Washington, Oct.13 : 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 favourable factors occurred simultaneously to yield these spectacular images of the Pluto system," said Dr. David Tholen, who performed the observations on September 5 using the adaptive optics system on one of Mauna Kea's twin Keck telescopes.

The new images, which have a resolution of about 35 milliarc seconds, 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.

In all, 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," he added.

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. (ANI)