Sharpest ever picture of Pluto

It might not look very much, but this is said to be the sharpest picture of Pluto and its moons that has ever been taken. The astronomer who produced it says that, by using special techniques with a giant telescope on Hawaii, he managed to exceed the detail possible with Hubble.

The image combines two sets of exposures - one that recorded Pluto and its biggest satellite Charon and another that picked up the much fainter companions Nix and Hydra, the upper two starlike points.

The telescope, one of the twin Keck instruments on Mauna Kea, was tracking the Pluto system which means the background stars appear as caterpillar-like trails in the picture.

University of Hawaii astronomer David Tholen took 16 photos during a one-hour period on September 5. The 10-metre telescope was fitted with so-called adaptive optics which combines sensors and a flexible mirror to compensate for blurring turbulence in the Earth's atmosphere.

Dr Tholen said: "Several favourable factors occurred simultaneously to yield these spectacular images of the Pluto system. 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."

Pluto is so faint - more than a thousand times fainter than the faintest star you can see with your eyes alone - that most amateur astronomers have never seen it. It has an apparent magnitude of 14 and so is shown here in a cutaway disk alongside Nix and Hydra, which are much, much fainter at magnitude 23.

Dr Tholen added: "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."

He said that by measuring the tiny moons' precise positions, it would be possible to calculate their masses, or how much they "weigh". Astronomers have estimated that Nix and Hydra are less than 100 km in diameter, compared with 1,212 km for Charon and about 2,300 km for Pluto.

Pluto was discovered in 1930 and was considered the ninth planet until astronomy's controlling authority, the International Astronomical Union, demoted it to the rank of dwarf planet in 2006. But many scientists have protested the decision, particularly those in the United States from where it was the only planet to be discovered.

A leading protester against the IAU's decision was Alan Stern, chief scientist for the first mission from Earth to Pluto, New Horizons. You can see a Hubble image of Pluto and its satellites here.