) as the small dots in the upper right. Both Nix and Hydra are about 5000 times fainter than Pluto, thus both Pluto and Charon are washed out in this image. The Pluto system moved with respect to the background stars during the one hour of observations, leaving the stars trailed in this image. Photo by David Tholen.

Figure 2: Pluto and Charon. This image is just one of the 16 exposures, with the contrast optimized to show Pluto (left) and Charon (right). Photo by David Tholen.

Figure 3: Charon's motion. This two-frame animation shows Charon moving slightly counterclockwise with respect to Pluto during the one hour of observation. Photo by David Tholen.

Figure 4: The Pluto system. The region around Pluto and Charon was reduced in brightness so that all four objects could be shown individually in a single image. Photo by David Tholen.

For comparison, the discovery image of Charon (1978, James Christy) is available at http://updatecenter.britannica.com/eb/image?binaryId=4297&rendTypeId=4

Founded in 1967, the Institute for Astronomy at the University of Hawaii at Manoa conducts research into galaxies, cosmology, stars, planets, and the sun. Its faculty and staff are also involved in astronomy education, deep space missions, and in the development and management of the observatories on Haleakala and Mauna Kea.

Established in 1907 and fully accredited by the Western Association of Schools and Colleges, the University of Hawaii is the state's sole public system of higher education. The UH System provides an array of undergraduate, graduate, and professional degrees and community programs on 10 campuses and through educational, training, and research centers across the state. UH enrolls more than 50,000 students from Hawaii, the U.S. mainland, and around the world.

Electronic version with images: http://www.ifa.hawaii.edu/info/press-releases/PlutoPictures/Pluto-Tholen-10-07.html

Contacts:

Dr. David Tholen
Institute for Astronomy
University of Hawaii at Manoa
Honolulu, Hawaii 96822
1-808-956-6039 (best time to call is afternoon of Monday, October 15, HST)
tholen@ifa.hawaii.edu

Mrs. Karen Rehbock
Assistant to the Director
Institute for Astronomy
University of Hawaii at Manoa
1-808-956-6829
rehbock@ifa.hawaii.edu