Monday, October 18, 1999 Published at 15:32 GMT 16:32 UK
Neptune brought into focus
The Keck telescope image shows Neptune's clouds
By BBC News Online Science Editor Dr David Whitehouse
New cameras mounted on two of the world's largest telescopes have provided astronomers with some of the sharpest ground-based images of the planet Neptune.
The images show the importance of the technique called "adaptive optics" (AO). This allows astronomers using ground-based telescopes to produce images at least as sharp as those from the orbiting Hubble Space Telescope.
The AO system used a mirror positioned between the main mirror and the light detector. The secondary mirror is adjusted up to 500 times per second by computer to compensate for the rapid distortions caused by the Earth's turbulent atmosphere.
The images reveal a major cloud feature about the size of Europe in the planet's southern hemisphere. Many other clouds and cloud bands are also seen.
A similar device has also been mounted on one of the pair of much larger 10-metre (400-inch) telescopes at the Keck Observatory in Hawaii. It has provided the best images of Neptune taken from the ground.
Those images, although taken three months earlier on 24 May 1999 also show the very powerful storm in the upper atmosphere.
Both observatories are planning follow-up observations to study the changing dynamics of Neptune's cloud systems and to follow the expected decay of the Europe-sized cloud mass.
Gas-giant Neptune is the eight planet from the Sun and orbits it every 165 years from a distance of 4.5 billion kilometres. It was almost completely unknown before the Voyager 2 spacecraft flew past it in 1989. Voyager saw numerous storms in the planet's atmosphere including a large "Great Dark Spot".
The atmosphere is chiefly composed of hydrogen but just one percent helium is enough to turn the atmosphere a vivid blue. Subsequent observations made with the Hubble Space Telescope have shown changes in the cloud patterns, with even large features like the Great Dark Spot disappearing over time.