Saturn, as observed with the VLT NAOS-CONICA Adaptive Optics instrument on December 8 2001 at a distance of 1209 million km. The intricate, banded structure of the planetary atmosphere and the rings can be seen along with the dark spot at the south pole at the bottom of the image. (Image: © European Southern Observatory) Astronomers have used the Very Large Telescope (VLT) to look at Saturn and Jupiter's moon Io, obtaining the sharpest-ever images taken with an Earth-based telescope.

The VLT is part of the European Southern Observatory (ESO) that the UK is set to join in the next few years. It consists of four 8-m telescopes, which combined make up the largest optical telescope in the world. Here one of the telescopes is used to observe objects in near-infrared light. The images are sharpened up by the NAOS / CONICA adaptive optics system which flexes the mirror to compensate for the turbulence of the Earth's atmosphere.

NAOS-CONICA image of Io obtained on December 5 2001. Many surface features can be identified including volcanoes and lava fields. (Image: © European Southern Observatory) The results are spectacular – the images of Saturn and Io are better than those obtained by the Hubble Space Telescope. Saturn's South Pole is in full view and has a recently discovered dark spot around 300 km across. Near the equator the remains of a five-year-old storm are visible as a bright spot.

Structures in Saturn's beautiful ring system are clearly visible with the two bright A- and B- rings and dark C-ring closer to the planet. Each ring section is divided into many further narrow rings.

Io has a diameter of 3660 km and takes 42.5 hours to complete each orbit around Jupiter. Just like the Earth's moon, it always turns the same side towards the planet. In 1979 the Voyager 1 spacecraft showed that it was the most volcanically active place in the Solar system with a surface covered in volcanoes and lava fields. Volcanic plumes throw material into space and the surface of Io is continually reshaped. All this activity is caused by its proximity to Jupiter – the gravitational force from the giant planet on one side of Io is less than on the other so the whole moon is stretched by tidal forces which melt the interior.