The Hubble Space Telescope that peers across the cosmos also is giving astronomers new insight into our own solar system.
It has, for example, ''turned our understanding [of Neptune] upside down,'' says planetary physicist Heidi Hammel of the Massachusetts Institute of Technology in Cambridge.
Dr. Hammel leads the team that took the recently released images of that planet that are reproduced here. They show the changing cloudscape of an alien atmosphere that is more dynamic than scientists had expected. Neptune was 4.5 billion kilometers (2.8 billion miles) from Earth at the time.
From its discovery in 1846 until the Voyager 2 spacecraft swept past in 1989, this most distant of major planets was a tantalizing mystery. That mission has turned what even the best telescopes had shown as a fuzzy blob into a recognizable world. Now astronomers can continue to study that world to learn how an atmosphere circulates around a planet that is 30 times more distant from the sun than is Earth and whose diameter is four times as large as that of our own world.
The spacecraft sent back a stream of images showing an atmosphere with large cloud systems and a prominent southern hemisphere dark spot. With only weak solar heating, scientists expected the general appearance of this atmosphere to change slowly. But the Hubble telescope's follow-up studies are showing surprising changeability.
Images made last July by Dave Crisp and his colleagues of the Jet Propulsion Laboratory in Pasadena, Calif., showed that the southern dark spot had vanished. They also showed a major northern-hemisphere cloud where Voyager had seen only small cloud systems. By the time the Hammel-led team took pictures in October, that cloud had brightened considerably. And a new dark spot had suddenly appeared in the northern hemisphere.
Moreover, the team imaged clouds that appeared within as little as 17 hours.
The ability to see such changes astounds even the scientists. ''Just resolving cloud structure at all [at Neptune's distance] is a challenge,'' Hammel explains. Yet, she adds, ''it's remarkable how much you can see.''
The three images reproduced here illustrate this. They were taken last fall respectively on Oct. 10 (upper left), Oct. 18 (upper right), and Nov. 2 (lower center).
The smallest detail the space telescope can resolve is 1,000 kilometers (620 miles) across. Thus the cloud systems and dark spots are major circulation features in Neptune's atmosphere, which consists mainly of hydrogen and helium seasoned with acetylene, ethane, methane, and other hydrocarbons. Dark spots probably represent high-altitude regions where this soupy mix has cleared enough to reveal lower atmospheric levels.
Hammel points out that, unlike Jupiter's famous persistent red spot, Neptune's dark spots that come and go are ''a global phenomenon.'' They are much larger relative to the planet's size than is Jupiter's spot. The major cloud systems Hubble sees also have planet-wide significance. This makes Neptune a better planet for studying global atmospheric circulation than Jupiter, Hammel says. It can provide a reality check for scientists as they try to apply basic principles of atmospheric circulation learned on Earth to explain what happens in the atmospheres of alien worlds.
Meanwhile, the Hubble telescope is helping scientists learn more about asteroids. In releasing the Neptune images last month, the Space Telescope Science Institute in Baltimore also released a set of 24 images of the asteroid Vesta. These show a full 5.34-hour rotation of the asteroid, which is about 1,000 kilometers (600 miles) long by 525 kilometers (326 miles) wide. They were taken between Nov. 28 and Dec. 1 last year when Vesta was 251 million kilometers (156 million miles) away.
At that distance, the space telescope can resolve details as small as 80 kilometers (50 miles) across. Mapping the asteroid on this scale and using ground-based observations as well, scientists should be able to construct a geochemical map of the asteroid's surface.
Benjamin Zellner of Georgia Southern University in Statesboro, who leads this Hubble investigation, has explained that Vesta has remained essentially unchanged since the formation of the solar system. It could retain a ''record'' of an important part of our solar system's history.